JPH08154381A - Power factor improvement converter - Google Patents

Abstract

PURPOSE: To prevent the efficiency of a power factor improvement converter from being lowered even when the self-inductance of a reactor is large by a method wherein surplus energy stored in the reactor is output to a secondary winding of a transformer through a second switching element. CONSTITUTION: When a first FET 21 is OFF and a second FET 22 is ON, energy stored in a reactor 5 and a primary winding 9a of a transformer 9 is output to a load 12 via a secondary winding 9b of the transformer 9. Then, surplus energy stored in the reactor 5 is supplied to a second capacitor 23, and it is discharged to the secondary winding 9b of the transformer 9 through the second FET 22. As a result, even when the self-inductance of the reactor 5 is large, a period in which a current flowing in the reactor becomes zero can be ensured. As a result, a switching loss is reduced as compared with conventional examples, and a converter can be operated with high efficiency.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power factor correction converter, and more particularly to a power factor correction converter for improving a power factor by approximating an input current waveform to a rectifier circuit to an input voltage waveform.

[0002]

2. Description of the Related Art FIG. 4 shows a conventional power factor correction converter disclosed in Japanese Patent Laid-Open No. 3-207268. This power factor correction converter includes a rectifier circuit 4 connected to an AC power supply 1 via a power switch 2 and an AC filter 3. The AC filter 3 includes a pair of reactors 3a and 3b and a small-capacity capacitor 3c. A series circuit including a reactor 5 and a transistor 6 as a switching element is connected to the output terminals 4a and 4b of the rectifier circuit 4, and a control circuit 7 for controlling ON / OFF of the transistor 6 is connected to the base of the transistor 6. A series circuit composed of a primary winding 9a of a transformer 9 and a smoothing capacitor 10 is connected to the emitter and collector of the transistor 6.
A diode 8a is provided on both ends of the primary winding 9a of the transformer 9,
The magnetic reset circuit 8 of the transformer 9 including the capacitor 8b and the resistor 8c is connected. Secondary winding 9 of transformer 9
b is connected to the load 12 via the output rectifying / smoothing circuit 11. The one end 9c of the secondary winding 9b of the transformer 9 is connected to the transformer 9 via the diode 11a and the smoothing reactor 11d.
The other end 9d of the secondary winding 9b of the transformer 9 is connected to one end 12a of the load 12 via the diode 11b.
The center tap 9e of the secondary winding 9b of the
2b is connected. The feedback diode 11c is connected between the input end 11f of the smoothing reactor 11d and the other end 12b of the load 12, and the output smoothing capacitor 11e is connected to both ends of the load 12.

When the power switch 2 is turned on, power is supplied from the AC power supply 1 and the power factor correction converter starts operating. The smoothing capacitor 10 is discharged when the transistor 6 is on, and charged when the transistor 6 is off. As a result, energy is given to the output rectifying / smoothing circuit 11 via the transformer 9, and the stabilized DC output is supplied to the load 12. As shown in FIG. 5, the on / off cycle of the transistor 6 is set to the input voltage e from the AC power supply 1.
The input current i flows intermittently by setting the cycle shorter than the cycle (1/4 or less). The maximum value of the amplitude of the input current i flowing intermittently depends on the input voltage e, and if the input voltage e is a sine wave, the change in the maximum value of each input current i also follows the sine wave. Therefore, the input current waveform is approximated to the input voltage waveform, and the power factor is improved.

The AC filter 3 turns on the transistor 6
The current interrupted by the off operation is smoothed into an approximate sine wave. The magnetic reset circuit 8 magnetically resets the transformer 9 after the current flowing through the reactor 5 becomes zero. Further, the output rectifying / smoothing circuit 11 promotes smooth supply of output to the load 12.

[0005]

In the above-mentioned conventional example, the reactor 5 stores the current flowing through the reactor 5 as magnetic energy in the ON period of the transistor 6 (T _{1 in} FIG. 5) and in the OFF period of the transistor 6 (see FIG. 5 is discharged as a current again at T ₂ + T ₃ ). In FIG. 5, after the current flows in the period T ₂ , a period in which no current flows is set as the period T ₃ . This is because when the current is continuous, the current waveform is not a sine wave and the power factor is reduced, so that it is necessary for each current to be intermittent. In order to secure the period T ₃ , allow a margin before the transistor 6 is turned on and the reactor 5 is turned on.
It is desirable that the current flowing through the line be zero. However, for that purpose, the self-inductance of the reactor 5 must be reduced, and as a result, the peak of the current flowing through the transistor 6 becomes large, and switching loss increases.

In addition, in the circuit of FIG.
The duty ratio of, that is, the ratio of the [ON period] and the [ON period + OFF period] determines the ratio between the voltage of the capacitor 10 and the output voltage supplied to the load 12. Further, the length of the ON period of the transistor 6 determines the magnitude of the voltage of the capacitor 10. Therefore, to obtain the desired value of output voltage,
It is necessary to control both the frequency and the duty ratio of the control signal given from the control circuit 7. Therefore, the setting of the control signal becomes complicated, and a high-performance control IC must be used for the control circuit 7, which tends to increase the cost.

Therefore, an object of the present invention is to provide a power factor correction converter in which the efficiency is not reduced even when the self-inductance of the reactor is large and the control signal can be set easily.

[0008]

A power factor correction converter according to the present invention comprises a rectifier circuit connected to an AC power source, and a series circuit composed of a reactor and a first switching element connected to an output terminal of the rectifier circuit. A transformer including a control circuit for controlling ON / OFF of the first switching element and a series circuit including a primary winding of a transformer and a first capacitor connected to both ends of the first switching element, A stabilized DC output is generated from the secondary winding through the output rectifying and smoothing circuit. A series circuit including a second switching element and a second capacitor is connected to both ends of the primary winding of the transformer, a diode is connected in parallel with the second switching element, and the first circuit is connected by the control circuit. The second switching element is controlled to be turned on / off in a phase opposite to that of the switching element.

[0009]

When the first switching element is in the ON state, the second
The switching element of is in the off state. At this time, the energy supplied from the rectifier circuit is accumulated in the reactor, and the energy charged in the first capacitor is accumulated in the primary winding of the transformer. When the first switching element is turned off, the second switching element is turned on, and the energy accumulated in the reactor and the primary winding of the transformer is output to the secondary winding of the transformer. At this time, a part of the energy accumulated in the leakage inductance of the transformer and the energy of the reactor is supplied to the second capacitor through the diode, and the energy of the second capacitor passes through the second switching element and the transformer. Is output from the primary winding to the secondary winding. That is, since the excess energy stored in the reactor is output to the secondary winding of the transformer through the second switching element, the efficiency of the power factor correction converter does not decrease even if the reactor has a large self-inductance.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT An embodiment of a power factor correction converter according to the present invention will be described below with reference to FIGS. In FIGS. 1 and 3, the same parts as those shown in FIG. 4 are designated by the same reference numerals, and the description thereof will be omitted.

As shown in FIG. 1, a power factor correction converter according to the present invention includes a rectifier circuit 4 connected to an AC power source 1 and a reactor 5 connected to output terminals 4a and 4b of the rectifier circuit 4.
And a first FET 21 as a first switching element
And a control circuit 20 for ON / OFF controlling the first FET 21, and a series circuit including the primary winding 9a of the transformer 9 and the first capacitor 10 connected to both ends of the first FET 21. Equipped with a secondary winding 9 of the transformer
A stabilized DC output is generated from b through the output rectifying / smoothing circuit 26. The output rectifying / smoothing circuit 26 is a diode 26.
a and a capacitor 26b. A series circuit composed of a second FET 22 and a second capacitor 23 as a second switching element is connected to both ends of the primary winding 9a of the transformer 9, and a diode 25 is connected in parallel with the second FET 22. The second FET 22 is on / off controlled by the control circuit 20 in a relationship with the first FET 21 in the opposite phase. The resistor 24 is connected to the source terminal of the second FET 22.

The control circuit 20 includes a first FET 21 and a second FET 21.
Is connected to each gate terminal of the FET 22 and both ends of the load 12. The control circuit 20 detects the voltage output to the load 12, and the detected output voltage approaches a predetermined value.
The first FET 21 and the second FET 22 are on / off controlled in a phase relationship opposite to each other. That is, as shown in FIG.
When the first FET 21 is on, the second FET is off, and when the first FET 21 is off, the second FE
T is controlled to the ON state.

When the first FET 21 is in the ON state, the second FE
When T is in the off state, the energy supplied from the rectifier circuit 4 is stored in the reactor 5, the power of the first capacitor 10 is discharged, and the energy is stored in the primary winding 9a of the transformer 9. Next, when the first FET 21 is turned off and the second FET 22 is turned on, the energy stored in the reactor 5 and the primary winding 9a of the transformer 9 is loaded through the secondary winding 9b of the transformer 9. 12 is output. At this time, a part of the energy accumulated in the leakage inductance of the transformer 9 and the energy of the reactor 5 is supplied to the second capacitor 23 through the diode 25. When the energy stored in the reactor 5 and the primary winding 9a of the transformer 9 becomes zero, the second FET 22 is turned on, the diode 25 is reverse biased, and the primary winding 9a of the transformer 9 is excited in the reverse direction. A current starts to flow and is output to the load 12 via the secondary winding 9b. At this time, a voltage drop occurs in the resistor 24. Resistance 24
When the voltage drop of 2 becomes above a certain level, the first FET 21 is turned on again and the second FET 22 is turned off.

As described above, the excess energy accumulated in the reactor 5 is supplied to the second capacitor 23 and also discharged to the secondary winding 9b of the transformer 9 through the second FET 22. Even when the self-inductance of the reactor is large, a period (T _{3 in} FIG. 2) in which the current flowing through the reactor 5 becomes zero can be secured. As a result, the switching loss is reduced as compared with the conventional example described above, and the converter operates efficiently.

The present invention is not limited to the above embodiments, but various modifications can be made. For example, as shown in FIG. 3, a diode 27 may be provided between the reactor 5 and the first FET 21. In the case where the circuit of FIG. 3 is operated, when the first FET 21 is off and the second FET 22 is on, the energy stored in the leakage inductance of the transformer 9 is reverse biased by the diode 27. . Therefore, unlike the case of the circuit of FIG. 1, the energy accumulated in the leakage inductance of the transformer 9 is not supplied to the second capacitor 23 through the diode 25.

[0016]

According to the present invention, the excess energy stored in the reactor is supplied to the second capacitor and is also discharged to the secondary winding of the transformer through the second switching element. Even if the self-inductance is large, the efficiency of the power factor correction converter does not decrease.
Further, since it is possible to operate with a simple control circuit without the need to set complicated control signals, it is possible to reduce the cost of the entire device.

[Brief description of drawings]

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

FIG. 2 is a time chart showing an input voltage, an input current, and a control signal in the circuit of FIG.

FIG. 3 is an electric circuit diagram showing an embodiment of another power factor correction converter according to the present invention.

FIG. 4 is an electric circuit diagram showing a conventional power factor correction converter.

5 is a time chart showing an input voltage, an input current, and a control signal in the circuit of FIG.

[Explanation of symbols]

1 ... AC power supply, 4 ... Rectifier circuit, 4a, 4b ...
・ Output terminals of rectifier circuit, 5 ... Reactor, 9 ...
Transformer, 9a ... Primary winding of transformer, 9b ...
Secondary winding of transformer, 10 ... First capacitor, 2
0 ... Control circuit, 21 ... First FET (first switching element), 22 ... Second FET (second switching element), 23 ... Second capacitor, 25
... Diode, 26 ... Output rectifying and smoothing circuit

Claims (1)

[Claims] 1. A rectifier circuit connected to an AC power supply, a series circuit composed of a reactor and a first switching element connected to an output terminal of the rectifier circuit, and on / off control of the first switching element. 1 of a control circuit and a transformer connected to both ends of the first switching element
A power factor correction converter for generating a stabilized direct current output from a secondary winding of the transformer via an output rectifying / smoothing circuit, the second switching element comprising a secondary winding and a series circuit including a first capacitor; And a second circuit connected to both ends of the primary winding of the transformer, a diode connected in parallel with the second switching element, and a phase opposite to the first switching element by the control circuit. The power factor correction converter is characterized in that the second switching element is controlled to be turned on / off in accordance with the above relation.