JPH1014232A - Switching power source - Google Patents

Abstract

PROBLEM TO BE SOLVED: To perform improvement in power factor, reduction in switching loss, and prevention of its overvoltage, by the addition of few parts, in a monolithic switching power source. SOLUTION: A transformer 4 is provided with a second secondary winding NS2, and a part of the energy of the transformer energized in the switch on period is returned from the secondary winding NS2 to the smoothing capacitor 10, being the primary power source of a transformer, through a capacitor and a line AC power source 1. Then, during the next switch-on period, the capacitor is charged and reset reversely. In the next switch-off period, when the voltage across the secondary winding NS2 reaches the voltage of the smoothing capacitor 10, the current of the secondary winding NS2 is returned directly to the smoothing capacitor 10, through a rectifier 3.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention has as its main object to improve the power factor of a switching power supply and to reduce switching loss.
The present invention relates to a method for preventing overvoltage of a switch during a switch-off period.

[0002]

2. Description of the Related Art There are various power factor improving methods.
A system in which an active filter, for example, a booster composed of a reactor, a capacitor, and a switch is provided before a switching power supply that has not been subjected to power factor improvement has a high power factor, but generally has two converters, is expensive, and has a large size.

A system in which a reactor is provided before a switching power supply has a simple configuration, but is heavy. One-converter flyback type shown in FIG. 6 (for example, Nikkan Kogyo New Page (2) Monsha Electronics Inc., 1994 Vol. 36 No. 3)
P51-56) has a simple configuration and a good power factor, but because there is no smoothing capacitor on the primary side, the
Double ripples occur.

There are other examples of simple power factor improving circuits (for example, refer to the above-mentioned literature), but many of them have a high cost such as providing a reactor for improving the power factor or using a two-switch type.

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems of the prior art, and has as its object to improve the power factor by adding a small number of components. Another object of the present invention is to provide a switching power supply capable of reducing switching loss and preventing overvoltage of a switch.

[0006]

In order to achieve the above object, the present invention provides a transformer having a second secondary winding NS2,
During the switch-off period, part of the energy of the transformer that is energized during the switch-on period is returned from the secondary winding NS2 to the smoothing capacitor CR that is the primary power source of the transformer via the capacitor C and the line AC power supply. Let it. The secondary winding NS2 and the line AC power supply are connected so that the energy of the transformer energizes the line AC power supply.

During the switch-on period, the capacitor C is charged and reset in the reverse direction.

In the switch-off period, the secondary winding NS
When the voltage across the second terminal 2 reaches the voltage of the smoothing capacitor CR, the current of the secondary winding NS2 may flow directly to the smoothing capacitor CR via the rectifier.

[0009]

[Action]

Page (3) The load on the secondary winding NS2 is a series circuit of a capacitor C, a line AC power supply, and a smoothing capacitor CR.

When the switch is turned off, the secondary winding NS
In order to maintain the ampere-turn of the transformer immediately before the turn-off, the voltage and the current of No. 2 change to a voltage value equal to the sum of the voltages of the series circuit, and the capacitor C and the smoothing capacitor CR are changed by the current corresponding to the ampere-turn. Start charging. That is, the energy of the transformer is returned to the smoothing capacitor CR via the line AC power supply.

When the voltage of the line AC power supply is
Even in a region lower than the voltage of R, the voltage of the secondary winding NS2 is applied to this, and a line current flows.

The voltage of the secondary winding NS2 immediately after the turn-off is the voltage of the line AC power supply, the reset voltage during the switch-on period of the capacitor C, and the smoothing capacitor CR.
Voltage. Thereafter, it changes at the resonance frequency due to the inductance of the capacitor C and the transformer. The switch voltage is regulated by a voltage change of the secondary winding NS2.
When the reset voltage of the capacitor C is equal to the voltage of the smoothing capacitor CR, the turn-off when the voltage of the line AC power supply is near the voltage of the smoothing capacitor CR is close to zero voltage switching.

During the switch-off period, the current of the secondary winding NS2 of the transformer is supplied to the smoothing capacitor C through the rectifier.
When the current is directly returned to R, the current flowing through the line AC power supply disappears thereafter. Since the voltage of the secondary winding NS2 is clamped to the voltage of the smoothing capacitor CR, the switch voltage is also clamped.

[0014]

FIG. 1 shows a main part of an embodiment in which a power factor improving circuit of the present invention is added to a flyback type converter. FIG. 2 is a schematic diagram of the operation waveform.

Page (4) The smoothing capacitor 10 (corresponding to the smoothing capacitor CR) is a DC power supply for the transformer 4 and includes rectifiers 3 and 5.
Is charged from the line AC power supply 1 via the.

The switch 7 is an electronic switch such as a transistor, and is self-excited or separately excited, for example, with a frequency of 100 kHz.
Turn on and off with z. The output power is output from the secondary winding NS of the transformer 4 to the load 9 via the rectifier 8 during the off period of the switch 7. The duty of the switch 7 is controlled so that the voltage of the load 9 becomes constant, for example. Filter 2 is a low-pass filter. Secondary winding NS and second secondary winding NS2
As described later, the turns ratio is selected so that the secondary winding NS conducts first during the off period of the switch 7.

At time T0, when the switch 7 is turned on, the primary winding N1 of the transformer 4 is energized by the smoothing capacitor 10 so that the primary current I1 flows, and the capacitor 11 (corresponding to the capacitor C) turns on the secondary winding. It is charged to the polarity shown by the line NS2 via the rectifier 5. V in FIG. 2 is the voltage of the capacitor 11.

Immediately after the switch 7 is turned off at time T1, the voltage of the secondary winding NS2 changes to a value such that the sum of the voltage and the voltage of the capacitor 11 and the line AC power supply 1 becomes equal to the voltage of the smoothing capacitor 10. , A current IF flows through the secondary winding NS2, and is returned to the smoothing capacitor 10 via the capacitor 11, the rectifier 3, the filter 2, and the line AC power supply 1. During that time, the rectifier 5 is reverse biased.

The reverse polarity of the capacitor 11 advances, the voltage polarity of each winding of the transformer reverses, and at time T2, the secondary winding NS
Is higher than that of the load 9, the secondary winding NS becomes conductive and the secondary current I2 flows through the rectifier 8.

When the reverse charging of the capacitor 11 further proceeds and the voltage of the secondary winding NS2 reaches the voltage of the smoothing capacitor 10 at time T3, the current of the secondary winding NS2 is directly passed through the rectifier 6 to the smoothing capacitor. Refluxed to 10. At this point, the voltage VC across the switch 7 Page (5) E becomes maximum. The return current IF via the rectifier 3, the filter 2, and the line AC power supply 1 disappears.

At time T4, the magnetic flux of the transformer 4 is reset, and at this time, the voltage of the capacitor 11 is inverted. Time T
One cycle ends at 5. In the region where the voltage of the line AC power supply 1 is large, when the voltage of the capacitor 11 is inverted at the time T4 and the sum of the voltage and the voltage of the line AC power supply 1 exceeds the voltage of the smoothing capacitor 10, the excess amount is output from the capacitor 11. Discharged to the smoothing capacitor 10.

The power factor improving circuit of FIG. 1 can be applied to a feed-forward type converter.

FIG. 3 shows a main part of another embodiment in which the power factor improving circuit of the present invention is added to a half bridge type converter. FIG. 4 is a schematic diagram of the operation waveform.

The smoothing capacitors 101 and 102 (each corresponding to the smoothing capacitor CR) are connected to a transformer 40.
And is charged from the line AC power supply 1 via the rectifiers 3, 51, 52.

When the switches 71 and 72 are turned on and off alternately, an alternating current flows through the primary winding N11 of the transformer 40,
The output power is output to the load 90 via the secondary winding NSP.

The capacitors 111 and 112 respectively correspond to the capacitor C, and operate alternately in a manner similar to the capacitor 11 of FIG. 1 according to the current direction of the primary winding N11 of the transformer 40. Now, the capacitors 111, 1
12 is charged to the polarity shown in FIG. FIG.
V1 and V2 are voltages of the capacitors 111 and 112, respectively.

At time T0, when the switch 71 is turned on,
The primary winding N11 of the transformer 40 is energized by the flat-page (6) smoothing capacitor 101, and the output power is output to the load 90 via the secondary winding NSP. Capacitor 11
2 is reset to the voltage of the secondary winding NS2 via the rectifier 52. The capacitor 111 is blocked by the rectifier 51 so that no current flows and its voltage does not change.

At time T1, when the switch 71 is turned off, the voltage of the secondary winding NS2 changes, and the return current IF to the smoothing capacitors 101 and 102 is changed to the secondary winding NS2.
Flows through the rectifier 51, the rectifier 3, the line AC power supply 1, the filter 2, and the capacitor 112. Rectifier 5
No. 2 is reverse biased and no current flows. If rectifier 5
If 1 is reverse biased, the voltage on the secondary winding NS2 will change until its conduction is restored. In addition, capacitor 1
When the series sum of the voltages of the lines 11 and 112 and the line AC power supply 1 exceeds the series sum of the voltages of the smoothing capacitors 101 and 102, the capacitors 111 and 112 discharge and the rectifier 51
And the return current IF flows.

When the reverse charging of the capacitor 112 proceeds, the voltage polarity of each winding of the transformer is reversed, and the rectifier 62 conducts at time T2, the residual energy of the transformer 40 is reduced to the primary winding N.
11, the smoothing capacitor 102 directly via the rectifier 62
Refluxed to Return current IF from secondary winding NS2 to smoothing capacitors 101 and 102 via line AC power supply 1
Disappears. The capacitor 111 is charged to the illustrated polarity via the rectifier 51, and its voltage V1 becomes maximum. The voltage VCE1 across the switch 71 also becomes maximum.

At time T3, the magnetic flux of the transformer 40 is reset. At this time, depending on the turn ratio between the primary winding N11 and the secondary winding NS2 and the magnitude of the voltage of the line AC power supply 1 at that time, the capacitor 112 is connected to the secondary winding NS2.
And the voltage is inverted. At this time, when the series sum of the voltages of the capacitors 111 and 112 and the voltage of the line AC power supply 1 exceeds the series sum of the voltages of the smoothing capacitors 101 and 102, the capacitors 111 and 112 are connected via the line AC power supply 1 to the smoothing capacitor. Discharge to 101 and 102. Page (7)

At time T4, the switch 72 is turned on, and the first half cycle ends and the second half cycle begins. The operation in the second half cycle T4 to T8 is symmetric with the first half cycle. For example, the operation of the capacitor 111 is the same as the operation of the capacitor 112 in the first half cycle. Time T
One cycle ends at 8.

The power factor improving circuit of FIG. 3 can be applied to a full bridge type, push-pull type converter and the like.

The configuration of the power factor correction circuit according to the present invention is not limited to the embodiment shown in FIGS. For example, FIG.
This is an embodiment in which another power factor improving circuit of the present invention is added to the same half-bridge type converter as in FIG. Except for having two second secondary windings NS21 and NS22, the power factor correction circuit operates in a manner similar to that of FIGS. The line AC power supply may be single-phase or three-phase, and the rectification method may be full-wave or half-wave.

[0034]

Since the present invention is configured as described above, the following effects can be obtained.

In one cycle of the line AC power supply, even when the power supply voltage is lower than the voltage of the smoothing capacitor CR,
The voltage of the secondary winding NS2 is added to the power supply voltage, and a line current flows. Therefore, the line current flows in a wide range of the AC power supply cycle, and the power factor is improved.

Due to the clamping action of the capacitor C, the rise of the switch voltage immediately after the turn-off is moderate, and the turn-off loss of the switch is small. In particular, when the reset voltage of the capacitor C is equal to the voltage of the smoothing capacitor CR, the turn-off when the voltage of the line AC power supply is near the voltage of the smoothing capacitor CR is close to zero voltage switching. Page (8)

In the switching power supply according to the second aspect,
As a result of the voltage of the secondary winding NS2 being clamped during the switch-off period, the voltage of the switch is also clamped and protected from overvoltage.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of a main part of a first embodiment of the present invention.

FIG. 2 is an operation waveform schematic diagram of the circuit of FIG. 1;

FIG. 3 is a circuit diagram of a second embodiment of the present invention.

FIG. 4 is an operation waveform schematic diagram of the circuit of FIG. 3;

FIG. 5 is a circuit diagram of another embodiment of the present invention relating to FIG. 3;

FIG. 6 is a circuit diagram of a main part of a conventional example.

[Explanation of symbols]

 N1, N11 Primary winding NS, NSP, NS2, NS21, NS22 Secondary winding 1 AC power supply 2 Filter 3, 5, 6, 8, 51, 52, 61, 62 Rectifier 4, 40 Transformer 7, 71, 72 Switch 9, 90 Load 10, 101, 102 Smoothing capacitor 11, 111, 112 capacitor VCE switch 7 voltage waveform Page (9) VCE1 switch 71 voltage waveform VCE2 switch 72 voltage waveform V capacitor 11 voltage waveform V1 capacitor 111 voltage Waveform V2 Voltage waveform of capacitor 112

Claims (2)

[Claims] 1. A switching power supply using a smoothing capacitor CR to which DC power is supplied from a line AC power supply via a rectifier as a primary side power supply of a transformer, the energy of the transformer energized from the smoothing capacitor CR during a switch-on period. During the switch-off period, a part of the current is returned to the smoothing capacitor CR from the second secondary winding NS2 of the transformer via the capacitor C and the line AC power so as to energize the line AC power, and the switch is turned on. A switching power supply having a power factor improving circuit for charging and resetting the capacitor C in the opposite direction during the period. 2. During the switch-off period, the secondary winding N
2. The method according to claim 1, wherein when the voltage across S2 reaches the voltage of the smoothing capacitor CR, the current of the secondary winding NS2 of the transformer is directly returned to the smoothing capacitor CR via a rectifier.
The described switching power supply.