JP2001178131A - High power factor flyback converter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize a circuit system for improving power factor of a flyback converter at a low cost. SOLUTION: This flyback converter comprises a series circuit of an AC power supply 31 and a bridge rectifier 12, a filter capacitor 13, a primary winding 14a of a transformer 14 for forming a closed circuit with the capacitor 13 and the main switch element 15, and an oscillation control circuit 16 connected to the control electrode of the element 15. In this case, a series circuit of first and second diodes 1 and 2, connected in series with the same polarity poles opposed, is connected in parallel with an AC power supply 31. Then, a third diode 3 is connected between the connecting point of the first and second diodes 1 and 2 and a prescribed terminal of the capacitor 13, and a first snubber capacitor 4 is connected between the connecting point of the first and second diodes 1 and 2 and the connecting point of the winding 14 and the element 15.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a switching power supply, and more particularly to a switching power supply having a power factor correction circuit.

[0002]

2. Description of the Related Art FIG. 8 is a circuit diagram showing an example of a conventional system. In FIG. 8, the AC voltage of the AC power supply 31 is full-wave rectified by the bridge rectifier 12, passes through the reactor 24 and the diode 23, and charges the smoothing capacitor 13.
The DC voltage of the smoothing capacitor 13 is
Is repeatedly intermittently applied to the primary winding 14a of the transformer 14. The current flowing through the primary winding 14a during the ON period of the main switch element 15 includes a load current for supplying power to the secondary-side load and an excitation current for exciting the core of the transformer 14. The stored excitation energy is applied to the primary winding 14a and the snubber capacitor 22 when the main switch element 15 is turned off.
Flows through a closed circuit composed of a diode 23 and the snubber capacitor 22 to charge and accumulate energy in the form of electric charge. When the main switch element 15 is turned on from the off state, the charge of the snubber capacitor 22 is
, Bridge rectifier 12, AC power supply 31, and reactor 24
Flows through a closed circuit consisting of At this time, the AC current also flows when the absolute value of the AC voltage of the AC power supply 31 is lower than the voltage of the smoothing capacitor 13. In a general capacitor input type rectifier circuit, AC current does not flow in the section where the absolute value of the AC voltage is low with respect to the voltage of the smoothing capacitor, so the conduction angle of the AC current is narrowed and the power factor is reduced, In the circuit of FIG. 8, an alternating current flows each time the main switch element 15 is turned on, so that the conduction angle is widened and the power factor is improved.

[0003]

In the circuit shown in FIG.
While the main switch element 15 is turned on, a current flows through a series resonance circuit composed of the snubber capacitor 22 and the reactor 24. However, since the bridge rectifier 12 allows the current to flow in only one direction, a half-wave rising from zero and ending at zero is obtained. Current resonance occurs. However, when the ON period of the main switch element 15 is shorter than one half of the resonance period, the resonance ends in an incomplete state, the conduction angle of the AC input current becomes narrow, and when the main switch element 15 is turned off, The effect of the snubber capacitor 22 for suppressing the surge voltage generated in the primary winding 14a also decreases.

[0004] Therefore, it has been difficult to apply the present invention to a flyback converter in which the ON period of the switch element changes greatly depending on the input voltage and the load current. Therefore, the present invention uses a resonance current generated not in the ON period of the switch element but in the OFF period to flow the resonance current to the snubber capacitor,
As a result, even when the ON period of the switch element is short,
It is an object of the present invention to provide a high power factor flyback converter that increases the conduction angle of an AC input current and does not reduce the effect of a snubber capacitor.

[0005]

In order to achieve the above object, the present invention provides an AC power supply, a full-wave rectifier, a smoothing capacitor, and a transformer for forming a closed circuit with the smoothing capacitor.
In a flyback converter including a series circuit including a secondary winding and a main switch element and an oscillation control circuit connected to a control electrode of the main switch element, a first diode and a second diode connected in series with the same electrodes facing each other are connected. A series circuit consisting of two diodes is connected in parallel to the AC power supply, a third diode is connected between a connection point between the first diode and the second diode and a predetermined terminal of the smoothing capacitor,
Further, the connection point between the first diode and the second diode and 1
A first snubber capacitor was connected between the connection point of the next winding and the main switch element.

When the cathodes of the first diode and the second diode face each other, the third diode
Is connected to the positive terminal of the smoothing capacitor. When the anodes of the first diode and the second diode face each other, the anode of the third diode is connected to the negative terminal of the smoothing capacitor.

When the main switch element is turned off from the on state, the first snubber capacitor is charged by the exciting energy of the transformer. In the discontinuous section from when the transformer excitation energy becomes zero to when the main switch element is turned on, the first snubber capacitor and the transformer
Resonance is generated by the next winding, and the charge of the first snubber capacitor is discharged first, and is charged again if the discontinuous section is longer than a half cycle of resonance. Then, if the charge remains in the first snubber capacitor when the main switch element is turned on, it flows through the main switch element and is discharged.

As described above, there is at least one reciprocating current in the first snubber capacitor between the turn-off and the turn-on of the main switch element, so that the current always flows to the AC power supply. Since this AC current flows every time the main switch element is switched, regardless of the voltage of the AC power supply, the conduction angle of the AC input current is widened and the power factor is improved.

According to a second aspect of the present invention, in the first aspect of the invention, an auxiliary switch element is inserted in series in a circuit in which the current of the first snubber capacitor reciprocates, and a fourth switch element is connected in parallel with the auxiliary switch element. A diode is connected, and a delay circuit for delaying the signal of the oscillation control circuit of the main switch element for a predetermined time and adding it to the control electrode of the auxiliary switch element is added, thereby providing the following effects. The current in the direction of discharging the first snubber capacitor can always flow through the fourth diode, but the current in the direction of charging the first snubber capacitor is turned off after the main switch element is turned off until the auxiliary switch element is turned off. Therefore, the first snubber capacitor is charged only when the main switch element is turned off, and is only discharged in the discontinuous section. As a result, since the first snubber capacitor has the lowest voltage when the main switch element is turned on, the loss due to the discharge of the first snubber capacitor when the main switch element is turned on is improved.

The effect of the auxiliary switching element, the fourth diode, and the delay circuit described above was first provided by the present applicant by a soft switching circuit of a separately-excited switching power supply in discontinuous current mode (Japanese Patent No. 2835899). ing.

According to a third aspect of the present invention, the oscillation control circuit according to the first aspect of the present invention is a self-excited oscillation type having a positive feedback winding of a transformer, wherein the oscillation control circuit is provided between the positive feedback winding and a control electrode of the main switch element. A saturable inductor is inserted in series, which has the following effects. Since the saturable inductor has a high impedance until saturation and drops to an impedance close to zero after saturation, the pulse signal is delayed. If time is required due to the delay action of the saturable inductor from when the transformer excitation energy becomes zero to when the main switch element is turned on, the resonance period of the first snubber capacitor and the primary winding also becomes longer. , By appropriately selecting the delay time due to the saturable inductor,
Since the main switch element can be turned on when the voltage of the snubber capacitor becomes lowest, the loss due to the electric charge of the snubber capacitor flowing through the main switch element and discharging is improved.

The effect of the saturable inductor described above has been previously provided by the present applicant by a soft switching circuit (utility model registration No. 2560208) of a self-excited switching power supply.

According to a fourth aspect of the present invention, the oscillation control circuit of the first aspect is a self-excited oscillation type having a positive feedback winding of a transformer, and is provided between the positive feedback winding and the control electrode of the main switch element. , A saturable inductor is inserted in series, a MOSFET is inserted in series with the first snubber capacitor, a second snubber capacitor is connected in parallel with the MOSFET, an auxiliary winding is wound around the transformer, and both ends are connected via resistors. To the gate and source of the MOSFET. This has the following effects. Since the signal applied between the gate and the source of the MOSFET is inverted in phase from the signal applied to the control electrode of the main switch element, the MOSFET is in the off state when the main switch element is in the on period, and the main switch element is in the off period during the off period. At this time, the MOSFET is on. Here, the ON period of the main switch element refers to the time during which the excitation energy of the transformer primary winding increases linearly, and the OFF period of the main switch element refers to the time during which the excitation energy of the transformer primary winding is increased. It refers to the period during which it is decreasing linearly. Although the discontinuous section is neither the off period nor the on period, the main switch element is in the off state. In a discontinuous section from when the excitation energy of the transformer becomes zero to when the main switch element is turned on, the MOS is maintained while the main switch element remains off.
The FET changes from the on state to the off state.

In the discontinuous section, the voltage of the first snubber capacitor decreases by resonance, but the current flowing through the primary winding increases by resonance. The MOSFET is turned off when the voltage of the auxiliary winding becomes close to the gate threshold of the MOSFET, but the main switching element has not yet been turned on due to the delay effect of the saturable inductor. Therefore, the series resonance circuit that has been formed by the first snubber capacitor and the primary winding until now has a series resonance capacitance that is formed by the series combined capacitance including the first snubber capacitor and the second snubber capacitor and the primary winding. Turns into a circuit. Second
If the capacity of the snubber capacitor is made sufficiently smaller than the capacity of the first snubber capacitor, the momentum of the current flowing through the primary winding remains, so that the second snubber capacitor is connected to the first snubber capacitor. It is charged to the voltage equal to the capacitor. When the voltage of the second snubber capacitor and the voltage of the first snubber capacitor become equal to each other, the voltage across the main switch element becomes zero. By appropriately selecting the delay by the saturable inductor, The first snubber capacitor can be turned on, and the loss caused by the first snubber capacitor flowing through the main switch element and discharging can be reduced to zero.

MOSFET and MOS inserted in series with the saturable inductor and the first snubber capacitor shown above.
The effect of the second snubber capacitor connected in parallel with the FET has been previously provided by the present applicant through a low-loss circuit (Japanese Patent Application No. 10-291274) of a partial resonance type self-excited switching power supply.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described with reference to FIGS. 1 and 2 showing an embodiment and FIG. 3 showing waveforms of main parts of FIG. In FIG. 1, the main switch element 15 is repeatedly turned on and off, a current flows through the primary winding 14a of the transformer 14 during the on period, and the excitation energy is stored in the transformer, and most of the excitation energy during the off period. Is taken out as a current by the secondary winding 14b, rectified and smoothed by the diode 17 and the capacitor 18, and supplied to the load 19 as a DC voltage. Immediately after the main switch element 15 is turned off, part of the excitation energy is supplied to the first snubber capacitor 4 and the third diodes 3 and 1.
The first snubber capacitor 4 is charged by flowing through a closed circuit composed of the next winding 14a. The embodiment shown in FIG. 1 is a flyback converter in the discontinuous current mode. However, since there is a time called a discontinuous section between the time when the excitation energy is completely released and the time when the switch element enters the ON period, the discontinuous section is used. The following resonance occurs due to the first snubber capacitor 4 and the primary winding 14a.

When the release of the excitation energy is completed and the first snubber capacitor 4 becomes empty, the electric charge of the first snubber capacitor 4 is reduced to the first diode 1 or the second diode 2 shown by a dotted line in FIG. 1 and the first snubber capacitor. 4, a primary winding 14a, a smoothing capacitor 13, a bridge rectifier 12, and an AC power supply. That is, a current flows to the AC power supply 31 due to the discharge of the first snubber capacitor 4, but since this current always flows in the discontinuous section, the conduction angle of the AC current is widened and the power factor is improved. The positive side of the current of the first snubber capacitor 4 shown in FIG. 3 is the current flowing through the third diode 3. The shaded negative side is the first diode 1
Or a current flowing through one of the second diodes 2.
This increases the conduction angle of the alternating current.

In FIG. 2, the directions of the first diode 1 and the second diode 2 are opposite to those in FIG. 1, and the terminal of the smoothing capacitor 13 to which the third diode 3 is connected is also on the opposite side. In FIG. 2, the first snubber capacitor 4
Is performed in a closed circuit formed by the third diode 3, the first snubber capacitor 4, the primary winding 14a, and the smoothing capacitor 13. However, when the main switching element 15 is turned off, the first switching element 15 discharges the first energy to release the first energy. The current for charging the snubber capacitor 4 is equal to the first diode 1 or the second diode 2 indicated by the dotted line in FIG.
1, a bridge rectifier 12, a primary winding 14 a, and a first snubber capacitor 4. That is, the first
Since the current flows through the AC power supply 31 by the current charging the snubber capacitor 4, the conduction angle of the AC current is increased and the power factor is improved.

An embodiment of the present invention will be described with reference to FIG. 4 showing an embodiment and FIG. 5 showing waveforms of main parts of a circuit. In FIG. 4, the main switch element 15 repeats on / off in response to the signal of the oscillation control circuit 16, and the auxiliary switch element 5 receives the signal of the oscillation control circuit 16 through the delay circuit 6, so that the main switch element 15 is delayed by the main switch element 15. On and off. Main switch element 15
When the switch is turned off from the on state, the auxiliary switching element 5 is still in the on state, so that the excitation energy of the transformer 14 includes the primary winding 14a, the auxiliary switching element 5, the first snubber capacitor 4, and the first diode. One or second diode 2, AC power supply 31, and bridge rectifier 1
2 and charges the first snubber capacitor 4.

Since the delay time of the delay circuit 6 is a short period until the charging of the first snubber capacitor 4 is completed, the auxiliary switch element 5 is turned off when a discontinuous section is reached. However, since the fourth diode 7 is connected in parallel to the auxiliary switch element 5, the first snubber capacitor 4 is composed of the fourth diode 7, the primary winding 14a, the smoothing capacitor 13, and the third diode 3. Discharge through a closed circuit. This discharge becomes a series resonance current since the auxiliary switching element 5 is in the off state, and rises at zero and stops when it returns to zero. Therefore, the voltage of the first snubber capacitor 4 is maintained at the lowest point during the discontinuous section, and the electric charge remaining in the first snubber capacitor 4 when the main switch element 15 is turned on is small. Loss is small.

The hatched positive side of the current of the first snubber capacitor 4 shown in FIG. 5 flows through either the first diode 1 or the second diode 2 to increase the conduction angle of the alternating current. The negative side is a current flowing through the fourth diode 7,
Most flows into the discontinuous section and returns energy to the smoothing capacitor 13, and the rest flows when the main switch element 15 is turned on. In the discontinuous section in FIG. 5, a resonance having a long period is followed by a resonance having a short period.
It depends on the stray capacitance existing between the auxiliary switch element 5 and the electrode of the fourth diode 7.

An embodiment of the present invention will be described with reference to FIG. 6 showing an embodiment. In FIG. 6, excitation energy is accumulated in the transformer 14 during the ON period of the main switch element 15, and the excitation energy is released to the load side through the secondary winding 14b during the OFF period, and a part of the excitation energy is The first snubber capacitor 4 is charged. The current for charging the first snubber capacitor 4 is either the first diode 1 or the second
Pass through the AC power supply 31, the bridge rectifier 12, and the primary winding 14a. At this time, since the AC input current flows, the conduction angle increases. When the excitation energy is released, the voltage generated by each winding of the transformer 14 becomes zero, and the difference between the voltage of the first snubber capacitor 4 and the voltage of the smoothing capacitor 13 is applied to the primary winding 14a. Series resonance by the snubber capacitor 4, the primary winding 14a, the smoothing capacitor 13, and the third diode 3 starts. At the same time, the voltage stored in the capacitor 20 is applied to the control electrode of the main switch element 15, so that the main switch element 15 turns on. The extent to which the series resonance by the first snubber capacitor 4 and the primary winding 14a progresses before the main switch element 15 is turned on depends on the saturable inductor 8 inserted in series with the control electrode. Later, that is, when the voltage of the first snubber capacitor 4 becomes the lowest, the main switch element 1
The saturable inductor 8 is selected so that 5 turns on. As a result, most of the electric charge of the first snubber capacitor 4 is regenerated to the smoothing capacitor 13, so that the loss caused by the electric charge of the first snubber capacitor 4 flowing through the main switch element 15 is improved. By utilizing this effect, the capacity of the first snubber capacitor 4 can be increased without increasing the loss so much, so that the current flowing through the first snubber capacitor 4 when the main switch element 15 is turned off increases, and the conduction angle increases. Becomes wider.

An embodiment of the present invention will be described with reference to FIG. 7 showing an embodiment. In FIG.
Assuming that there is no SFET 9, the operation can be considered that the series combined capacitance of the first snubber capacitor 4 and the second snubber capacitor 11 corresponds to the first snubber capacitor 4 in FIG. 6. The following differences occur due to the insertion of the MOSFET 9. Auxiliary winding 14d is MOSFE
The signal sent to the gate of T9 has an opposite phase to the signal sent from the positive feedback winding 14c to the control electrode of the main switch element 15. When the main switch element 15 is in the ON period, the MOSFET 9 is turned off and the main switch element 15 is turned off. Is in the off period, the MOSFET 9 is turned on. However, in the discontinuous section, the main switch element 15 is in the off state but the MOSFET
For T9, it is a transition period when the ON state changes to the OFF state. Before the main switch element 15 is turned on, the MOS
When the FET 9 is turned off, the capacity of the capacitor forming the series resonance is changed from the capacity of the first snubber capacitor 4 to the capacity of the first snubber capacitor 4 and the second snubber capacitor 11.
To the series combined capacity. If the capacity of the second snubber capacitor 11 is selected to be sufficiently smaller than the capacity of the first snubber capacitor 4, the current flowing through the primary winding 14a immediately before the MOSFET 9 is turned off is dragged. The second snubber capacitor 11 is charged and reaches a value equal to the voltage of the first snubber capacitor 4 in a short time. At this time, the amount of charge that moves from the first snubber capacitor 4 to the second snubber capacitor 11 does not significantly lower the voltage of the first snubber capacitor 4. If the delay time of the saturable inductor 8 is selected so that the main switch element 15 is turned on when the sum of the voltages of the first snubber capacitor 4 and the second snubber capacitor 11 becomes zero, a zero volt switch is possible. Become. As a result, the capacity of the first snubber capacitor 4 can be increased, the conduction angle of the AC input current becomes wider, and the effect of improving the power factor increases.

In the embodiment shown in FIGS. 4, 6, and 7, the first
The anodes of the diode 1 and the second diode 2 are connected to face each other. However, this is connected to the cathodes of the diode 1 and the cathode of the third diode 3 as in the embodiment shown in FIG. To the smoothing capacitor 1
The anode may be connected to the connection point of the first diode 1 and the second diode 2 on the positive side of 3.

[0025]

As described above, the power factor can be improved by simply adding a snubber capacitor and three diodes for separately dividing the charging and discharging paths of the snubber capacitor to the conventional flyback converter. Great economic effect.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a high power factor flyback converter according to an embodiment of the present invention.

FIG. 2 is a circuit diagram showing a high power factor flyback converter according to another embodiment of the present invention.

FIG. 3 is a waveform diagram of a main part of the circuit of FIG. 1;

FIG. 4 is a circuit diagram showing a high power factor flyback converter according to an embodiment of the present invention.

FIG. 5 is a waveform chart of a main part of the circuit of FIG. 4;

FIG. 6 is a circuit diagram showing a high power factor flyback converter according to an embodiment of the present invention.

FIG. 7 is a circuit diagram showing a high power factor flyback converter according to an embodiment of the present invention.

FIG. 8 is a circuit diagram showing one example of a conventional system.

[Description of Signs] 1 First diode 2 Second diode 3 Third diode 4 First snubber capacitor 5 Auxiliary switch element 6 Delay circuit 7 Fourth diode 8 Saturable inductor 9 MOSFET 10 Resistance 11 Second Snubber capacitor 12 Bridge rectifier 13 Smoothing capacitor 14 Transformer 15 Main switch element 16 Oscillation control circuit 17 Diode 18 Capacitor 19 Load 20 Capacitor 21 Resistance 22 Snubber capacitor 23 Diode 24 Reactor 25, 26 Diode 27 Reactor 28 Capacitor 29 Load 31 AC power supply 14a1 Secondary winding 14b Secondary winding 14c Positive feedback winding 14d Auxiliary winding

Claims (4)

[Claims] 1. A series circuit comprising an AC power supply, a full-wave rectifier, a smoothing capacitor, a primary winding of a transformer and a main switch element forming a closed circuit with the smoothing capacitor, and a control electrode of the main switch element. In a flyback converter having an oscillation control circuit connected thereto, a first diode and a second diode connected in series with the same electrodes facing each other.
Are connected in parallel to the AC power supply, and a third circuit is provided between a connection point between the first diode and the second diode and a predetermined terminal of the smoothing capacitor.
And a first snubber capacitor is connected between a connection point between the first diode and the second diode and a connection point between the primary winding and the main switch element. A high power factor flyback converter characterized by widening the conduction angle of input current and improving power factor. 2. An auxiliary switch element is inserted in series with the first snubber capacitor, a fourth diode is connected in parallel with the auxiliary switch element, and an output signal of the oscillation control circuit is delayed for a predetermined time. 2. The high power factor flyback converter according to claim 1, further comprising a delay circuit for sending a signal to a control electrode of the auxiliary switch element. 3. The self-excited oscillation type oscillation control circuit having a positive feedback winding electromagnetically coupled to the primary winding, wherein the oscillation control circuit is a self-oscillation type oscillation control circuit. The flyback converter according to claim 1, wherein a saturable inductor is inserted in series between the control electrodes of the switch element. 4. The self-excited oscillation type oscillation control circuit having a positive feedback winding electromagnetically coupled to the primary winding, wherein the oscillation control circuit is a self-oscillation type oscillation control circuit. Insert a saturable inductor in series between the control electrodes of the switch element,
A MOSFET is inserted in series with the first snubber capacitor, a second snubber capacitor is connected in parallel with the MOSFET, and an auxiliary winding that is electromagnetically coupled to the primary winding is wound around both ends. The MOSFET through a resistor
2. The high power factor flyback converter according to claim 1, wherein said high power factor flyback converter is connected to said gate and source.