U.S. patent application number 10/033412 was filed with the patent office on 2002-05-09 for power conversion apparatus.
This patent application is currently assigned to TDK CORPORATION. Invention is credited to Itoh, Kazuyuki, Okita, Yoshihisa, Tanaka, Katsuaki.
Application Number | 20020054499 10/033412 |
Document ID | / |
Family ID | 26591202 |
Filed Date | 2002-05-09 |
United States Patent
Application |
20020054499 |
Kind Code |
A1 |
Tanaka, Katsuaki ; et
al. |
May 9, 2002 |
Power conversion apparatus
Abstract
The present invention discloses a power conversion apparatus
comprising a control circuit for generating a switching signal at
the timing allowing soft-switching to be achieved, and free from
any occurrence of ripple. The power conversion apparatus includes a
first main switch (Q1) and a second main switch (Q2) which are
connected in series with each other. One of the ends of the first
main switch is connected with the positive side of a DC power
supply, and one of the ends of the second main switch is connected
to the negative side of the DC power supply. A diode (D1, D2) is
connected in parallel with each of the main switches so as to
become reverse biased with respect to the DC power supply. A
main-switch snubber capacitor (C1, C2) is connected in parallel
with each of the main switches. A load is connected with the
junction between the pair of main switches, and the main switches
are controllably switched according a switching signal from a
control circuit to generate an output. A first auxiliary resonant
circuit including serial-connected first and second auxiliary
switches (Q3, Q4, Q5, Q6) and a resonant inductor (L1) connected in
series with the second auxiliary switch is connected with each of
the positive side of the DC power supply and the junction between
the two main switches. A diode is connected to each of the first
and second auxiliary switches so as to become reverse biased with
respect to the DC power supply. The control circuit provides a
turn-on signal to the first and second auxiliary switches according
to a voltage signal as an input representing the voltage across
each of the main switches and auxiliary switches from voltage
detecting means before a turn-on signal as the switching signal is
provided to the first main switch.
Inventors: |
Tanaka, Katsuaki; (Tokyo,
JP) ; Okita, Yoshihisa; (Tokyo, JP) ; Itoh,
Kazuyuki; (Tokyo, JP) |
Correspondence
Address: |
FRISHAUF, HOLTZ, GOODMAN, LANGER & CHICK, P.C.
767 Third Avenue - 25th Floor
New York
NY
10017
US
|
Assignee: |
TDK CORPORATION
Tokyo
JP
|
Family ID: |
26591202 |
Appl. No.: |
10/033412 |
Filed: |
December 26, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10033412 |
Dec 26, 2001 |
|
|
|
PCT/JP01/03663 |
Apr 26, 2001 |
|
|
|
Current U.S.
Class: |
363/132 |
Current CPC
Class: |
H02M 7/5233 20130101;
Y02B 70/10 20130101; H02M 1/0048 20210501; H02M 1/12 20130101; H02M
7/4826 20130101 |
Class at
Publication: |
363/132 |
International
Class: |
H02M 007/5387 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 28, 2000 |
JP |
2000-130513 |
Aug 11, 2000 |
JP |
2000-243563 |
Claims
What is claimed is:
1. A power conversion apparatus including: at least a pair of main
switches composed of serial-connected first and second main
switches, one of the ends of said first main switch being connected
with the positive side of a DC power supply, one of the ends of
said second main switch being connected to the negative side of
said DC power supply; a diode connected in parallel with each of
said main switches so as to become reverse biased with respect to
said DC power supply; a main-switch snubber capacitor connected in
parallel with each of said main switches; a load connected with the
junction between said pair of main switches; and a control circuit
for forming a switching signal for controlling the switching
operation of said main switches by using a load voltage and/or a
load current as an input thereof, wherein said main switches are
controllably switched according said switching signal from said
control circuit so as to generate an output, said power conversion
apparatus comprising: a first auxiliary resonant circuit including
serial-connected first and second auxiliary switches and a resonant
inductor connected in series with said second auxiliary switch,
said first auxiliary resonant circuit being connected with each of
the positive side of said DC power supply and the junction between
said pair of main switches; a diode connected to each of said first
and second auxiliary switches so as to become reverse biased with
respect to said DC power supply; and voltage detecting means for
detecting the voltage across each of said main switches and
auxiliary switches, wherein said control circuit is applied with a
voltage signal as an input representing said voltage across each of
said main switches and auxiliary switches from said voltage
detecting means, said control circuit being adapted to provide a
turn-on signal to said first and second auxiliary switches
according to said input before a turn-on signal as the switching
signal is provided to said first main switch, said control circuit
being adapted to provide the turn-on signal to said first and
second auxiliary switches when the load current passes through said
diode connected in parallel with said second main switch, so as to
turn on said first and second auxiliary switches to direct the
current from said DC power supply to said resonant inductor,
whereby a resonant circuit is formed by said resonant inductor and
said snubber capacitors connected in parallel with said main
switches when the current of said resonant inductor goes up
approximately to the load current, wherein said control circuit is
adapted to output a signal for turning on said first main switch
when the voltage across said first main switch goes down
approximately to zero through the resonance in said resonance
circuit.
2. A power conversion apparatus as defined in claim 1, which
further includes: serial-connected third and fourth auxiliary
switches which are connected between the negative side of said DC
power supply and said inductor so as to form a second auxiliary
resonant circuit; an auxiliary-switch snubber capacitor connected
between the junction between said first and second auxiliary
switches and the junction between said third and fourth auxiliary
switches; and a diode connected to each of said third and fourth
auxiliary switches so as to become reverse biased with respect to
said DC power supply, wherein said control circuit is adapted to
provide a turn-off signal to said first auxiliary switch when the
charged voltage of said auxiliary-switch snubber capacitor is
approximately equal to the voltage of said DC power supply after
said first main switch is turned on, and to provide the turn-off
signal to said second auxiliary switch when the charged voltage of
said auxiliary-switch snubber capacitor is approximately equal to
zero after said first main switch is turned on, so as to achieve
soft-switching of said first and second auxiliary switches.
3. A power conversion apparatus including: at least a pair of main
switches composed of serial-connected first and second main
switches, one of the ends of said first main switch being connected
with the positive side of a DC power supply, one of the ends of
said second main switch being connected to the negative side of
said DC power supply; a diode connected in parallel with each of
said main switches so as to become reverse biased with respect to
said DC power supply; a main-switch snubber capacitor connected in
parallel with each of said main switches; a load connected with the
junction between said pair of main switches; and a control circuit
for forming a switching signal for controlling the switching
operation of said main switches by using a load voltage and/or a
load current as an input thereof, wherein said main switches are
controllably switched according said switching signal from said
control circuit so as to generate an output, said power conversion
apparatus comprising: a second auxiliary resonant circuit including
serial-connected third and fourth auxiliary switches and a resonant
inductor connected in series with said fourth auxiliary switch,
said second auxiliary resonant circuit being connected with each of
the negative side of said DC power supply and the junction between
said pair of main switches; a diode connected to each of said third
and fourth auxiliary switches so as to become reverse biased with
respect to said DC power supply; and voltage detecting means for
detecting the voltage across each of said main switches and
auxiliary switches, wherein said control circuit is applied with a
voltage signal as an input representing said voltage across each of
said main switches and auxiliary switches from said voltage
detecting means, said control circuit being adapted to provide a
turn-on signal to said third and fourth auxiliary switches
according to said input, before a turn-on signal as the switching
signal is provided to said second main switch, when said first main
switch is in ON-state to allow the load current to pass through
said first main switch and said load current is less than a
threshold associated with the product of multiplying the capacity
of said main-switch snubber capacitor by the power supply voltage
of said DC power supply, so as to turn on said third and fourth
auxiliary switches to direct the current from said DC power supply
to said resonant inductor, wherein said control circuit is adapted
to provide a turn-off signal to said first main switch when the
current of said resonant inductor goes up approximately to said
threshold, so as to turn off said first main switch.
4. A power conversion apparatus as defined in claim 3, wherein said
control circuit is adapted to provide the turn-on signal to said
second main switch, when said third and fourth auxiliary switches
are in ON-state and the current passing through said resonant
inductor is refluxed from said third and fourth auxiliary switches
through said diode connected in parallel with said second main
switch.
5. A power conversion apparatus as defined in claim 4, wherein said
control circuit is adapted to provide the turn-off signal to said
third auxiliary switch after said second main switch is turned on,
when the initial voltage of said auxiliary-switch snubber capacitor
is approximately equal to the voltage of said DC power supply, and
to provide the turn-off signal to said fourth auxiliary switch
after said second main switch is turned on, when the initial
voltage of said auxiliary-switch snubber capacitor is approximately
equal to zero.
6. A power conversion apparatus as defined in claim 3, wherein said
control circuit is adapted to provide the turn-off signal to said
first main switch without providing any turn-on signal to said
third and fourth auxiliary switches, when said load current is
larger than said threshold.
7. A power conversion apparatus as defined in either one of claims
3 to 6, wherein said threshold is defined by the following
formula;I.sub.th=Cr.times.V.sub.in/t.sub.maxwhere I.sub.th is said
threshold, Cr being the capacity of said main-switch snubber
capacitor connected in parallel with said main switch, V.sub.in
being the voltage of said DC power supply, and t.sub.max being the
maximum allowable value of the time required for the load current
to commutate from one of said first and second main switches to the
other main switch.
8. A power conversion apparatus including: at least a pair of main
switches composed of serial-connected first and second main
switches, one of the ends of said first main switch being connected
with the positive side of a DC power supply, one of the ends of
said second main switch being connected to the negative side of
said DC power supply; a diode connected in parallel with each of
said main switches so as to become reverse biased with respect to
said DC power supply; a main-switch snubber capacitor connected in
parallel with each of said main switches; a load connected with the
junction between said pair of main switches; and a control circuit
for forming a switching signal for controlling the switching
operation of said main switches by using a load voltage and/or a
load current as an input thereof, wherein said main switches are
controllably switched according said switching signal from said
control circuit so as to generate an output, said power conversion
apparatus comprising: a second auxiliary resonant circuit including
serial-connected third and fourth auxiliary switches and a resonant
inductor connected in series with said fourth auxiliary switch,
said second auxiliary resonant circuit being connected with each of
the negative side of said DC power supply and the junction between
said pair of main switches; a diode connected to each of said third
and fourth auxiliary switches so as to become reverse biased with
respect to said DC power supply; and voltage detecting means for
detecting the voltage across each of said main switches and
auxiliary switches, wherein said control circuit is applied with a
voltage signal as an input representing said voltage across each of
said main switches and auxiliary switches from said voltage
detecting means, said control circuit being adapted to provide a
turn-on signal to said third and fourth auxiliary switches and
provide a turn-off signal to said first main switch according to
said input, before a turn-on signal as the switching signal is
provided to said second main switch, when said first main switch is
in ON-state to allow the load current to pass through said first
main switch and said load current is less than a threshold
associated with the product of multiplying the capacity of said
main-switch snubber capacitor by the power supply voltage of said
DC power supply, so as to turn on said third and fourth auxiliary
switches to generate a resonance between said resonant inductor and
said snubber capacitors connected in parallel with said main
switches.
9. A power conversion apparatus including: at least a pair of main
switches composed of serial-connected first and second main
switches, one of the ends of said first main switch being connected
with the positive side of a DC power supply, one of the ends of
said second main switch being connected to the negative side of
said DC power supply; a diode connected in parallel with each of
said main switches so as to become reverse biased with respect to
said DC power supply; a main-switch snubber capacitor connected in
parallel with each of said main switches; a load connected with the
junction between said pair of main switches; and a control circuit
for forming a switching signal for controlling the switching
operation of said main switches by using a load voltage and/or a
load current as an input thereof, wherein said main switches are
controllably switched according said switching signal from said
control circuit so as to generate an output, said power conversion
apparatus comprising: a first auxiliary resonant circuit including
serial-connected first and second auxiliary switches and a resonant
inductor connected in series with said second auxiliary switch,
said first auxiliary resonant circuit being connected with each of
the positive side of said DC power supply and the junction between
said pair of main switches; a diode connected to each of said first
and second auxiliary switches so as to become reverse biased with
respect to said DC power supply; a second auxiliary resonant
circuit formed by connecting serial-connected third and fourth
auxiliary switches between the negative side of said DC power
supply and said inductor; an auxiliary-switch snubber capacitor
connected between the junction between said first and second
auxiliary switches and the junction between said third and fourth
auxiliary switches; a diode connected to each of said third and
fourth auxiliary switches so as to become reverse biased with
respect to said DC power supply; and voltage detecting means for
detecting the voltage across each of said main switches and
auxiliary switches, wherein said control circuit is applied with a
voltage signal as an input representing said voltage across each of
said main switches and auxiliary switches from said voltage
detecting means, said control circuit being adapted to provide a
turn-on signal to said first and second auxiliary switches
according to said input before a turn-on signal as the switching
signal is provided to said first main switch, said control circuit
being adapted to provide the turn-on signal to said first and
second auxiliary switches when the load current passes through said
diode connected in parallel with said second main switch, so as to
turn on said first and second auxiliary switches to direct the
current from said DC power supply to said resonant inductor,
wherein said control circuit is adapted to output a signal for
turning on said first main switch when the voltage across said
first main switch goes down approximately to zero through the
resonance in a resonance circuit formed by said resonant inductor
and said snubber capacitors connected in parallel with said main
switches when the current of said resonant inductor goes up
approximately to the load current.
10. A power conversion apparatus as defined in claim 9, wherein
said control circuit is adapted to provide a turn-off signal to
said first auxiliary switch when the charged voltage of said
auxiliary-switch snubber capacitor is approximately equal to the
voltage of said DC power supply after said first main switch is
turned on, and to provide the turn-off signal to said second
auxiliary switch when the charged voltage of said auxiliary-switch
snubber capacitor is approximately equal to zero after said first
main switch is turned on, so as to achieve soft-switching of said
first and second auxiliary switches.
11. A power conversion apparatus as defined in either one of claims
9 and 10, wherein said control circuit is adapted to provide a
turn-on signal to said third and fourth auxiliary switches, before
a turn-on signal as the switching signal is provided to said second
main switch, when said first main switch is in ON-state to allow
the load current to pass through said first main switch and said
load current is less than a threshold associated with the product
of multiplying the capacity of said main-switch snubber capacitor
by the power supply voltage of said DC power supply, so as to turn
on said third and fourth auxiliary switches to direct the current
from said DC power supply to said resonant inductor, wherein said
control circuit is adapted to provide a turn-off signal to said
first main switch when the current of said resonant inductor goes
up approximately to said threshold, so as to turn off said first
main switch.
12. A power conversion apparatus as defined in claim 11, wherein
said control circuit is adapted to provide the turn-on signal to
said second main switch, when said third and fourth auxiliary
switches are in ON-state and the current passing through said
resonant inductor is refluxed from said third and fourth auxiliary
switches through said diode connected in parallel with said second
main switch.
13. A power conversion apparatus as defined in either one of claims
9 and 10, wherein said control circuit is adapted to provide a
turn-on signal to said third and fourth auxiliary switches and
provide a turn-off signal to said first main switch, before a
turn-on signal as the switching signal is provided to said second
main switch, when said first main switch is in ON-state to allow
the load current to pass through said first main switch and said
load current is less than a threshold associated with the product
of multiplying the capacity of said main-switch snubber capacitor
by the power supply voltage of said DC power supply, so as to turn
on said third and fourth auxiliary switches to generate a resonance
between said resonant inductor and said snubber capacitor connected
in parallel with said first main switch.
14. A power conversion apparatus including: at least a pair of main
switches composed of serial-connected first and second main
switches, one of the ends of said first main switch being connected
with the positive side of a DC power supply, one of the ends of
said second main switch being connected to the negative side of
said DC power supply; a diode connected in parallel with each of
said main switches so as to become reverse biased with respect to
said DC power supply; a main-switch snubber capacitor connected in
parallel with each of said main switches; a load connected with the
junction between said pair of main switches; and a control circuit
for forming a switching signal for controlling the switching
operation of said main switches by using a load voltage and/or a
load current as an input thereof, wherein said main switches are
controllably switched according said switching signal from said
control circuit so as to generate an output, said power conversion
apparatus comprising: a second auxiliary resonant circuit including
serial-connected third and fourth auxiliary switches and a resonant
inductor connected in series with said fourth auxiliary switch,
said second auxiliary resonant circuit being connected with each of
the negative side of said DC power supply and the junction between
said pair of main switches; a diode connected to each of said third
and fourth auxiliary switches so as to become reverse biased with
respect to said DC power supply; and voltage detecting means for
detecting the voltage across each of said main switches and
auxiliary switches, wherein said control circuit is applied with a
voltage signal as an input representing said voltage across each of
said main switches and auxiliary switches from said voltage
detecting means, said control circuit being adapted to provide a
turn-on signal to said third and fourth auxiliary switches
according to said input before a turn-on signal as the switching
signal is provided to said second main switch, said control circuit
being adapted to provide the turn-on signal to said third and
fourth auxiliary switches when the load current passes through said
diode connected in parallel with said first main switch, so as to
turn on said third and fourth auxiliary switches to direct the
current from said DC power supply to said resonant inductor,
whereby a resonant circuit is formed by said resonant inductor and
said snubber capacitors connected in parallel with said main
switches when the current of said resonant inductor goes up
approximately to the load current, wherein said control circuit is
adapted to output a signal for turning on said second main switch
when the voltage across said second main switch goes down
approximately to zero through the resonance in said resonance
circuit.
15. A power conversion apparatus as defined in claim 14, which
further includes: serial-connected first and second auxiliary
switches which are connected between the positive side of said DC
power supply and said inductor so as to form a first auxiliary
resonant circuit; an auxiliary-switch snubber capacitor connected
between the junction between said first and second auxiliary
switches and the junction between said third and fourth auxiliary
switches; and a diode connected to each of said first and second
auxiliary switches so as to become reverse biased with respect to
said DC power supply, wherein said control circuit is adapted to
provide a turn-off signal to said third auxiliary switch when the
charged voltage of said auxiliary-switch snubber capacitor is
approximately equal to the voltage of said DC power supply after
said second main switch is turned on, and to provide the turn-off
signal to said fourth auxiliary switch when the charged voltage of
said auxiliary-switch snubber capacitor is approximately equal to
zero after said second main switch is turned on, so as to achieve
soft-switching of said third and fourth auxiliary switches.
16. A power conversion apparatus including: at least a pair of main
switches composed of serial-connected first and second main
switches, one of the ends of said first main switch being connected
with the positive side of a DC power supply, one of the ends of
said second main switch being connected to the negative side of
said DC power supply; a diode connected in parallel with each of
said main switches so as to become reverse biased with respect to
said DC power supply; a main-switch snubber capacitor connected in
parallel with each of said main switches; a load connected with the
junction between said pair of main switches; and a control circuit
for forming a switching signal for controlling the switching
operation of said main switches by using a load voltage and/or a
load current as an input thereof, wherein said main switches are
controllably switched according said switching signal from said
control circuit so as to generate an output, said power conversion
apparatus comprising: a first auxiliary resonant circuit including
serial-connected first and second auxiliary switches and a resonant
inductor connected in series with said second auxiliary switch,
said first auxiliary resonant circuit being connected with each of
the negative side of said DC power supply and the junction between
said pair of main switches; a diode connected to each of said first
and second auxiliary switches so as to become reverse biased with
respect to said DC power supply; and voltage detecting means for
detecting the voltage across each of said main switches and
auxiliary switches, wherein said control circuit is applied with a
voltage signal as an input representing said voltage across each of
said main switches and auxiliary switches from said voltage
detecting means, said control circuit being adapted to provide a
turn-on signal to said first and second auxiliary switches
according to said input, before a turn-on signal as the switching
signal is provided to said first main switch, when said second main
switch is in ON-state to allow the load current to pass through
said second main switch and said load current is less than a
threshold associated with the product of multiplying the capacity
of said main-switch snubber capacitor by the power supply voltage
of said DC power supply, so as to turn on said first and second
auxiliary switches to direct the current from said DC power supply
to said resonant inductor, wherein said control circuit is adapted
to provide a turn-off signal to said second main switch when the
current of said resonant inductor goes up approximately to said
threshold, so as to turn off said second main switch.
17. A power conversion apparatus as defined in claim 16, wherein
said control circuit is adapted to provide the turn-on signal to
said first main switch, when said first and second auxiliary
switches are in ON-state and the current passing through said
resonant inductor is refluxed from said first and second auxiliary
switches through said diode connected in parallel with said first
main switch.
18. A power conversion apparatus as defined in claim 17, wherein
said control circuit is adapted to provide the turn-off signal to
said first auxiliary switch after said first main switch is turned
on, when the initial voltage of said auxiliary-switch snubber
capacitor is approximately equal to the voltage of said DC power
supply, and to provide the turn-off signal to said second auxiliary
switch after said first main switch is turned on, when the initial
voltage of said auxiliary-switch snubber capacitor is approximately
equal to zero.
19. A power conversion apparatus as defined in claim 16, wherein
said control circuit is adapted to provide the turn-off signal to
said second main switch without providing any turn-on signal to
said first and second auxiliary switches, when said load current is
larger than said threshold.
20. A power conversion apparatus as defined in either one of claims
16 to 19, wherein said threshold is defined by the following
formula;I.sub.th=Cr.times.Vr.sub.in/t.sub.maxwhere I.sub.th is said
threshold, Cr being the capacity of said main-switch snubber
capacitor connected in parallel with said main switch, V.sub.in
being the voltage of said DC power supply, and t.sub.max being the
maximum allowable value of the time required for the load current
to commutate from one of said first and second main switches to the
other main switch.
21. A power conversion apparatus including: at least a pair of main
switches composed of serial-connected first and second main
switches, one of the ends of said first main switch being connected
with the positive side of a DC power supply, one of the ends of
said second main switch being connected to the negative side of
said DC power supply; a diode connected in parallel with each of
said main switches so as to become reverse biased with respect to
said DC power supply; a main-switch snubber capacitor connected in
parallel with each of said main switches; a load connected with the
junction between said pair of main switches; and a control circuit
for forming a switching signal for controlling the switching
operation of said main switches by using a load voltage and/or a
load current as an input thereof, wherein said main switches are
controllably switched according said switching signal from said
control circuit so as to generate an output, said power conversion
apparatus comprising: a first auxiliary resonant circuit including
serial-connected first and second auxiliary switches and a resonant
inductor connected in series with said second auxiliary switch,
said first auxiliary resonant circuit being connected with each of
the negative side of said DC power supply and the junction between
said pair of main switches; a diode connected to each of said first
and second auxiliary switches so as to become reverse biased with
respect to said DC power supply; and voltage detecting means for
detecting the voltage across each of said main switches and
auxiliary switches, wherein said control circuit is applied with a
voltage signal as an input representing said voltage across each of
said main switches and auxiliary switches from said voltage
detecting means, said control circuit being adapted to provide a
turn-on signal to said first and second auxiliary switches and
provide a turn-off signal to said second main switch according to
said input, before a turn-on signal as the switching signal is
provided to said first main switch, when said second main switch is
in ON-state to allow the load current to pass through said second
main switch and said load current is less than a threshold
associated with the product of multiplying the capacity of said
main-switch snubber capacitor by the power supply voltage of said
DC power supply, so as to turn on said first and second auxiliary
switches to generate a resonance between said resonant inductor and
said snubber capacitor connected in parallel with said second main
switch.
22. A power conversion apparatus including: at least a pair of main
switches composed of serial-connected first and second main
switches, one of the ends of said first main switch being connected
with the positive side of a DC power supply, one of the ends of
said second main switch being connected to the negative side of
said DC power supply; a diode connected in parallel with each of
said main switches so as to become reverse biased with respect to
said DC power supply; a main-switch snubber capacitor connected in
parallel with each of said main switches; a load connected with the
junction between said pair of main switches; and a control circuit
for forming a switching signal for controlling the switching
operation of said main switches by using a load voltage and/or a
load current as an input thereof, wherein said main switches are
controllably switched according said switching signal from said
control circuit so as to generate an output, said power conversion
apparatus comprising: a first auxiliary resonant circuit including
serial-connected first and second auxiliary switches and a resonant
inductor connected in series with said second auxiliary switch,
said first auxiliary resonant circuit being connected with each of
the positive side of said DC power supply and the junction between
said pair of main switches; a diode connected to each of said first
and second auxiliary switches so as to become reverse biased with
respect to said DC power supply; a second auxiliary resonant
circuit formed by connecting serial-connected third and fourth
auxiliary switches between the negative side of said DC power
supply and said inductor; an auxiliary-switch snubber capacitor
connected between the junction between said first and second
auxiliary switches and the junction between said third and fourth
auxiliary switches; a diode connected to each of said third and
fourth auxiliary switches so as to become reverse biased with
respect to said DC power supply; and voltage detecting means for
detecting the voltage across each of said main switches and
auxiliary switches, wherein said control circuit is applied with a
voltage signal as an input representing said voltage across each of
said main switches and auxiliary switches from said voltage
detecting means, said control circuit being adapted to provide a
turn-on signal to said third and fourth auxiliary switches
according to said input before a turn-on signal as the switching
signal is provided to said second main switch, said control circuit
being adapted to provide the turn-on signal to said third and
fourth auxiliary switches when the load current passes through said
diode connected in parallel with said first main switch, so as to
turn on said third and fourth auxiliary switches to direct the
current from said DC power supply to said resonant inductor,
wherein said control circuit is adapted to output a signal for
turning on said second main switch when the voltage across said
second main switch goes down approximately to zero through the
resonance in a resonance circuit formed by said resonant inductor
and said snubber capacitors connected in parallel with said main
switches when the current of said resonant inductor goes up
approximately to the load current.
23. A power conversion apparatus as defined in claim 22, wherein
said control circuit is adapted to provide a turn-off signal to
said third auxiliary switch when the charged voltage of said
auxiliary-switch snubber capacitor is approximately equal to the
voltage of said DC power supply after said second main switch is
turned on, and to provide the turn-off signal to said fourth
auxiliary switch when the charged voltage of said auxiliary-switch
snubber capacitor is approximately equal to zero after said second
main switch is turned on, so as to achieve soft-switching of said
third and fourth auxiliary switches.
24. A power conversion apparatus as defined in either one of claims
22 and 23, wherein said control circuit is adapted to provide a
turn-on signal to said first and second auxiliary switches, before
a turn-on signal as the switching signal is provided to said first
main switch, when said second main switch is in ON-state to allow
the load current to pass through said second main switch and said
load current is less than a threshold associated with the product
of multiplying the capacity of said main-switch snubber capacitor
by the power supply voltage of said DC power supply, so as to turn
on said first and second auxiliary switches to direct the current
from said DC power supply to said resonant inductor, wherein said
control circuit is adapted to provide a turn-off signal to said
second main switch when the current of said resonant inductor goes
up approximately to said threshold, so as to turn off said second
main switch.
25. A power conversion apparatus as defined in claim 24, wherein
said control circuit is adapted to provide the turn-on signal to
said first main switch, when said first and second auxiliary
switches are in ON-state and the current passing through said
resonant inductor is refluxed from said first and second auxiliary
switches through said diode connected in parallel with said first
main switch.
26. A power conversion apparatus as defined in either one of claims
22 and 23, wherein said control circuit is adapted to provide a
turn-on signal to said first and second auxiliary switches and
provide a turn-off signal to said second main switch, before a
turn-on signal as the switching signal is provided to said first
main switch, when said second main switch is in ON-state to allow
the load current to pass through said second main switch and said
load current is less than a threshold associated with the product
of multiplying the capacity of said main-switch snubber capacitor
by the power supply voltage of said DC power supply, so as to turn
on said first and second auxiliary switches to generate a resonance
between said resonant inductor and said snubber capacitor connected
in parallel with said second main switch.
Description
TECHNICAL FIELD
[0001] The present invention generally relates to a power
conversion apparatus. More specifically, the present invention
relates to a power conversion apparatus having a control circuit
capable of achieving soft-switching of a switching element.
BACKGROUND ART
[0002] In a power conversion apparatus such as a converter or an
inverter, various circuits having a soft-switching function are
currently under development in order to reduce a switching loss in
each switching element and increase a switching frequency.
[0003] For example, U.S. Pat. No. 5,047,913 discloses a converter
in which the voltage of a DC (Direct Current) power supply is split
into two one-half voltages, by a pair of series-connected
capacitors, and the junction between the capacitors is connected to
the junction between a pair of series-connected main switches,
through a circuit including a series connection of a bidirectional
switch and an inductor. A load is connected to the junction between
the main switches. A diode is connected in parallel with each of
the main switches to allow each of the diodes to become reverse
biased with respect to the DC power supply. A snubber capacitor is
connected in parallel with each of the main switches. In the power
conversion apparatus described in this U.S. patent, it is intended
to obtain a specific condition for achieving soft-switching of all
switches by making up an auxiliary resonant commutation circuit
with the circuit of the bidirectional switch and the inductor makes
up and performing resonant operation through the auxiliary resonant
commutation circuit.
[0004] In the above circuit, while such soft-switching is achieved
when each of the main switches are turned on, a turn-off loss is
caused by turning off each of the main switches. Specifically, in
this circuit, for commutating from one of the main switches to the
other main switch, the bidirectional switch is first turned on in
the state when a load current is refluxed to the diode connected in
parallel with the one main switch, so as to generate a resonant
state. Then, when the current of the inductor increases up to a
sufficient extent for commutation, the above one main switch is
turned off. However, the control taught in this U.S. patent is
inevitably involved with turn-off loss of the main switches. The
necessity for detecting of a resonant current required for
commutation also forces a complicated control.
[0005] Furthermore, in this prior art circuit, for preventing the
accumulated energy in the snubber capacitor connected to each of
the main switches from being consumed as short-circuit loss in the
main switches under light load, the auxiliary switch is turned on
before switching the main switches to commutate from one of the
main switches to the other main switch. In this control, upon
turning on the auxiliary switch, the inductor current starts
passing through the one main switch along with the load current.
When the current goes up to a certain threshold, the one main
switch is turned off to charge or discharge the energy in each of
the snubber capacitors. After the completion of commutation, the
other main switch is turned on. Thus, a turn-on at zero current is
achieved in the other main switch, and the energy of the snubber
capacitor does not become a loss. However, the control taught in
this U.S. patent is undesirably involved with complexity in control
due to the switching control according to detecting the inductor
current required for commutation.
[0006] Moreover, in the control taught by the U.S. patent, for
commutating with passing the load current through the diode
connected in parallel to the other main switch in the state when
the load current passes through the other main switch, the
commutation in large load current is achieved based on the load
current without activating the auxiliary resonant commutation
circuit. In small load current, the commutation is achieved based
on the sum of the resonant current and the load current with
activating the auxiliary resonant commutation circuit. This control
process undesirably involves a ripple voltage caused by operating
the power conversion apparatus as an inverter. Specifically, when
the power conversion apparatus is operated as an inverter according
to this control process, the current of the auxiliary resonant
commutation circuit generates a ripple having the same cycle as
that of the output voltage of the inverter at the midpoint of the
potential of the capacitor connected in series with the DC power
supply. If it is attempted to suppress this ripple voltage within
the allowable range of voltage variation, it will be required to
employ capacitors having larger capacity than those of conventional
circuits, resulting in larger size components.
[0007] Japan Patent Laid-Open Publication No. Hei 07-115775
discloses an inverter in which one ends of auxiliary switches is
connected respectively to a first split point having a first
potential and a second split point having a second potential, the
other ends of the auxiliary switches being connected with each
other, and the junction between the auxiliary switches being
connected with the junction between a pair of main switches through
a resonant inductor. A snubber capacitor is connected in parallel
with each of the main switches. A diode is connected in parallel
with each of the main switches and in the reverse bias direction
with respect to a DC power supply. In the circuit for a power
conversion apparatus disclosed in this Patent Laid-Open
Publication, an auxiliary resonant commutation circuit is formed of
the auxiliary switches, the resonant inductor, and the snubber
capacitors each connected in parallel with the main switches so as
to achieve soft-switching based on resonant current passing through
the formed resonant circuit.
[0008] The power conversion apparatus described in this Patent
Laid-Open Publication employs a battery to obtain the first and
second potentials. This undesirably makes the circuit larger in
size. If a capacitor is used to provide smaller size apparatus, a
ripple voltage having the same cycle as that of the output voltage
of the inverter will be generated between the first and second
potentials. This causes the same problem as that of the circuit
described in the above U.S. patent occurs.
DISCLOSURE OF THE INVENTION
[0009] In view of the aforementioned problem in conventional power
conversion apparatuses intended for achieving soft-switching, it is
therefore a primary object of the present invention to provide an
improved power conversion apparatus comprising a control circuit
for generating a switching signal at the timing allowing
soft-switching to be achieved, and free from any occurrence of
ripple.
[0010] In order to achieve this object, a power conversion
apparatus according to the present invention includes at least a
pair of main switches composed of serial-connected first and second
main switches, wherein one of the ends of the first main switch is
connected with the positive side of a DC power supply, and one of
the ends of the second main switch is connected to the negative
side of the DC power supply. The power conversion apparatus further
includes a diode connected in parallel with each of the main
switches so as to become reverse biased with respect to the DC
power supply, a main-switch snubber capacitor connected in parallel
with each of the main switches, a load connected with the junction
between the pair of main switches, and a control circuit for
forming a switching signal for controlling the switching operation
of the main switches by using a load voltage and/or a load current
as an input thereof, wherein the main switches are controllably
switched according the switching signal from the control circuit so
as to generate an output. Based on the above construction, the
power conversion apparatus of the present invention comprises a
first auxiliary resonant circuit including serial-connected first
and second auxiliary switches and a resonant inductor connected in
series with the second auxiliary switch, wherein the first
auxiliary resonant circuit is connected with each of the positive
side of the DC power supply and the junction between the pair of
main switches. The power conversion apparatus further includes a
diode connected to each of the first and second auxiliary switches
so as to become reverse biased with respect to the DC power supply,
and voltage detecting means for detecting the voltage across each
of the main switches and auxiliary switches. The control circuit is
applied with a voltage signal as an input representing the voltage
across each of the main switches and auxiliary switches from the
voltage detecting means, and then the control circuit provides a
turn-on signal to the first and second auxiliary switches according
to the input before a turn-on signal as the switching signal is
provided to the first main switch. The control circuit also
provides the turn-on signal to the first and second auxiliary
switches when the load current passes through the diode connected
in parallel with the second main switch, so as to turn on the first
and second auxiliary switches to direct the current from the DC
power supply to the resonant inductor. Then, a resonant circuit is
formed by the resonant inductor and the snubber capacitors
connected in parallel with the main switches when the current of
the resonant inductor goes up approximately to the load current,
and the control circuit outputs a signal for turning on the first
main switch when the voltage across the first main switch goes down
approximately to zero through the resonance in the resonance
circuit.
[0011] In another aspect of the present invention, the power
conversion apparatus may includes serial-connected third and fourth
auxiliary switches which are connected between the negative side of
the DC power supply and the inductor so as to form a second
auxiliary resonant circuit. Further, an auxiliary-switch snubber
capacitor is connected between the junction between the first and
second auxiliary switches and the junction between the third and
fourth auxiliary switches, and a diode is connected to each of the
third and fourth auxiliary switches so as to become reverse biased
with respect to the DC power supply. In this case, the control
circuit provides a turn-off signal to the first auxiliary switch
when the charged voltage of the auxiliary-switch snubber capacitor
is approximately equal to the voltage of the DC power supply after
the first main switch is turned on, and to provide the turn-off
signal to the second auxiliary switch when the charged voltage of
the auxiliary-switch snubber capacitor is approximately equal to
zero after the first main switch is turned on, so as to achieve
soft-switching of the first and second auxiliary switches.
[0012] In another aspect of the present invention, the power
conversion apparatus comprises a second auxiliary resonant circuit
including serial-connected third and fourth auxiliary switches and
a resonant inductor connected in series with the fourth auxiliary
switch, wherein the second auxiliary resonant circuit is connected
with each of the negative side of the DC power supply and the
junction between the pair of main switches. The power conversion
apparatus further includes a diode connected to each of the third
and fourth auxiliary switches so as to become reverse biased with
respect to the DC power supply, and voltage detecting means for
detecting the voltage across each of the main switches and
auxiliary switches. In this case, the control circuit is applied
with a voltage signal as an input representing the voltage across
each of the main switches and auxiliary switches from the voltage
detecting means. The control circuit provides a turn-on signal to
the third and fourth auxiliary switches according to the input,
before a turn-on signal as the switching signal is provided to the
second main switch, when the first main switch is in ON-state to
allow the load current to pass through the first main switch and
the load current is less than a threshold associated with the
product of multiplying the capacity of the main-switch snubber
capacitor by the power supply voltage of the DC power supply, so as
to turn on the third and fourth auxiliary switches to direct the
current from the DC power supply to the resonant inductor. Further,
the control circuit provides a turn-off signal to the first main
switch when the current of the resonant inductor goes up
approximately to the threshold, so as to turn off the first main
switch. The control circuit may be adapted to provide the turn-on
signal to the second main switch, when the third and fourth
auxiliary switches are in ON-state and the current passing through
the resonant inductor is refluxed from the third and fourth
auxiliary switches through the diode connected in parallel with the
second main switch.
[0013] The control circuit according another aspect of the present
invention may be adapted to provide the turn-off signal to the
third auxiliary switch after the second main switch is turned on,
when the initial voltage of the auxiliary-switch snubber capacitor
is approximately equal to the voltage of the DC power supply, and
to provide the turn-off signal to the fourth auxiliary switch after
the second main switch is turned on, when the initial voltage of
the auxiliary-switch snubber capacitor is approximately equal to
zero, so as to achieve soft-switching of the third and fourth
auxiliary switches. The control circuit may also be adapted to
provide the turn-off signal to the first main switch without
providing any turn-on signal to the third and fourth auxiliary
switches, when the load current is larger than the threshold, so as
to achieve soft-switching of the first main switch. The
aforementioned threshold may be defined by the following
formula;
I.sub.th=Cr.times.V.sub.in/t.sub.max
[0014] where I.sub.th is the threshold, Cr being the capacity of
the main-switch snubber capacitor connected in parallel with the
main switch, V.sub.in being the voltage of the DC power supply, and
t.sub.max being the maximum allowable value of the time required
for the load current to commutate from one of the first and second
main switches to the other main switch.
[0015] The control circuit according to another aspect of the
present invention provides a turn-on signal to the third and fourth
auxiliary switches according to a voltage signal as an input
representing the voltage across each of the main switches and the
auxiliary switches from the voltage detecting means, before a
turn-on signal as the switching signal is provided to the second
main switch, and the control circuit also provides the turn-on
signal to the third and fourth auxiliary switches when the load
current passes through the diode connected in parallel with the
first main switch, so as to turn on the third and fourth auxiliary
switches to direct the current from the DC power supply to the
resonant inductor. Then, a resonant circuit is formed by the
resonant inductor and the snubber capacitors connected in parallel
with the main switches when the current of the resonant inductor
goes up approximately to the load current, and the control circuit
outputs a signal for turning on the second main switch when the
voltage across the second main switch goes down approximately to
zero through the resonance in the resonance circuit.
[0016] According to another aspect of the present invention, the
control circuit may be adapted to provide a turn-off signal to the
third auxiliary switch when the charged voltage of the
auxiliary-switch snubber capacitor is approximately equal to the
voltage of the DC power supply after the second main switch is
turned on, and to provide the turn-off signal to the fourth
auxiliary switch when the charged voltage of the auxiliary-switch
snubber capacitor is approximately equal to zero after the second
main switch is turned on, so as to achieve soft-switching of the
third and fourth auxiliary switches. In this case, the control
circuit may also be adapted to provide a turn-on signal to the
first and second auxiliary switches, before a turn-on signal as the
switching signal is provided to the first main switch, when the
second main switch is in ON-state to allow the load current to pass
through the second main switch and the load current is less than a
threshold associated with the product of multiplying the capacity
of the main-switch snubber capacitor by the power supply voltage of
the DC power supply, so as to turn on the first and second
auxiliary switches to direct the current from the DC power supply
to the resonant inductor, and then to provide a turn-off signal to
the second main switch when the current of the resonant inductor
goes up approximately to the threshold, so as to turn off the
second main switch. Further, the control circuit may be adapted to
provide the turn-on signal to the first main switch, when the first
and second auxiliary switches are in ON-state and the current
passing through the resonant inductor is refluxed from the first
and second auxiliary switches through the diode connected in
parallel with the first main switch.
[0017] In another aspect, a power conversion apparatus according to
the present invention comprises a second auxiliary resonant circuit
including serial-connected third and fourth auxiliary switches and
a resonant inductor connected in series with the fourth auxiliary
switch, wherein the second auxiliary resonant circuit is connected
with each of the negative side of the DC power supply and the
junction between the pair of main switches. The power conversion
apparatus further includes a diode connected to each of the third
and fourth auxiliary switches so as to become reverse biased with
respect to the DC power supply, and voltage detecting means for
detecting the voltage across each of the main switches and
auxiliary switches. In this case, the control circuit is applied
with a voltage signal as an input representing the voltage across
each of the main switches and auxiliary switches from the voltage
detecting means, and the control circuit provides a turn-on signal
to the third and fourth auxiliary switches according to the input
before a turn-on signal as the switching signal is provided to the
second main switch. The control circuit also provides the turn-on
signal to the third and fourth auxiliary switches when the load
current passes through the diode connected in parallel with the
second main switch, so as to turn on the third and fourth auxiliary
switches to direct the current from the DC power supply to the
resonant inductor. Then, a resonant circuit is formed by the
resonant inductor and the snubber capacitors connected in parallel
with the main switches when the current of the resonant inductor
goes up approximately to the load current, and the control circuit
outputs a signal for turning on the second main switch when the
voltage across the second main switch goes down approximately to
zero through the resonance in the resonance circuit.
[0018] In another aspect of the present invention, the power
conversion apparatus may further includes serial-connected first
and second auxiliary switches which are connected between the
negative side of the DC power supply and the inductor so as to form
a first auxiliary resonant circuit, an auxiliary-switch snubber
capacitor connected between the junction between the first and
second auxiliary switches and the junction between the third and
fourth auxiliary switches, and a diode connected to each of the
first and second auxiliary switches so as to become reverse biased
with respect to the DC power supply. In this case, the control
circuit provides a turn-off signal to the third auxiliary switch
when the charged voltage of the auxiliary-switch snubber capacitor
is approximately equal to the voltage of the DC power supply after
the second main switch is turned on, and to provide the turn-off
signal to the fourth auxiliary switch when the charged voltage of
the auxiliary-switch snubber capacitor is approximately equal to
zero after the second main switch is turned on, so as to achieve
soft-switching of the third and fourth auxiliary switches.
[0019] In another aspect of the present invention, a power
conversion apparatus comprises a first auxiliary resonant circuit
including serial-connected first and second auxiliary switches and
a resonant inductor connected in series with the second auxiliary
switch, wherein the first auxiliary resonant circuit is connected
with each of the positive side of the DC power supply and the
junction between the pair of main switches. The power conversion
apparatus further includes a diode connected to each of the first
and second auxiliary switches so as to become reverse biased with
respect to the DC power supply, and voltage detecting means for
detecting the voltage across each of the main switches and
auxiliary switches. In this case, the control circuit is applied
with a voltage signal as an input representing the voltage across
each of the main switches and auxiliary switches from the voltage
detecting means, and the control circuit then provides a turn-on
signal to the first and second auxiliary switches according to the
input, before a turn-on signal as the switching signal is provided
to the first main switch, when the second main switch is in
ON-state to allow the load current to pass through the second main
switch and the load current is less than a threshold associated
with the product of multiplying the capacity of the main-switch
snubber capacitor by the power supply voltage of the DC power
supply, so as to turn on the first and second auxiliary switches to
direct the current from the DC power supply to the resonant
inductor. Further, the control circuit provides a turn-off signal
to the second main switch when the current of the resonant inductor
goes up approximately to the threshold, so as to turn off the
second main switch. The control circuit may be adapted to provide
the turn-on signal to the first main switch, when the first and
second auxiliary switches are in ON-state and the current passing
through the resonant inductor is refluxed from the first and second
auxiliary switches through the diode connected in parallel with the
first main switch.
[0020] According to another aspect of the present invention, the
control circuit may be adapted to provide the turn-off signal to
the first auxiliary switch after the first main switch is turned
on, when the initial voltage of the auxiliary-switch snubber
capacitor is approximately equal to the voltage of the DC power
supply, and to provide the turn-off signal to the second auxiliary
switch after the first main switch is turned on, when the initial
voltage of the auxiliary-switch snubber capacitor is approximately
equal to zero, so as to achieve soft-switching of the first and
second auxiliary switches. The control circuit can also achieve
soft-switching in the second main switch by providing the turn-off
signal to the second main switch without providing any turn-on
signal to the first and second auxiliary switches, when the load
current is larger than a threshold. The threshold in this case may
be defined by the same formula as described above.
[0021] According to another aspect of the present invention, the
control circuit is applied with a voltage signal as an input
representing the voltage across each of the main switches and
auxiliary switches from the voltage detecting means. Then, the
control circuit provides a turn-on signal to the first and second
auxiliary switches according to the input before a turn-on signal
as the switching signal is provided to the first main switch, and
provides the turn-on signal to the first and second auxiliary
switches when the load current passes through the diode connected
in parallel with the second main switch, so as to turn on the first
and second auxiliary switches to direct the current from the DC
power supply to the resonant inductor. Then, the control circuit
outputs a signal for turning on the first main switch when the
voltage across the first main switch goes down approximately to
zero through the resonance in a resonance circuit formed by the
resonant inductor and the snubber capacitors connected in parallel
with the main switches when the current of the resonant inductor
goes up approximately to the load current.
[0022] According another aspect of the present invention, the
control circuit may be adapted to provide a turn-off signal to the
first auxiliary switch when the charged voltage of the
auxiliary-switch snubber capacitor is approximately equal to the
voltage of the DC power supply after the first main switch is
turned on, and to provide the turn-off signal to the second
auxiliary switch when the charged voltage of the auxiliary-switch
snubber capacitor is approximately equal to zero after the second
main switch is turned on, so as to achieve soft-switching of the
first and second auxiliary switches. In this case, the control
circuit may also be adapted to provide a turn-on signal to the
third and fourth auxiliary switches, before a turn-on signal as the
switching signal is provided to the second main switch, when the
first main switch is in ON-state to allow the load current to pass
through the first main switch and the load current is less than a
threshold associated with the product of multiplying the capacity
of the main-switch snubber capacitor by the power supply voltage of
the DC power supply, so as to turn on the third and fourth
auxiliary switches to direct the current from the DC power supply
to the resonant inductor, and then to provide a turn-off signal to
the first main switch when the current of the resonant inductor
goes up approximately to the threshold, so as to turn off the first
main switch. Further, the control circuit may be adapted to provide
the turn-on signal to the second main switch, when the third and
fourth auxiliary switches are in ON-state and the current passing
through the resonant inductor is refluxed from the third and fourth
auxiliary switches through the diode connected in parallel with the
second main switch.
[0023] In another aspect of the present invention, the control
circuit provides a turn-on signal to the third and fourth auxiliary
switches and provide a turn-off signal to the first main switch,
before a turn-on signal as the switching signal is provided to the
second main switch, when the first main switch is in ON-state to
allow the load current to pass through the first main switch and
the load current is less than a threshold associated with the
product of multiplying the capacity of the main-switch snubber
capacitor by the power supply voltage of the DC power supply, so as
to turn on the third and fourth auxiliary switches to generate a
resonance between the resonant inductor and the snubber capacitor,
to achieve the commutation between the main switches. Similarly, in
light load, the control circuit provides a turn-on signal to the
third and fourth auxiliary switches and provide a turn-off signal
to the second main switch, before a turn-on signal as the switching
signal is provided to the first main switch, when the second main
switch is in ON-state to allow the load current to pass through the
second main switch and the load current passes through the second
main switch the control circuit provides a turn-on signal to the
third and fourth auxiliary switches and provide a turn-off signal
to the first main switch, before a turn-on signal as the switching
signal is provided to the second main switch, when the first main
switch is in ON-state to allow the load current to pass through the
first main switch and the load current is less than a threshold
associated with the product of multiplying the capacity of the
main-switch snubber capacitor by the power supply voltage of the DC
power supply, so as to turn on the third and fourth auxiliary
switches to generate a resonance between the resonant inductor and
the snubber capacitor, to achieve the commutation between the main
switches.
[0024] The control according to the above aspect can advantageously
eliminate the need for detecting current and thereby facilitate
simplifying the control. Further, since no indicator current is
passed through the main switches, the turn-off loss in the mains
switches is not increased.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] FIG. 1 is a circuit diagram showing the circuitry of a power
conversion apparatus of the present invention;
[0026] FIG. 2 illustrates waveforms in ON-state of a main switch
when the initial voltage of a snubber capacitor of an auxiliary
resonant commutation circuit in the circuitry of FIG. 1 is
approximately equal to the voltage of a DC power supply;
[0027] FIG. 3 illustrates waveforms in ON-state of the main switch
when the initial voltage of the snubber capacitor of the auxiliary
resonant commutation circuit in the circuitry of FIG. 1 is
approximately equal to zero;
[0028] FIG. 4 illustrates waveforms in OFF-state of the main switch
when the initial voltage of the snubber capacitor of the auxiliary
resonant commutation circuit in the circuitry of FIG. 1 is
approximately equal to the voltage of the DC power supply;
[0029] FIG. 5 illustrates waveforms in OFF-state of the main switch
when the initial voltage of the snubber capacitor of the auxiliary
resonant commutation circuit in the circuitry of FIG. 1 is
approximately equal to zero;
[0030] FIG. 6 is a circuit diagram showing an application of the
circuit of the present invention;
[0031] FIG. 7 is a circuit diagram showing another application of
the circuit of the present invention;
[0032] FIG. 8 is a circuit diagram showing still another
application of the circuit of the present invention;
[0033] FIG. 9 is a circuit diagram showing a modification of the
circuit of FIG. 1.
[0034] FIG. 10 illustrates waveforms in ON-state of a main switch
when the initial voltage of a snubber capacitor of an auxiliary
resonant commutation circuit in the circuitry of FIG. 9 is
approximately equal to the voltage of a DC power supply;
[0035] FIG. 11 illustrates waveforms in ON-state of the main switch
when the initial voltage of the snubber capacitor of the auxiliary
resonant commutation circuit in the circuitry of FIG. 9 is
approximately equal to zero;
[0036] FIG. 12 illustrates waveforms in OFF-state of the main
switch when the initial voltage of the snubber capacitor of the
auxiliary resonant commutation circuit in the circuitry of FIG. 9
is approximately equal to the voltage of the DC power supply;
[0037] FIG. 13 illustrates waveforms in OFF-state of the main
switch when the initial voltage of the snubber capacitor of the
auxiliary resonant commutation circuit in the circuitry of FIG. 9
is approximately equal to zero;
[0038] FIG. 14 illustrates waveforms showing another example of the
control according to the present invention, in the same state as
that in FIG. 4; and
[0039] FIG. 15 illustrates waveforms showing still another example
of the control according to the present invention, in the same
state as that in FIG. 15.
BEST MODE FOR CARRYING OUT THE INVENTION
[0040] Various embodiments of the present invention will now be
described with referring to drawings. FIG. 1 is a circuit diagram
showing a first embodiment of the present invention. This circuit
includes first and second main switches Q1, Q2 which are connected
in series with each other. One Q1 of the main switches is connected
with the positive side of a DC power supply V.sub.in, and the other
one Q2 of main switches is connected with the negative side of the
DC power supply V.sub.in. A Diode D1 is connected in parallel with
the main switches Q1, and a diode D2 is connected in parallel with
the main switches Q2. Each of the diodes D1, D2 is arranged to
become reverse biased with respect to the DC power supply, that is,
their forward directions are aligned with respect to the positive
side of the DC power supply V.sub.in. Main-switch snubber
capacitors C1, C2 are connected in parallel with the main switches
Q1, Q2, respectively. A capacitor C4 serving as the DC power supply
V.sub.in is connected in parallel with the series-connected main
switches Q1, Q2. An output V.sub.out is picked up from the junction
between the main switches Q1, Q2.
[0041] The illustrated circuit employs an auxiliary resonant
commutation circuit. This auxiliary resonant commutation circuit
includes first and second auxiliary switches Q5, Q3 connected in
series with each other. The first auxiliary switch Q5 is connected
with the positive side of the DC power supply V.sub.in. The second
auxiliary switch Q3 is connected with the junction between the main
switches Q1, Q2 through a resonant inductor L1. Diodes D5, D3 are
connected in parallel with the first and second auxiliary switches
Q5, Q3, respectively, so as to become reverse biased with respect
to the DC power supply V.sub.in.
[0042] The auxiliary resonant commutation circuit includes third
and fourth auxiliary switch Q6, Q4 connected in series with each
other. One of ends of the third auxiliary switch Q6 is connected
with the negative side of the DC power supply V.sub.in. The fourth
auxiliary switch Q4 is connected with one of the ends of the
resonant inductor L1. Diodes D6, D4 are connected in parallel with
the third and fourth auxiliary switches Q6, Q4, respectively, so as
to become reverse biased with respect to the DC power supply
V.sub.in. An auxiliary resonant commutation circuit snubber
capacitor (hereinafter referred to as "auxiliary resonant snubber
capacitor) C3 is connected between the first and third auxiliary
switches Q5, Q6.
[0043] The illustrated circuit further includes a control circuit S
for generating a switching signal for controlling each switching
operation of the switches. The control circuit S receives a load
current I.sub.out and a load voltage V.sub.out as an input signal,
and then calculates the switching timing of the main switches Q1,
Q2 to generate a switching signal. A voltage detector is provided
to each of the switches to detect the voltage across each of them.
The control circuit S receives the voltage signal as an input
signal from these voltage detectors. Then, the control circuit S
outputs the switching signal for controlling the switching
operation of each of the switches.
[0044] FIG. 2 shows waveforms of the above circuit. In this figure,
given that the load current I.sub.out is refluxed from the
direction indicated by the arrows in FIG. 1 through the diode D2
connected in parallel with the second main switch Q2, in the period
between the time the second main switch Q2 is turned off from its
ON-state and the time before the first main switch Qi is
subsequently turned on. When the initial voltage of the snubber
capacitor C3 in the auxiliary resonant commutation circuit is
approximately equal to the voltage of the DC power supply V.sub.in,
if the first and second auxiliary switches Q5, Q3 are turned on at
the time t.sub.0, the second auxiliary switch Q3 is turned on, the
voltage of the DC power supply V.sub.in, is applied to the resonant
inductor L1 and thereby the inductor current Ir is linearly
increased. Simultaneously, the current passing through the diode D2
is reduced by this increased amount.
[0045] At the time t.sub.1, the inductor current Ir becomes equal
to the load current I.sub.out. At this moment, resonation is caused
by the resonant inductor L1 and main-switch snubber capacitors C1,
C2. Thus, the voltage across the diode D2 connected in parallel
with the second main switch Q2 starts increasing. At the time
t.sub.2, the diode D1 connected in parallel with the first main
switch Q1 is biased in the forward direction and thereby the
inductor current Ir is refluxed to the inductor L1 from the diode
D1 through the first and second auxiliary switches Q5, Q3. On and
As shown in FIG. 2, after the time t.sub.2, the voltage across the
first main switch Q1 is approximately zero. Thus, "zero-voltage
turn-on" of the first main switch Q1 can be achieved by turning on
the first main switch Q1 after the time t.sub.2.
[0046] When the first auxiliary switch Q5 is turned off after
turning on the first main switch Q1, the current flows along the
path from the diode D6 connected in parallel with the third
auxiliary switch Q6, through the auxiliary resonant snubber
capacitor C3 to the second auxiliary switch Q3. This leads to
discharge in the snubber capacitor C3, and thereby the voltage
across the first auxiliary switch Q5 is increased with gradient.
Thus, the soft-switching can also be achieved in the first
auxiliary switch Q5.
[0047] At the time t.sub.3, the diode D6 connected in parallel with
the third auxiliary switch Q6 is biased in the forward direction.
Thus, the exited energy in the resonant inductor L1 is
regeneratively returned to the DC power supply V.sub.in through the
diodes D6, D4, D1 connected in parallel with the third auxiliary
switch Q6, the fourth auxiliary switch Q4 and the first main switch
Q1, respectively.
[0048] FIG. 3 shows waveforms of each part of the circuit when the
initial voltage Vcr of the auxiliary resonant snubber capacitor C3
is approximately zero. Operations from the time to t.sub.o the time
t.sub.2 are the same as those shown in FIG. 2. When the second
auxiliary switch Q3 is turned off after turning on the first main
switch Q1 on and after the time t.sub.2, the current flows along
the path from the first auxiliary switch Q5 through the snubber
capacitor C3 and the diode D4 connected in parallel with the fourth
auxiliary switch Q4 to the resonant inductor L1. Thus, the inductor
current Ir charges the snubber capacitor C3 with passing
therethrough, and thereby the voltage across the second auxiliary
switch Q3 is increased with gradient. Consequently, the
soft-switching can also be achieved in the second auxiliary switch
Q3.
[0049] At the time t.sub.3, the diode D6 connected in parallel with
the third auxiliary switch Q6 is biased in the forward direction.
Then, the accumulated energy in the inductor L1 is regeneratively
returned to the DC power supply V.sub.in along the path from said
diode 6 through the diode D4 connected in parallel with the fourth
auxiliary switch Q4 and the resonant inductor L1 to the diode D1
connected in parallel with the first main switch Q1.
[0050] FIG. 4 is a waveform diagram showing the commutating
operation in ON-state of the first main switch Q1. When the load
current is less than a threshold currentIth defined by the
following formula, the following operation is performed.
I.sub.th=Cr.times.V.sub.in/t.sub.max
[0051] where, I.sub.th is the threshold, Cr being the capacity of
the main-switch snubber capacitor when the snubber capacitor is
connected in parallel with the main switch, V.sub.in being the
voltage of the DC power supply, and t.sub.max max being the maximum
allowable value of the time required for the load current to
commute from one of the first and second main switches to the other
of them.
[0052] First, given that the auxiliary resonant snubber capacitor
C3 is charged at the initial voltage approximately equal to the
voltage of the DC power supply V.sub.in. In this state, when the
third and fourth auxiliary switches Q6, Q4 are turned on at the
time T.sub.0, the voltage of the DC power supply V.sub.in is
applied additionally to the path along the first main switch Q1,
the resonant inductor L1, the fourth auxiliary switch Q4 and the
third auxiliary switch Q3, and thereby the inductor current Ir is
linearly increased. At the time T.sub.1, the inductor current Ir
goes up to the threshold current t.sub.th. At this moment, if the
first main switch Q1 is turned off, resonant is caused by the
resonant inductor L1 and snubber capacitors C1, C2, and the voltage
across the first main switch Q1 is increased with gradient. Thus,
the soft-switching can be achieved in the first main switch Q1.
[0053] At the time T.sub.2, the diode D2 connected in parallel with
the second main switch Q2 is biased in the forward direction. Thus,
the current is refluxed along the path from the diode D2 through
the inductor Ir, the fourth auxiliary switch Q4 and the third
auxiliary switch Q6 to the diode D2. By turning on the second main
switch Q2 on and after the time T.sub.2, the zero-voltage turn-on
can be achieved in the main switch Q2.
[0054] Subsequently, when the third auxiliary switch Q6 is turned
off at the time T.sub.3, the current flows along the path from the
fourth auxiliary switch Q4 through the snubber capacitor C3 to the
diode D5 connected in parallel with the first auxiliary switch Q5,
and thereby the charged voltage of the snubber capacitor C3 is
discharged. Thus, the voltage across the third auxiliary switch Q6
is increased with gradient, and the soft-switching can be achieved
in the third auxiliary switch Q6.
[0055] At the time T.sub.4, the diode D3 connected in parallel with
the second auxiliary switch Q3 is biased in the forward direction,
and the accumulated energy in the resonant inductor L1 is
regeneratively returned to the DC power supply V.sub.in along the
path from the second main switch Q2 through the inductor Ir, the
diode D3 and the diode D5 connected in parallel with the first
auxiliary switch Q5.
[0056] FIG. 5 shows waveforms in the state when the initial voltage
Vcr of the auxiliary resonant snubber capacitor C3 is approximately
zero. The operations between the time T.sub.0 and the time T.sub.3
are the same as those shown in FIG. 4. In the operations of FIG. 5,
the fourth auxiliary switch Q4 is turned off after the second main
switch Q2 is turned on at the time T.sub.3. As a result, the
current flows along the path from the resonant inductor L1 through
the diode D3 connected in parallel with the second auxiliary switch
Q3 and the snubber capacitor C3 to the third auxiliary switch Q6,
and thereby the snubber capacitor C3 is charged. Thus, the voltage
across the fourth auxiliary switch Q4 is increased with gradient,
and the soft-switching can be achieved in said auxiliary switch
Q4.
[0057] At the time T.sub.4, the diode D5 connected in parallel with
the first auxiliary switch Q5 is biased in the forward direction,
and the accumulated energy in the resonant inductor L1 is
regeneratively returned along the path from the second main switch
Q2 through the inductor L1, the diode D3 and the diode D5.
[0058] When the load current is larger than the aforementioned
threshold, the auxiliary resonant commutation circuit is not
activated in the time period from the ON-state of the first main
switch Q16 to the completion of the reflux of the load current
through the diode D2. In this case, when the first main switch Q1
is turned off, the voltage at the junction between the first and
second main switches Q1, Q2 is varied with gradient by the action
of the snubber capacitors C1, C2. Thus, the soft-switching can be
achieved in the main switch Q1.
[0059] As described above, in the course of the commutation of the
load current not only from the diode D2 to the main switch Q1 but
also from the main switch Q1 to the diode D2, the soft-switching
can be achieved in all of the switches. Furthermore, all of the
accumulated energy in the resonant inductor for the commutation is
regeneratively returned to the DC power supply after the completion
of the commutation. Thus, in the circuit using the capacitor C4 as
the DC power supply, even if the load current is varied at low
frequency due to the operation of the inverter, no low-frequency
ripple is generated at the capacitor C4 by the operation of the
auxiliary resonant commutation circuit. This eliminates the need
for increasing the capacity of the capacitor C4.
[0060] FIG. 6 shows a second embodiment of the present invention.
This embodiment is a single-phase inverter arrangement in which a
pair of inverters each having the circuitry as shown in FIG. 1 are
connected in parallel with each other. In FIG. 6, an inverters X
and inverter Y are identical in circuitry. The same elements or
components as those of the circuit in FIG. 1 are defined by the
same numerals or codes, and the codes of the inverter Y are defined
by adding an affix "a". The Inverters X, Y have output terminals
T1, T2, respectively.
[0061] FIG. 7 shows a third embodiment of the present invention.
This embodiment is an example of a three-phase inverter in which
three inverters each having the circuitry as shown in FIG. 1 are
connected in parallel with each other. Each of the inverters X, Y
is the same as that of FIG. 6, and is defined by the same codes as
those of FIG. 6. In the example of FIG. 7, a third inverter Z
having an output terminal T3 is additionally provided. Each
elements of the third inverter Z is defined by adding an affix "b"
to the same code as that of FIG. 1, and their description will be
omitted.
[0062] FIG. 8 shows an example in which a down converter is formed
by using the circuitry of FIG. 1. A filter composed of a smoothing
reactor L0 and a smoothing capacitor C0 is connected with both ends
of the second main switch Q2 of the circuit shown in FIG. 1, and
output terminals T1, T2 are provided to both ends of the smoothing
capacitor C0, respectively. According to this circuit, the
auxiliary resonant commutation circuit can prevent undesirable
variance in voltage of the input capacitor C4, such as ripple, in
any load conditions ranging from light load to heavy load.
[0063] FIG. 9 is a circuit diagram showing a modification of the
first embodiment of the present invention. This modification is
different from the circuit of FIG. 1 just in that the output is
picked up from both ends of the first main switch Q1. Thus, the
detailed description about this circuitry will be omitted.
[0064] FIG. 10 shows waveforms of the circuit of FIG. 9. In FIG.
10, given that the load current I.sub.out is fluxed in the
direction indicated by the arrow shown in FIG. 9 through the diode
D1 connected in parallel to the first main switch Q1, in the period
between the time the first main switch Q1 is turned off from its
ON-state and the time before the second main switch Q2 is
subsequently turned on. When the initial voltage of the snubber
capacitor C3 of the auxiliary resonant commutation circuit is
approximately equal to the voltage of the DC power supply V.sub.in,
if the third auxiliary switches Q6 and the fourth auxiliary
switches Q4 are turn on at the time t'.sub.0, the voltage of the DC
power supply V.sub.in will be applied to the resonant inductor L1.
Thus, the inductor current Ir is linearly increased, and the
current of the diode D1 is simultaneously reduced by the
increment.
[0065] At the time t'.sub.1, the inductor current Ir becomes equal
to the load current I.sub.out. At this time, resonance is caused by
the resonant inductor L1 and the main-switch snubber capacitors C1,
C2. Consequently, the voltage across the diode D1 connected in
parallel to the first main switch Q1 starts going up. At the time
t'.sub.2, the diode D2 connected in parallel to the second main
switch Q2 is biased in the forward direction, and the inductor
current Ir is refluxed from the diode D2 to the inductor L1 through
the second auxiliary switch Q6 and the second auxiliary switch Q4.
On and after the time t'.sub.2, the voltage across the second main
switch Q2 is approximately zero as shown in FIG. 10. Thus, by
turning on the second main switch Q2 on and after the time
t'.sub.2, the "zero-voltage turn-on" can be achieved in the second
main switch Q2.
[0066] When the second auxiliary switch Q6 is turned off after
turning on the second main switch Q2, the current flows along the
path from the resonant inductor L1 through the fourth auxiliary
switch Q4 and the auxiliary resonant snubber capacitor C3 to the
diode D5 connected in parallel with the first auxiliary switch Q5,
and thereby the snubber capacitor C3 is discharged. Thus, the
voltage across the third auxiliary switch Q6 is increased with
gradient, and the soft-switching can also be achieved in the third
auxiliary switch Q6.
[0067] At the time t'.sub.3, the diode D3 connected in parallel
with the second auxiliary switch Q3 is biased in the forward
direction. Thus, the excited energy in the resonant inductor L1 is
regeneratively returned to the DC power supply V.sub.in through the
diodes D5, D3, D2 connected in parallel with the first auxiliary
switch Q5, the second auxiliary switch Q3, the second main switch
Q2, respectively.
[0068] FIG. 11 shows waveforms of each part of the circuit when the
initial voltage Vcr of the auxiliary resonant snubber capacitor C3
is approximately zero. The operations from the time t'.sub.0 to the
time t'.sub.2 are the same as those shown in FIG. 10. When the
fourth auxiliary switch Q4 is turned off after turning on the
second main switch Q2 on and after the time t'.sub.2, the current
flows along the path from the resonant inductor L1 through the
diode D3 connected in parallel with the second auxiliary switch Q3
and the snubber capacitor C3 to the third auxiliary switch Q6.
Thus, the inductor current Ir passes through the snubber capacitor
C3 with charging it, and the voltage across the fourth auxiliary
switch Q4 is increased with gradient. Consequently, the
soft-switching can also be achieved in the fourth auxiliary switch
Q4.
[0069] At the time t'.sub.3, the diode D5 connected in parallel
with the first auxiliary switch Q5 is biased in the forward
direction. Thus, the accumulated energy in the inductor L1 is
regeneratively returned to the DC power supply V.sub.in along the
path from said diode D5 through the diode D3 connected in parallel
with the second auxiliary switch Q3 and the resonant inductor L1 to
the diode D2 connected in parallel with the second main switch
Q2.
[0070] FIG. 12 is a waveform diagram showing a commutation
operation in ON-state of the second main switch Q2. When the load
current is less than a threshold current Ith defined by following
formula, the following operation is performed;
I.sub.th=Cr.times.V.sub.in/t.sub.max
[0071] where, I.sub.th is the threshold, Cr being the capacity of
the main-switch snubber capacitor when the snubber capacitor is
connected in parallel to the main switch, V.sub.in being the
voltage of the DC power supply, t.sub.max being the maximum
allowable value of time required for the load current to commutate
from one of the first and second main switches to the other of
them.
[0072] First, given that the auxiliary resonant snubber capacitor
C3 is charged at the initial voltage approximately equal to the
voltage of the DC power supply V.sub.in. In this state, when the
first auxiliary switch Q5 and the second auxiliary switch Q3 are
turn on at the time T'.sub.0, the voltage of the DC power supply
V.sub.in is applied to the path through the first auxiliary switch
Q5, the second auxiliary switch Q3, the resonant inductor L1 and
the second main switch Q2. Thus, the inductor current Ir is
linearly increased. At the time T'.sub.1, the inductor current Ir
goes up to the threshold current I.sub.th. At this moment, when the
second main switch Q2 is turned off, resonance is caused by the
resonant inductor L1 and the snubber capacitors C1, C2, and thereby
the voltage across the second main switch Q2 is increased with
gradient. Consequently, the soft-switching can be achieved in the
second main switch Q2.
[0073] At the time T'.sub.2, the diode D1 connected in parallel
with the first main switch Q1 is biased in the forward direction.
Thus, the current is refluxed along the path from the diode D1
through the first auxiliary switch 05, the second auxiliary switch
Q3 and the inductor Ir to the diode D1. By turning on the first
main switch Q1 on and after the time T'.sub.2, the "zero-voltage
turn-on" can be achieved in said main switch Q1.
[0074] Subsequently, when the third auxiliary switch Q5 is turned
off at the time T'.sub.3, the current flows along the path from the
diode D6 connected in parallel with the third auxiliary switch Q6,
through the snubber capacitor C3 to the second auxiliary switch Q3.
Thus, the charged voltage of the snubber capacitor C3 is
discharged, and thereby the voltage across the first auxiliary
switch Q5 is increased with gradient. Consequently, the
soft-switching can be achieved in the first auxiliary switch
Q5.
[0075] At the time T'.sub.4, the diode D4 connected in parallel
with the fourth auxiliary switch Q4 is biased in the forward
direction. Thus, the accumulated energy in the resonant inductor L1
is regeneratively returned to the DC power supply V.sub.in along
the path from the diode D6 connected in parallel with the third
auxiliary switch Q6 and said diode D4, through the inductor Ir and
the first main switch Q1.
[0076] FIG. 13 shows waveforms when the initial voltage Vcr of the
auxiliary resonant snubber capacitor C3 is approximately zero. The
operations from the time T'.sub.0 to the time T'.sub.3 are the same
as those shown in FIG. 12. In the operations of FIG. 13, after
turning on the first main switch Q1 at the time T'.sub.3, the third
auxiliary switch Q3 is turned off. As a result, the current flows
along the path from the first auxiliary switch Q5 through the
snubber capacitor C3 and the diode D4 connected in parallel with
the fourth auxiliary switch Q4 to the resonant inductor L1, and
thereby the snubber capacitor C3 is charged. Thus, the voltage
across the second auxiliary switch Q3 is increased with gradient.
Consequently, the soft-switching can be achieved in said auxiliary
switch Q3.
[0077] At the time T'.sub.4, the diode D6 connected in parallel
with the third auxiliary switch Q6 is biased in the forward
direction. Thus, the accumulated energy in the resonant inductor L1
is regeneratively returned along the path from the first main
switch Q1 through the inductor L1, the diode D4 and the diode
D6.
[0078] In the load current lager than the above threshold, the
auxiliary resonant commutation circuit is not activated in the time
period between the ON-state of the second main switch Q2 and the
completion of the reflux of the load current through the diode D1.
In this case, when the second main switch Q2 is turned off, the
voltage at the junction between the first and second main switches
Q1, Q2 is varied with gradient by the action of snubber capacitors
C1, C2. Thus, the soft-switching can be achieved in the main switch
Q2.
[0079] As described above, the soft-switching can be achieved in
all of the switches in the course of the commutation of the load
current not only from the diode D1 to the main switch Q2 but also
from the main switch Q2 to the diode D1. Moreover, after
commutation, all of the accumulated energy in the resonant inductor
for commutation is regeneratively returned to the DC power supply.
Therefore, in the circuit using the capacitor C4 as the DC power
supply, even if the load current is varied at low frequency by the
operation of the inverter, the auxiliary resonant commutation
circuit operates to prevent the occurrence of any low frequency
ripple in the capacitor C4. This eliminates the need for increasing
the capacity of the capacitor C4.
[0080] FIGS. 14 and 15 show examples of another control according
to the present invention. FIG. 14 corresponding to the
aforementioned FIG. 4 shows a control in the circuit of FIG. 1.
FIG. 15 corresponding to FIG. 11 shows a control in the circuit of
FIG. 9.
[0081] In FIG. 14, Vg indicates a driving signal for the switches.
First, given that the main switch Q1 is in ON-state. When the main
switch Q1 is turned off, the main switch starts the commutation of
the load current. However, when the load current is less than the
threshold I.sub.th, it takes time to commutate due to the snubber
capacitor connected in parallel with the main switch, and thereby
the main switch can be turned on with leaving voltage in the
snubber capacitor. This causes a short-circuit loss because the
accumulated energy in the capacitor is consumed by the main switch.
In order to prevent this, the fourth and sixth auxiliary switches
Q4, Q6 are turn on in conjunction with the turn-on of the main
switch Q1 at the time T.sub.0 in the waveform shown in FIG. 14.
Then, resonance is caused by the resonant inductor L1 and the
snubber capacitors C1, C2. By this resonance, the voltage across
the main switch Q2 is reduced, and goes down to zero at the time
T.sub.1. Simultaneously, the resonant current Ir is refluxed along
the path through the auxiliary switch Q4, the auxiliary switch Q6
and the diode D2. The "zero-voltage turn-on" can be achieved by
turning on the main switch Q2 on and after the time T.sub.1.
Further, when the auxiliary switch Q4 is turned off on and after
the time T.sub.1, diodes D3, D5 are brought into conduction by the
current Ir of the resonant inductor L1. Thus, the excited energy in
the resonant inductor L1 is regeneratively returned to the input
V.sub.in, and the regeneration is completed at the time T.sub.2.
The above control prevents any short-circuit loss in light load
current otherwise caused by the snubber capacitor. This control
process is performed by voltage detection without detecting
current. Thus, the detecting circuit can be simplified.
[0082] Referring to FIG. 15, Vg indicates a driving signal of the
switches. At first, given that the main switch Q2 is in ON-state.
When the main switch Q2 is turned off, the main switch starts to
commutate by the load current. However, when the load current is
less than the threshold I.sub.th, it takes time to the commutation
due to the snubber capacitor connected in parallel with the main
switch, and thereby the main switch can be turned on with leaving
voltage in the snubber capacitor. This causes a short-circuit loss
because the accumulated energy in the capacitor is consumed by the
main switch. In order to prevent this, the third and fifth
auxiliary switches Q3, Q5 are turned in conjunction with the
turn-off of the main switch Q2 at the time T.sub.0 in the waveform
shown in FIG. 15. Then, resonance is caused by the resonant
inductor L1 and the snubber capacitors C1, C2. By this resonance,
the voltage across the main switch Q1 is reduced, and goes down to
zero at the time T.sub.1. Simultaneously, the resonant current Ir
is refluxed along the path through the auxiliary switches Q3, Q5
and the diode D1. By turning on the main switch Q1 on and after the
time T.sub.1, the zero-voltage turn-on can be achieved. When the
auxiliary switch Q3 is turned off on and after the time T.sub.1,
the diodes D4, D6 are brought into conduction by the current Ir of
the resonant inductor L1. Then, the excited energy in the resonant
inductor L1 is regeneratively returned to the input V.sub.in, and
the regeneration is completed at the time T.sub.2. The above
control can prevent any short-circuit loss in light load current
otherwise caused by the snubber capacitor. This control process can
provide the same effect as that of the control in FIG. 14.
* * * * *