U.S. patent application number 09/756954 was filed with the patent office on 2001-07-12 for switching power supply.
Invention is credited to Nakahira, Koji, Nishida, Akio, Tani, Ryota, Yamada, Tomohiro.
Application Number | 20010007529 09/756954 |
Document ID | / |
Family ID | 18531613 |
Filed Date | 2001-07-12 |
United States Patent
Application |
20010007529 |
Kind Code |
A1 |
Nishida, Akio ; et
al. |
July 12, 2001 |
Switching power supply
Abstract
A switching power supply having two or more DC outputs
comprising: a DC power supply, a transformer having a primary
winding, at least two secondary windings, and a feedback winding, a
main switching element having an off-state period and an on-state
period, connected in series to the primary winding and to be turned
on by a voltage generated in the feedback winding, the main
switching element having a control terminal and a threshold voltage
to turn the main switching element on; and a rectifying circuit
connected to each secondary winding, a starting circuit which
initially turns on the main switching element at startup of the
power supply and a switching circuit provided between the two DC
outputs, and wherein, when the switching circuit is turned on, a
voltage generated in the feedback winding is lowered during the
off-state period of the main switching element and a voltage to be
applied to the control terminal of the main switching element is
controlled so as to be less than the threshold voltage, and the
main switching element is turned on by the starting circuit.
Inventors: |
Nishida, Akio; (Kyoto-fu,
JP) ; Tani, Ryota; (Kyoto-fu, JP) ; Nakahira,
Koji; (Kyoto-fu, JP) ; Yamada, Tomohiro;
(Kyoto-fu, JP) |
Correspondence
Address: |
OSTROLENK FABER GERB & SOFFEN
1180 AVENUE OF THE AMERICAS
NEW YORK
NY
100368403
|
Family ID: |
18531613 |
Appl. No.: |
09/756954 |
Filed: |
January 9, 2001 |
Current U.S.
Class: |
363/19 |
Current CPC
Class: |
H02M 1/0032 20210501;
Y02B 70/10 20130101; H02M 3/3385 20130101 |
Class at
Publication: |
363/19 |
International
Class: |
H02M 003/335 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 11, 2000 |
JP |
2000-002516 |
Claims
What is claimed is:
1. A switching power supply having two or more DC outputs
comprising: a DC power supply; a transformer having a primary
winding, at least two secondary windings, and a feedback winding; a
main switching element having an off-state period and an on-state
period, connected in series to the primary winding and to be turned
on by a voltage generated in the feedback winding, the main
switching element having a control terminal and a threshold voltage
to turn the main switching element on; and a rectifying circuit
connected to each secondary winding, a starting circuit which
initially turns on the main switching element at startup of the
power supply and a switching circuit provided between the two DC
outputs, and wherein, when the switching circuit is turned on, a
voltage generated in the feedback winding is lowered during the
off-state period of the main switching element and a voltage to be
applied to the control terminal of the main switching element is
controlled so as to be less than the threshold voltage, and the
main switching element is turned on by the starting circuit.
2. The switching power supply of claim 1, wherein the at least two
DC outputs comprise respectively, a low voltage output as a
controlled output and a high voltage output as an uncontrolled
output, and wherein the switching circuit is connected between the
low voltage output and the high voltage output.
3. The switching power supply of claim 1, wherein the at least two
DC outputs comprise, respectively, a high voltage output as a
controlled output and a low voltage output as an uncontrolled
output, and wherein the switching circuit is connected between the
high voltage output and the low voltage output.
4. The switching power supply of claim 1, wherein both DC outputs
are connected to a single voltage output terminal of the switching
power supply.
5. The switching power supply of claim 1, wherein the switching
circuit is turned on and off by an external signal.
6. The switching power supply of claim 2, wherein the switching
circuit is turned on and off by an external signal.
7. The switching power supply of claim 3, wherein the switching
circuit is turned on and off by an external signal.
8. The switching power supply of claim 4, wherein the switching
circuit is turned on and off by an external signal.
9. The switching power supply of claim 1, wherein the switching
circuit is turned on by detection of lowered load power.
10. The switching power supply of claim 2, wherein the switching
circuit is turned on by detection of lowered load power.
11. The switching power supply of claim 3, wherein the switching
circuit is turned on by detection of lowered load power.
12. The switching power supply of claim 4, wherein the switching
circuit is turned on by detection of lowered load power.
13. The switching power supply of claim 1, wherein the main
switching element has a voltage control terminal.
14. A switching power supply of claim 1, wherein the main switching
element has a current control terminal.
15. The switching power supply of claim 5, further comprising a
load current detecting circuit for detecting reduced load current
and for supplying the external signal to turn on the switching
circuit.
16. The switching power supply of claim 9, further comprising a
load current detecting circuit for detecting reduced load current
and for supplying a signal to turn on the switching circuit.
17. The switching power supply of claim 1, wherein the switching
circuit comprises two switches, one connected between the DC
outputs and the other connected between one of the DC outputs and a
control input of a control circuit for controlling turn-off of said
main switching element.
18. The switching power supply of claim 17, wherein the control
input of the control circuit comprises a voltage sense input of a
voltage regulator circuit.
19. The switching power supply of claim 1, wherein the switching
circuit comprises a transistor switch.
20. The switching power supply of claim 17, wherein the two
switches comprise transistor switches.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to a switching power supply and more
particularly to a switching power supply of a self-excited ringing
choke converter (hereinafter referred to as RCC) type.
[0003] 2. Description of the Related Art
[0004] Generally, electronic equipment such as electronic computers
and communication devices require stable DC voltages. In order to
supply stable DC voltages for such electronic equipment using the
commercially available power source, switching power supplies of
RCC type which has a relatively simple construction and shows high
degrees of efficiency are widely used.
[0005] FIG. 7 is the circuit diagram of such a conventional
switching power supply of RCC type. In FIG. 7, a switching power
supply 1 is provided with an input circuit 2, a main operating
circuit 3, a voltage detecting circuit 4, a voltage output terminal
OUT, and a ground terminal GND. The input circuit 2 comprises a
diode bridge circuit DB for rectification, and a fuse F and a
filter circuit LF both of which are provided between an AC power
supply and the input terminal of the diode bridge circuit DB.
[0006] Furthermore, the main operating circuit 3 comprises a
capacitor C1 for smoothing provided between the output terminals a
and b of the diode bridge DB in the input circuit 2; a transformer
T containing a primary winding N1, a secondary winding N2 having
the opposite polarity to that of the primary winding N1, and a
feedback winding having the same polarity as the primary winding
N1; a FET Q1 as a switching element connected in series to one end
of the primary winding N1 of the transformer T; a resistor R1 for
starting connected between the other end of the primary winding N1
and the gate as a control terminal of the FET Q1, a resistor R8
connected between the gate and source of the FET Q1, a diode D1 for
rectification connected in series to one end of the secondary
winding N2 of the transformer T, and a capacitor C4 for smoothing
connected between the other end of the secondary winding N2 and the
voltage output terminal OUT.
[0007] Furthermore, the voltage detecting circuit 4 is provided on
the output side of the main operating circuit 3 and contains a
resistor R5, a light-emitting diode PD on the emission side of a
photo coupler PC, a shunt regulator Sr, and resistors R6 and R7.
The resistor R5, the light-emitting diode PD, and the anode and
cathode of the shunt regulator are connected in series and are
provided so as to be parallel to the capacitor C4 of the main
operating circuit 3. Furthermore, the resistors R6 and R7 are
connected in series and are also provided to be parallel to the
capacitor C4. The connection point of the resistors R6 and R7 is
connected to the control terminal of the shunt regulator Sr.
[0008] Furthermore, a control circuit 5 comprises a resistor R9 and
a capacitor C3 connected between one end of the feedback winding NB
and the gate of the FET Q 1, a transistor Q2 connected between the
gate and source of the FET Q1, a resistor R2 connected between one
end of the feedback winding NB and the base of the transistor Q2, a
resistor R3 and a capacitor C2 connected in parallel between the
base and emitter of the transistor Q2, and a resistor R4, a diode
D2, and a phototransistor PT on the light-receiving side of a photo
coupler PC connected in series between one end of the feedback
winding NB and the base of the transistor Q2.
[0009] Next, the operation of the switching power supply 1 thus
constructed is described. First of all, at start, a voltage is
applied to the gate of the FET Q1 through the resistor R1 and the
FET Q1 is turned on. When the FET Q1 is turned on, the voltage of
the power supply is applied to the primary winding N of the
transformer T, a voltage in the same direction as the voltage
generated in the primary winding N1 is generated in the feedback
winding NB, and then the FET Q1 is rapidly turned on because of
positive feedback. At this time, excitation energy is stored in the
primary winding N1.
[0010] When the capacitor C2 is charged through the resistor R2 and
the potential of the base of the transistor Q2 reaches the
threshold, the transistor Q2 is turned on and the FET Q1 is tuned
off. Because of this, the excitation energy stored in the primary
winding N1 of the transformer T, while the FET Q1 is turned on, is
discharged as electric energy through the secondary winding N2, and
the electric energy is rectified by the diode D1, smoothed by the
capacitor C4, and supplied to a load through the voltage output
terminal OUT.
[0011] Furthermore, when the excitation energy stored in the
primary winding N1 of the transformer T is discharged through the
secondary winding N2, a flyback voltage VNB is generated in the
feedback winding NB. The change of this flyback voltage VNB is
described with reference to FIG. 8. In FIG. 8, at the time t11, the
FET Q1 is turned off and the flyback voltage VNB is kept at a
nearly constant value Vb, that is, it goes into a so-called
off-state period. Then, at the time t12, the voltage of the diode
D1 becomes zero and the flyback voltage starts to oscillate, and
when the flyback voltage VNB rises and the gate voltage reaches the
threshold Vth at t13, the FET Q1 is turned on. Moreover, part of
the flyback voltage VNB shown by the chain line shows the case
where the flyback voltage VNB is assumed to continue to oscillate.
In this way, when the FET Q1 is turned off, a voltage is applied to
the primary winding N1 again and the excitation energy is
stored.
[0012] In the switching power supply 1, such an oscillation is
repeated. In a steady state, the output voltage on the load side is
divided by the resistors R6 and R7, and this divided detection
voltage and the reference voltage of the shunt regulator Sr are
compared. Then, the change of the output voltage is amplified at
the shunt regulator Sr, the current flowing in the light-emitting
diode PD of the photo coupler PC is changed, and in accordance with
the quantity of light emission of the light-emitting diode PD, the
impedance of the phototransistor PT changes. Thus, the charge and
discharge of the capacitor C2 can be changed and the output voltage
can be controlled so as to be constant.
[0013] In the conventional switching power supply 1 shown in FIG.
7, at light load the oscillation frequency increases and the
switching loss is large, which is a factor lowering the circuit
efficiency. In order to solve this problem, a method can be
considered whereby on the output side of the switching power supply
a circuit lowering the output voltage can be provided so that by
changing the impedance on the output side the output voltage Vo is
lowered.
[0014] In this case, by making use of a fact that the voltage VNB
generated in the feedback winding NB of the transformer T decreases
in proportion to the output voltage Vo, the degree of decrease in
the output voltage Vo is adjusted. By lowering the voltage VNB, the
gate voltage of the FET Q1 is made to oscillate in the range where
the gate voltage does not reach the threshold and the turn-on of
the FET Q1 is delayed, and by making the off-state period of the
FET Q I extended the oscillation frequency is lowered, and thus the
switching loss is reduced.
[0015] However, such a switching power supply of RCC type is
characterized in that at light load, the frequency increases
because the output current is small, and when a circuit lowering
the output voltage is provided as described above, all the output
voltages are decreased, and accordingly when a constant output
voltage is required, there is a problem that a constant-voltage
control circuit is required.
SUMMARY OF THE INVENTION
[0016] Therefore, it is an object of this invention to provide a
switching power supply where the increase of switching loss is
suppressed and a constant output voltage can be obtained.
[0017] According to the invention, a switching power supply is
provided having two or more DC outputs, the power supply comprising
a DC power supply; a transformer having a primary winding, at least
two secondary windings, and a feedback winding; a main switching
element connected in series to the primary winding and to be turned
on by a voltage generated in the feedback winding; and a rectifying
circuit connected to the secondary winding, further comprising a
starting circuit which turns on the main switching element at start
of the power supply and a switching circuit provided between two
outputs on the secondary side, and wherein by turning on the
switching means, the voltage generated in the feedback winding is
lowered during the off-state period of the main switching element,
a voltage to be applied to a control terminal of the main switching
element is controlled so as to be less than the threshold voltage,
and the main switching element is turned on by the starting
circuit.
[0018] According to the structure described above, the switching
frequency can be lowered during standby and the loss can be reduced
so that the switching circuit is provided between two outputs of
secondary windings of a transformer, and whereby, by turning on the
switching circuit, a voltage generated in a feedback winding during
the off-state period of a main switching element is lowered, and a
voltage to be applied to the control terminal of the main switching
element is controlled so as to be less than the threshold
level.
[0019] The at least two outputs on the secondary side contain a low
voltage output as a controlled output and a high voltage output as
an uncontrolled output, and the switching circuit may be connected
between the low voltage output and the high voltage output. In this
case, the accuracy of the voltage of the low output voltage can be
improved(increased).
[0020] Alternatively, the switching circuit may be connected
between the high voltage output and the low voltage output. In this
case, the accuracy of the voltage of the high output voltage can be
improved.
[0021] In addition to the aforementioned structure, one output may
be contained as an output on the secondary side and another output
may be connected to the DC output through a rectifying element and
the switching circuit connected to another secondary winding which
is different from the output. In this way, a switching power supply
can be made having a single output and the number of parts can be
reduced.
[0022] The switching circuit may be turned on and off by a signal
from the outside.
[0023] Alternatively, the switching circuit may be turned on by
detection of lowered load power, so that thus changing signals from
the outside becomes unnecessary Further the switching element may
have a voltage control terminal or a current control terminal. In
the case where the switching element contains a voltage control
terminal, when the gate of the switching element is controlled so
as to be less than the threshold level, a complete off state of the
switching element is maintained and accordingly very little the
loss is generated in the off state.
[0024] For the purpose of illustrating the invention, there is
shown in the drawings several forms which are presently preferred,
it being understood, however, that the invention is not limited to
the precise arrangements and instrumentalities shown.
BRIEF DESCRIPTION OF THE DRAWING(S)
[0025] FIG. 1 is the circuit diagram of a switching power supply of
one embodiment of this invention.
[0026] FIG. 2 is the circuit diagram of a switching power supply of
another embodiment of this invention.
[0027] FIG. 3 is a waveform showing the change of a flyback voltage
generated in a feedback winding.
[0028] FIG. 4 is the circuit diagram of a switching power supply of
yet another embodiment of this invention.
[0029] FIG. 5 is the circuit diagram of a switching power supply of
still another embodiment of this invention.
[0030] FIG. 6 is the circuit diagram showing one example of load
power detecting circuits.
[0031] FIG. 7 is the circuit diagram of a conventional switching
power supply of RCC type.
[0032] FIG. 8 is a waveform showing the change of a flyback voltage
VNB for the circuit of FIG. 7.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
[0033] Hereinafter, the preferred embodiments of the present
invention are explained in detail with reference to the
drawings.
[0034] FIG. 1 is the circuit diagram of a switching power supply of
one embodiment of this invention. In FIG. 1, two output terminals
are provided for a secondary winding N2 of a transformer T. One
output terminal outputs, for example, a low voltage of 5 V for
logical circuits controlled by a voltage detecting circuit 4 and a
control circuit 5, and a voltage in proportion to the turns ratio
of the transformer T is output from the other output terminal and,
for example, an uncontrolled high voltage of 24 V is output for
motor drive. Between these output terminals a switch SW is
connected.
[0035] To the low voltage output terminal of the secondary winding
N2, the anode of a rectifying diode D1 is connected in the same way
as in FIG. 7, and to the high voltage output terminal, the anode of
a rectifying diode D3 is connected. The cathode of the rectifying
diode D1 is connected to the voltage output terminal OUT1 through
the detecting circuit 4 and also connected to one terminal of the
switch SW. The cathode of the diode D3 is connected to one terminal
of a capacitor C5 for smoothing and the voltage output terminal
OUT2, and also connected to the other terminal of the switch SW.
The other terminal of the capacitor C5 for smoothing is grounded.
The remaining structure is the same as that in FIG. 7.
[0036] Next, the operation of the switching power supply shown in
FIG. 1 is described. When the switch SW is turned off, the same
operation as in FIG. 7 is carried out as described above. When the
switch SW is turned on, the high voltage output terminal and the
low voltage output terminal of the secondary winding T2 are short
circuited and at the same time 5 V is output. In this case, the
high output voltage is reduced to {fraction (5/24)}the former high
output voltage. Therefore, the voltage of the feedback winding NB
is also reduced at the same ratio, and the turn-on of an FET Q1 to
be caused by the feedback winding NB can be prevented.
[0037] FIG. 2 is the circuit diagram of a switching power supply of
another embodiment of this invention. In the embodiment shown in
FIG. 1, the low voltage output is controlled by the voltage
detecting circuit 4 and the control circuit 5 and the high output
voltage is uncontrolled, but in the embodiment shown in FIG. 2, the
high output voltage is controlled and the low output voltage is
uncontrolled.
[0038] That is, one output terminal of the secondary winding N2 is
controlled by a voltage detecting circuit 4 and a control circuit 5
and, for example, a high voltage of 24 V is output to a voltage
output terminal OUT2. From the other output terminal of the
secondary winding N2, for example, a low voltage of 5 V is output,
rectified through a diode D1, and output to a voltage output
terminal OUT1 after it has been smoothed through a capacitor
C4.
[0039] Then, in the same way as in the embodiment of FIG. 1,
between the voltage output terminals OUTI and OUT2 a switch SW1 is
connected. Furthermore, between the voltage output terminal OUT2
and the connection point of resistors R6 and R7 a series circuit of
a resistor R10 and a switch SW2 is connected.
[0040] The operation when the switches SW1 and SW2 are turned off
is nearly the same as in FIG. 7. For example, a voltage of 24 V is
output as a controlled output from the voltage output terminal
OUT2, and, for example, a voltage of 5 V is output as an
uncontrolled output from the voltage output terminal OUT1.
[0041] When the switch SW1 is closed and the switch SW2 is closed
at the same time, the voltage output terminals OUTI and OUT2 are
short circuited and a low voltage of 5 V divided by resistors R10
and R7 is provided to shunt regulator Sr, and accordingly voltages
to be output from the voltage output terminals OUTI and OUT2 are
forced to the low voltage so as to output a voltage of 5 V. In this
case, the high output voltage is reduced to {fraction (5/24)} the
former high output voltage. Therefore, the voltage of the feedback
winding is also reduced at the same ratio, and the turn-on of an
FET Q1 due to the feedback winding can be prevented.
[0042] FIG. 3 is a waveform showing the change of a flyback voltage
VNB generated in the feedback winding. In FIG. 3, at the time t1,
the FET QI is turned off and the flyback voltage VNB is maintained
nearly constant after the generation of surge voltage so as to
enter the so-called off-state period. Here, the absolute value of
the flyback voltage VNB in the off-state period is expressed by the
following formula (1) below. In the formula (1), NB is the number
of turns of the feedback winding NB of the transformer T, N2 is the
number of turns of the secondary winding, and VF is the forward
voltage of the rectifying diode D1 in the main operating circuit
3.
.vertline.VNB.vertline.=(NB/N2).times.(VO1+VF) (1)
[0043] Furthermore, because the secondary outputs are short
circuited and the output voltage VO1 in the formula (1) is lowered,
the absolute value of the flyback voltage VNB decreases. That is,
the absolute value of the flyback voltage VNB of the switching
power supply shown by Va in FIG. 3 is smaller than the absolute
value of the flyback voltage VNB of the conventional switching
power supply shown by Vb in FIG. 8. Then, at the time t2, the
flyback voltage VNB starts to resonate. As the voltage at this time
is expressed by V.sub.NB=Va.multidot.e.sup.-At sin (wt+B), because
a relatively low value is maintained up to time t2, the amplitude
is small and even the maximum of Vgs is lower than the threshold
Vth of the FETQ1, and accordingly the FETQ1 is prevented from
turning on by the flyback voltage VNB.
[0044] After that, at the time t3, a voltage is applied to the gate
of the FET Q1 through a starting circuit (a starting resistor RI in
this embodiment) of the FET Q1 constituting the main operating
circuit 3 and the FET Q1 is again turned on.
[0045] In this way, because the turn-on of the FET Q1 is delayed
and the off-state period of the FET Q1 is extended, the oscillation
frequency is decreased. Therefore, the increase of switching loss
due to the increase of the oscillation frequency is suppressed and
the circuit efficiency is improved.
[0046] FIG. 4 is the circuit diagram of a switching power supply
showing another embodiment of this invention. In the embodiment,
the voltage output terminal OUT2 and the capacitor C5 of the
embodiment shown in FIG. 1 are omitted, a voltage is taken only
from a voltage output terminal OUT1, and voltages to be output are
from a single output terminal. In this case, the turn-on of the FET
Q1 through the feedback winding is prevented by turning on the
switch SW.
[0047] FIG. 5 is the circuit diagram of a switching power supply
showing a further embodiment of this invention. Instead of the
switch SW1 shown in FIG. 2, a transistor Q3 is connected, and
instead of the switch SW2 a transistor Q4 is connected. That is, to
voltage output terminals OUT1 and OUT2 the collector and emitter of
the transistor Q3 are connected, and to the connection point of
resistors R6 and R7 and the voltage output terminal OUT2 the
collector and emitter of the transistor Q4 are connected. The
emitter may be connected through resistor R12. A control signal is
provided to a remote terminal from the outside, and this control
signal is provided to the base of the transistor Q4 through a
resistor R10 and at the same time provided to the base of the
transistor Q3 through a resistor R11.
[0048] When a level signal "L" is given to the remote terminal, the
transistors Q3 and Q4 are made conductive, and then the voltage
output terminals OUT1 and OUT2 are short circuited and at the same
time the voltage to be output from the voltage output terminal OUT2
is lowered. Because of this, the loss can be improved by lowering
the switching frequency.
[0049] Moreover, this embodiment can be also applied to the
embodiments shown in FIGS. 1, 2, and 3.
[0050] FIG. 6 is a circuit diagram showing one example of a load
power detecting circuit. In FIG. 6, the load power detecting
circuit 6 functions in such a way that when a current is supplied
to a load from the voltage output terminal OUT1, a control signal
is provided to the remote terminal shown in FIG. 5, depending on
whether the load is a light load or a heavy load.
[0051] The load current detecting circuit 6 contains an operational
amplifier 7, a voltage V+(V plus), that is, a voltage output from
the voltage output terminal OUT1 and to be input to a resistor R12
which is divided by resistors R13 and R14, is provided to the
+(plus) input terminal of the operational amplifier 7, and a
voltage V(voltage V minus), that is, a voltage output from a
resistor R12 and divided by resistors R15 and R16 and provided to
the -(minus) input terminal of the operational amplifier 7. At a
heavy load, V+(V plus) becomes larger than V-(V minus) and the
operational amplifier 7 outputs a level signal "H", and, at a light
load, V-(V minus) becomes larger than V+(V plus) and the
operational amplifier 7 outputs a level signal "L" to turn on the
transistors Q3 and Q4.
[0052] It should be considered that all of the embodiments
disclosed are illustrative in every respect and not restrictive.
The scope of the present invention is not given by the above
description, but given by the scope of the claims, and it is
intended that all modifications in the meaning and scope equivalent
to the scope of the claims should be included.
[0053] While preferred embodiments of the invention have been
disclosed, various modes of carrying out the principles disclosed
herein are contemplated as being within the scope of the following
claims. Therefore, it is understood that the scope of the invention
is not to be limited except as otherwise set forth in the
claims.
* * * * *