U.S. patent number RE37,898 [Application Number 09/464,190] was granted by the patent office on 2002-11-05 for self-oscillating switching power supply with output voltage regulated from the primary side.
This patent grant is currently assigned to STMicroelectronics S.r.l.. Invention is credited to Giordano Seragnoli.
United States Patent |
RE37,898 |
Seragnoli |
November 5, 2002 |
**Please see images for:
( Certificate of Correction ) ** |
Self-oscillating switching power supply with output voltage
regulated from the primary side
Abstract
Regulation of the output voltage of a power supply employing a
flyback-type self-oscillating DC--DC converter employing a
transformer. The primary winding circuit of the transformer senses
a current recirculation loop for discharging the energy cyclically
stored in an auxiliary winding of the self-oscillation loop of the
converter such as to represent a replica of the circuit of the
secondary winding of the transformer and by summing a signal
representative of the level of the energy stored in the auxiliary
winding with a drive signal on a control node of a driver of the
power switch of the converter.
Inventors: |
Seragnoli; Giordano (Imbersago,
IT) |
Assignee: |
STMicroelectronics S.r.l.
(Agrate Brianza, IT)
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Family
ID: |
8221941 |
Appl.
No.: |
09/464,190 |
Filed: |
December 16, 1999 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
658278 |
Jun 5, 1996 |
05699237 |
Dec 16, 1997 |
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Foreign Application Priority Data
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Jun 5, 1995 [EP] |
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95830235 |
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Current U.S.
Class: |
363/19; 363/131;
363/97 |
Current CPC
Class: |
H02M
3/3385 (20130101) |
Current International
Class: |
H02M
3/338 (20060101); H02M 3/24 (20060101); H02M
003/335 () |
Field of
Search: |
;363/18,19,97,131 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Sterrett; Jeffrey
Attorney, Agent or Firm: Jorgenson; Lisa K. Santarelli;
Bryan A.
Claims
I claim:
1. A self-oscillating, DC--DC converter, comprising: a transformer
having a primary winding coupled to a primary circuit and a
secondary winding coupled to a secondary circuit, said primary
circuit including a .[.first.]. switch, functionally connected in
series with the primary winding, having a first terminal thereof
coupled to an input node; a sensing resistance functionally
connected between said .[.first.]. switch and a common potential
node of the circuit, said .[.first.]. switch being driven by a
self-oscillation circuit composed of at least an auxiliary winding
having a first and a second terminal magnetically coupled to said
primary winding and a first capacitor connected between a control
element of said .[.first.]. switch and an intermediate connection
node between said auxiliary winding and said first capacitor; a
.[.second switch.]. .Iadd.device .Iaddend.capable of
.[.shortcircuiting said control element of.]. .Iadd.deactivating
.Iaddend.said .[.first.]. switch .[.to said common potential
node.]. when a current through the primary winding reaches a
preestablished level; at least a second capacitor connected between
a second terminal of said auxiliary winding and said common
potential node; at least a diode having an anode coupled to said
common potential node and a cathode coupled to said intermediate
connection node; and at least a zener diode connected between said
second terminal of said auxiliary winding and a control element of
said .[.second switch.]. .Iadd.device.Iaddend..
2. The self-oscillating, DC--DC converter, according to claim 1
wherein said .[.first.]. switch is an isolated-gate, field effect
device and said .[.second switch.]. .Iadd.device .Iaddend.is
bipolar NPN transistor.
3. A DC--DC voltage regulating circuit, comprising: an input
voltage terminal; a .[.first.]. switch; a primary winding serially
coupled between said input voltage terminal and said .[.first.].
switch; a secondary winding magnetically coupled to said primary
winding; a sensing resistance serially coupled between said
.[.first.]. switch and a common potential node; and a
self-oscillation circuit coupled to a first control terminal of
said .[.first.]. switch, wherein said self-oscillation circuit
comprises an auxiliary winding magnetically coupled to said primary
winding and having a first node and a first intermediate node; a
first capacitive element coupled between said first node and said
common potential node; a first diode having an anode coupled to
said common potential node and a cathode coupled to said first
intermediate node; a second capacitive element coupled between said
first intermediate node and said first control terminal of said
.[.first.]. switch; a first resistive element coupled between said
input voltage terminal and said first control terminal of said
.[.first.]. switch; a .[.second switch.]. .Iadd.device
.Iaddend.coupled between said first control terminal of said
.[.first.]. switch and said common potential node and having a
second control terminal coupled to a second intermediate node
connecting said .[.first.]. switch and said sensing resistance; and
a second diode coupled between said first node and said second
control terminal.
4. The circuit of claim 3 wherein said .[.second switch.].
.Iadd.device .Iaddend.is a bipolar NPN transistor.
5. The circuit of claim 3 wherein said second diode is a zener
diode.
6. The circuit of claim 3 wherein said self-oscillation circuit
further includes means for summing a signal representative of the
level of the energy stored in said auxiliary winding with a control
signal provided to said first control terminal of said .[.first.].
switch.
7. The circuit of claim 3, further including a filtering capacitive
element coupled between said input voltage terminal and said common
potential node.
8. The circuit of claim 3, wherein said .[.second switch
shortcircuits said first control terminal to said common potential
node.]. .Iadd.device deactivates said switch .Iaddend.when a
current through the primary winding reaches a predetermined
level.
9. The circuit of claim 3, further including a third resistive
element coupled between said second control terminal and said
second intermediate node.
10. The circuit of claim 3 wherein said .[.first.]. switch is an
isolated-gate, field effect device..Iadd.
11. A power-supply circuit, comprising: an output terminal; a
transformer having a primary winding, having an auxiliary winding,
and having a secondary winding coupled to the output terminal and
operable to generate an output voltage on the output terminal; a
device having a variable conductivity and operable to control a
flow of current through the primary winding; and a regulation
circuit including the auxiliary winding and a sense element coupled
to the device and operable to conduct the current the regulation
circuit operable to maintain the output voltage at a constant or an
approximately constant level by coupling a regulation signal to the
sense element during a period in which the device has a high
conductivity..Iaddend..Iadd.
12. The power-supply circuit of claim 11 wherein the primary and
auxiliary windings are electrically isolated from the secondary
winding..Iaddend..Iadd.
13. The power-supply circuit of claim 11, further comprising an
input terminal coupled to the primary winding and operable to
receive an unregulated AC power signal..Iaddend..Iadd.
14. The power-supply circuit of claim 11 wherein the device
comprises an N-channel MOS transistor..Iaddend..Iadd.
15. The power-supply circuit of claim 11 wherein the regulation
circuit is operable to generate the regulation
signal..Iaddend..Iadd.
16. The power-supply circuit of claim 11 wherein the regulation
circuit controls the conductivity of the device by periodically
varying the conductivity of the device from the high conductivity
to a low conductivity at a frequency that is proportional to a
flyback signal generated across the primary winding when the device
has the low conductivity..Iaddend..Iadd.
17. The power-supply circuit of claim 11 wherein the regulation
circuit controls the conductivity of the device by periodically
varying the conductivity of the device from the high conductivity
to a low conductivity at a frequency that is proportional to a
flyback signal generated across the auxiliary winding when the
device has the low conductivity..Iaddend..Iadd.
18. The power-supply circuit of claim 11 wherein: the secondary
winding is operable to provide an output current to the output
terminal; and the regulation circuit controls the conductivity of
the device by periodically varying the conductivity of the device
from the high conductivity to a low conductivity at a frequency
that is proportional to the output current..Iaddend..Iadd.
19. The power-supply circuit of claim 11 wherein the regulation
signal comprises a regulation current..Iaddend..Iadd.
20. A power-supply circuit, comprising: an input terminal; an
output terminal operable to provide an output voltage; a primary
transformer winding coupled to the input terminal; a secondary
transformer winding coupled to the output terminal, the secondary
transformer winding being electrically isolated from and
magnetically coupled to the primary transformer winding; a
switching device having a first drive terminal coupled to the
primary transformer winding, having a second drive terminal, and
having a control terminal; and a regulation circuit comprising, an
auxiliary transformer winding that is electrically isolated from
the secondary transformer winding and that is magnetically coupled
to the primary and secondary transformer windings, a sense element
coupled to the second drive terminal of the switching device, and
wherein the regulation circuit is operable to regulate the output
voltage by, generating a regulation signal, and coupling the
regulation signal to the sense element while energy is being stored
in the primary transformer winding..Iaddend..Iadd.
21. The power-supply circuit of claim 20, further comprising a
common core upon which the primary, secondary, and auxiliary
transformer windings are wound..Iaddend..Iadd.
22. The power-supply circuit of claim 20 wherein the device
comprises an N-channel power transistor..Iaddend..Iadd.
23. The power-supply circuit of claim 20 wherein: the regulation
signal comprises a regulation current; and the regulation circuit
is operable to cause the regulation current to flow through the
sense element..Iaddend..Iadd.
24. A method for regulating an output voltage, the method
comprising: storing flyback energy by allowing a charging current
to flow through a primary transformer winding and through a sense
element during a charging period; controlling the duration of the
charging period by coupling a regulating signal to the sense
element during the charging period; generating a primary flyback
voltage across the primary transformer winding after the charging
period; generating an auxiliary flyback voltage across an auxiliary
transformer winding in response to the primary flyback voltage;
generating the regulating signal from the auxiliary flyback
voltage; generating a secondary flyback voltage across a secondary
transformer winding in response to the primary flyback voltage; and
generating the output voltage from the secondary flyback
voltage..Iaddend..Iadd.
25. The method of claim 24 wherein: the regulating signal comprises
a regulating current; the sense element comprises an input
terminal; and controlling the duration of the charging current
comprises summing the regulating and charging currents at the input
terminal of the sense element..Iaddend..Iadd.
26. The method of claim 24 wherein: the regulating signal comprises
a regulating current; and controlling the duration of the charging
current comprises causing the regulating current to flow through
the sense element..Iaddend..Iadd.
27. The method of claim 24, wherein: the regulating signal
comprises a regulating current; and controlling the duration of the
charging period comprises causing the regulating current to flow
through the sense element..Iaddend..Iadd.
28. A power-supply circuit, comprising: an input terminal; a supply
terminal; a regulated output terminal operable to provide an output
voltage; a primary transformer winding coupled to the input
terminal; a secondary transformer winding coupled to the regulated
output terminal, the secondary transformer winding being
electrically isolated from and magnetically coupled to the primary
transformer winding; a switching device having a first drive
terminal coupled to the primary transformer winding, having a
second drive terminal, and having a control terminal; and a
regulation circuit comprising, an auxiliary transformer winding
that has first and second terminals, that is electrically isolated
from the secondary transformer winding, and that is magnetically
coupled to the primary and secondary transformer windings, a sense
element coupled between the supply terminal and the second drive
terminal of the switching device, a diode coupled between the
supply terminal and the first terminal of the auxiliary winding, a
capacitor coupled between the supply terminal and the second
terminal of the auxiliary winding, a transistor having a first
drive terminal coupled to the supply terminal, a second drive
terminal coupled to the control terminal of the switching device,
and a control terminal coupled to the second drive terminal of the
switching device, and a zener diode coupled between the second
terminal of the auxiliary transformer winding and the control
terminal of the transistor..Iaddend..Iadd.
29. A method for generating a regulated output voltage, the method
comprising: storing flyback energy by allowing a charging current
to flow through a primary transformer winding during a charging
period; controlling the duration of the charging period by
combining a regulating current with the charging current during the
charging period; generating a primary flyback voltage across the
primary transformer winding after the charging period; generating
an auxiliary flyback voltage across an auxiliary transformer
winding in response to the primary flyback voltage; generating the
regulating current from the auxiliary flyback voltage; generating a
secondary flyback voltage across a secondary transformer winding in
response to the primary flyback voltage; and generating the
regulated output voltage from the secondary flyback
voltage..Iaddend..Iadd.
30. The method of claim 29 wherein: storing the flyback energy
comprises reducing the impedance of a switching device coupled in
series with the primary transformer winding; and generating the
primary flyback voltage comprises increasing the impedance of the
switching device in response to the combination of the charging and
regulating currents..Iaddend..Iadd.
31. The method of claim 29 wherein controlling the duration of the
charging period comprises summing the charging and regulating
currents..Iaddend..Iadd.
32. The method of claim 29 wherein: controlling the duration of the
charging period comprises summing the charging and regulating
currents; and generating the primary flyback voltage comprises
increasing the impedance of a switching device coupled in series
with the primary transformer winding when the sum of the charging
and regulating currents equals or exceeds a predetermined
value..Iaddend.
Description
TECHNICAL FIELD
A flyback-type self-oscillating DC--DC converter uses the primary
winding side circuit of a transformer to regulate the output
voltage of a power supply.
BACKGROUND OF THE INVENTION
Switching power supplies offer remarkable advantages in terms of
volume, weight and electrical efficiency if compared with
traditional transformer-type power supplies functioning at the
mains frequency. However, due to the complexity of the electronic
circuitry employed, these switching power supplies are rather
costly. One of the architectures most frequently used is based on
the use of a flyback-type, DC--DC converter.
In a flyback system, energy is stored within the primary winding
inductance of the transformer during a conduction phase of a power
transistor (switch), functionally connected in series with the
primary winding and is transferred to the secondary winding of the
transformer during a subsequent phase of non-conduction of the
switch, which is driven at a relatively high frequency, for
example, by a local oscillator having a frequency in the order of
tens of kHz.
In switching power supplies, the voltage at the input of the DC--DC
converter is not regulated. Commonly, in a power supply connectable
to the mains, the input voltage of the converter is a nonregulated
voltage as obtained by rectifying the mains voltage by a Wien
bridge and leveling it by a filtering capacitor. Therefore this
voltage is a nonregulated DC voltage whose value depends on the
mains voltage that can vary from 180 VAC to 264 VAC in Europe and
from 90 VAC to 130 VAC in America.
A diagram of a flyback-type, self-oscillating primary side circuit
of a power supply connectable to the mains is shown in FIG. 1.
At the turning on instant, the voltage V.sub.INDC produces a
current i in the resistance R1 that has normally a high ohmic
value. This current charges the gate-source capacitance of the
power switch T1, which, in the example shown, is an isolated-gate,
field effect transistor. The gate-source voltage increases in time
according to the following approximate equation: ##EQU1##
where V.sub.GS indicates the voltage between the gate and the
source of transistor T1. C.sub.GS is the gate-source capacitance, i
is the current that flows through R1 and t is time.
When the voltage V.sub.GS reaches the threshold value V.sub.THR,
the transistor begins to drive a current I.sub.P while the drain
voltage V.sub.DS decreases because of the voltage drop provoked by
the current I.sub.P on the inductance L of the primary winding
N.sub.P of the transformer.
Therefore, a voltage equal to V.sub.INDC -V.sub.DS is generated at
the terminals of the primary winding N.sub.P. This voltage, reduced
according to the turn ratio N.sub.1 /N.sub.P between the primary
winding N.sub.P and the auxiliary winding N.sub.1 belonging to the
self-oscillating circuit, is also applied between the gate node G
and the common ground node of the circuit through a capacitor C2.
This voltage, which is in phase with the voltage present on the
primary winding N.sub.P, provokes a further increase of the voltage
between the gate node G and the source node S of the transistor T1,
which therefore is driven to a state of full conduction. Therefore
the voltage on the inductance L of the primary winding N.sub.P is
approximately equal to the rectified input voltage V.sub.INDC and
the current that flows through the primary winding of the
transformer has a value given by the following equation:
##EQU2##
On the other hand, the current I.sub.P also flows in the resistance
R2 provoking a voltage drop thereon given by I.sub.P.multidot.R2.
Even this voltage drop grows linearly in time until it reaches
conduction threshold value V.sub.BE of the second (transistor)
switch T2.
By entering into a state of conduction, the transistor T2
.[.shortcircuits toward the ground node and.]. .Iadd.reduces the
voltage at .Iaddend.the gate node G of the transistor T1, which
therefore turns off. Initially the current I.sub.P continues to
flow thus increasing the voltage V.sub.DS well above the input
voltage V.sub.INDC. Therefore, a flyback voltage develops on the
primary winding inductance L that has an opposite polarity to that
of the voltage present during a conduction phase of the switch T1.
This flyback voltage, reduced in terms of the turn ratio N.sub.1
/N.sub.P, is also applied between the gate node G of the transistor
T1 and the common ground node of the circuit and further
contributes to keep the transistor T1 in an off condition, having a
negative polarity as referred to the ground potential.
During a .Iadd.high .Iaddend.conduction phase of the transistor
.[.T1.]. .Iadd.T2.Iaddend., the energy accumulated in the
inductance L of the primary winding of the transformer transfers
completely into the secondary circuit that is only partially
depicted in FIG. 1. This occurs during such an OFF or FLYBACK phase
of operation of the transistor T1. When this phase of energy
transfer is over, the .[.voltage.]. .Iadd.voltages .Iaddend.on the
primary winding N.sub.P and on the winding N.sub.1 of the
self-oscillation circuit are null and therefore a new cycle can
start again.
The above-mentioned system typifies a common flyback architecture
where the primary current I.sub.P rises linearly from zero up to a
peak value given by the following equation: ##EQU3##
during a conduction phase of transistor T1.
The relevant waveforms of the circuit are shown in FIG. 3.
Upon a variation of the input voltage V.sub.INDC, the conduction
time T.sub.ON of transistor T1 varies according to the following
expression: ##EQU4##
Therefore, the frequency of oscillation is inversely proportional
to the rectified mains voltage.
In the majority of applications, the output voltage must be
regulated to make it independent from input voltage variation, in
other words from the value of the rectified mains voltage.
Commonly in the majority of applications, control of the output
voltage is implemented in the secondary circuit. These regulating
use feedback control loops that normally sense the secondary
voltage level provide this information to the primary circuit via
an electrical isolation device, for example, a photo-coupler. These
solutions are very efficient but they are also relatively
expensive. Even alternative known solutions implementing an output
voltage regulation by regulating the current flowing through the
primary winding of the transformer during conduction phase of the
switch, imply the realization of one or more auxiliary windings and
a remarkable complication of the primary circuit.
SUMMARY OF THE INVENTION
It has now been found a surprisingly simple and effective system
for regulating the output voltage through the primary circuit of
the transformer of a DC--DC converter. The system of this invention
does not require any additional winding because it exploits the
auxiliary winding N.sub.1 of the self-oscillation circuit for
implementing the desired regulation of the voltage output by the
secondary circuit of the transformer-type converter.
In practice, the method of this invention consists in realizing a
discharge current circulation loop of the energy transferred in the
auxiliary winding of the self-oscillation circuit during a phase of
conduction of the transistor that switches the primary winding and
in summing a signal representative of the level of energy on the
control node of a driving stage of the switch to regulate its
conduction interval.
Practically, the circulation loop of the discharge current relative
to the energy stored in the auxiliary winding of the
self-oscillation circuit, reproduces electrically the discharge
current circulation loop of the energy that is stored in the
secondary winding of the transformer. Through a process of
self-redistribution of the energy that is stored in the primary
winding inductance during the conduction phase of the switch, the
system regulates the energy that is transferred from the primary to
the secondary winding of the transformer so as to keep
substantially uniform the output voltage that develops on the
secondary winding of the transformer. This regulation occurs
following a change of the value of the nonregulated input voltage
and/or of the current absorbed by the secondary circuit.
The auxiliary winding of the self-oscillation circuit is in phase
with the primary winding of the transformer during the switch
conduction phase while is in phase with the secondary winding
during the following off phase of the switch (flyback phase).
According to an important embodiment of this invention, a power
supply includes a self-oscillating DC--DC converter having a power
transistor switch connected in series with a primary winding of a
transformer that is coupled to an input voltage, and a sensing
resistance functionally connected between the switch and a common
potential node of the circuit. The switch is driven by a
self-oscillating circuit composed of at least an auxiliary winding,
magnetically coupled to the primary winding, and a first capacitor
that is connectable in series between a control node of the switch
and common potential node. A second transistor switch is driven to
shortcircuit the control node of the first switch with common
potential node, when the current flowing through the primary
winding reaches a pre-established level. According to the
invention, the circuit includes also a second capacitor at least
one diode and at least one zener diode. The second capacitor is
connected between the auxiliary winding and common potential node.
The diode has the anode coupled to common potential node and a
cathode coupled to the intermediate connection node between the
auxiliary winding and the first capacitor so as to constitute a
discharge current recirculation loop for the energy stored in the
auxiliary winding inductance. The zener diode is functionally
connected between the intermediate connection node between the
auxiliary winding and the second capacitor and a control node of
the second transistor switch.
While the current recirculation loop realizes a replica in the
primary circuit of the current recirculation loop of the secondary
winding of the transformer, by means of the zener diode, a current
signal is injected on the control node of the switch connected in
series with the primary winding of the transformer so as to
regulate the timing of the turning off of the switch by the driver
stage, i.e., the interval of conduction of the switch. Through a
mechanism of energy self-redistribution, the circuit ensures
stability of the voltage, output by the secondary circuit of the
converter, when the input voltage and/or the current absorbed from
the secondary circuit change, as it will be further demonstrated in
the description below.
BRIEF DESCRIPTION OF THE DRAWINGS
The various aspects and advantages of this invention will become
more evident through the following description of some important,
through non-limitative embodiments and by referring to the annexed
drawings.
FIG. 1 shows, as already mentioned above, a basic scheme of a
self-oscillating primary circuit
FIG. 2 shows a basic scheme of a power supply connectable to the
mains that employs a self-oscillating, flyback-type DC--DC
converter made according to this invention.
FIG. 3 shows, as already mentioned above, the typical waveforms of
the flyback converter.
DETAILED DESCRIPTION OF THE INVENTION
With reference to the diagram of FIG. 2 and to the waveforms shown
in FIG. 3, during a phase of operation where the transistor T1
conducts T.sub.ON, the voltage between the cathode of the diode D1
and the ground node of the circuit is positive and therefore the
diode is not conductive and the positive voltage contributes to
keep the transistor T1 in a state of conduction by means of the
capacitor C2.
During the subsequent flyback phase, when the transistor T1 is not
conductive, the voltage on the diode D1 cathode becomes negative
and a recirculation current I.sub.S1 can circulate in the loop
composed of the diode D1, the winding N1 and the capacitor C3.
Therefore, the capacitor C3 is charged by the recirculation current
I.sub.S1 and the voltage on it rises. By calling V.sub.Z the
voltage of the zener diode D2, when the following condition is
fulfilled:
the diode D2 begins to conduct, thus forcing a current through the
resistance R3.
By calling i.sub.Z the current that flows through the diode D2, the
equation of the recirculation loop that includes the base-emitter
junction of the transistor .[.D2.]. .Iadd.T2 .Iaddend.and the
resistances R2 and R3 becomes:
If i.sub.Z increases due to an increment of the voltage V.sub.C3
reached by the capacitor C3 when charging, I.sub.P and consequently
T.sub.ON must proportionally decrease in value in accordance with
equation (2). Therefore, a lower amount of energy will be stored in
the inductance L of the primary winding of the transformer and, as
a consequence, a lower recirculation current I.sub.S1 will flow
during the next flyback phase in order to keep the voltage V.sub.C3
constant and equal to a value given by V.sub.Z +V.sub.BE.
Thus, during a flyback phase, the voltage applied to the N1 winding
is constant and equal to:
where V.sub.D1 represent the voltage drop through the diode D1 when
conducting.
Even the voltage V.sub.S2 that develops on the secondary winding
N.sub.S of the transformer will be constant during the flyback
phase and will have a value given by: ##EQU5##
by combining equations (3), (5) and (6), we obtain: ##EQU6##
and the output voltage V.sub.OUT becomes: ##EQU7##
wherein V.sub.D3 indicates the voltage drop on the diode D3 when
conducting.
Equation (8) contains only constant terms therefore the resulting
output voltage V.sub.OUT will be constant too. In particular, if
N.sub.2 =N.sub.1 and V.sub.D1 =V.sub.D3, the output voltage
becomes:
Therefore the above described circuit permits the regulation of the
output voltage of the secondary side circuit of the
transformer-type DC--DC converter by implementing the necessary
control in the primary side circuit of the converter by the
addition of only three components, namely: D1, D2 and C3, according
to the embodiment shown.
In practice, by the addition of the components D1 and C3, a
recirculation loop is realized for a discharge current of the
energy stored in the auxiliary winding N1 of the self-oscillation
circuit, which substantially replicates the secondary side output
loop of the converter. By means of the zener diode D2, a current
i.sub.Z is then injected on the driving node of the transistor T2,
the current is representative of the charge level reached by the
inductance of the auxiliary winding N1 of the self-oscillation
circuit, during a phase of conduction of the switch .[.D1.].
.Iadd.T1.Iaddend.. This current i.sub.Z produces a voltage drop on
the resistance R3, which is in turn summed to the voltage drop
I.sub.P.multidot.R2 during the conduction phase of the switch T1,
thus regulating the turn-on interval T.sub.ON and therefore the
energy stored in the inductance L of the primary winding of the
transformer.
The system is perfectly capable of regulating the output voltage
V.sub.OUT upon the changing of the input voltage V.sub.INDC as well
as of the output current I.sub.OUT.
In fact, if the output current increases, a larger amount of energy
must be transferred from the primary winding N.sub.P to the
secondary winding N.sub.S during the flyback phase so that a lower
amount of energy remains available from the inductance of the
auxiliary winding N1 to charge the capacitor C3. Therefore the
voltage reached by C3 upon charging will be lower. As a
consequence, the current i.sub.Z will also be lower and the current
I.sub.P will proportionally increase in order to fulfill the
following equation:
The increase of the current I.sub.P increments the energy stored in
the inductance L of the primary winding N.sub.P and this increased
energy will be available during the flyback phase. Therefore the
system is capable to supply the additional energy required by the
rise of the output current I.sub.OUT, thus keeping constant the
output voltage V.sub.OUT.
The way the increase of the output current I.sub.OUT provokes an
increase of the conduction interval T.sub.ON of the switch T1, and
therefore a consequent reduction of the converter switching
frequency, to allow the current I.sub.P to reach a higher peak
value should be remarked.
* * * * *