JP3661136B2 - Switching power supply with power factor correction function - Google Patents

Description

[0001]
[Industrial application fields]
The present invention relates to a switching power supply device, and more particularly to a switching power supply device having a power factor correction function.
[0002]
[Prior art]
In a switching power supply circuit that converts alternating current to direct current and obtains a stable direct current output voltage by switching the direct current, if a capacitor input type rectifier circuit is used for the circuit that converts alternating current to direct current, the AC input voltage is sinusoidal. On the other hand, a pulsed AC input current having a high peak value flows, the conduction angle of the input current waveform becomes narrow, and the power factor decreases. A choke input type rectifier circuit in which a choke coil is inserted in front of a capacitor is known as a method for preventing the power factor from decreasing. However, since the AC frequency of the commercial power supply is low, the choke coil inductance must be large. There are drawbacks. Therefore, a circuit called an active filter has been put into practical use by a combination of a reactor and a switch element. FIG. 5 shows a principle diagram of a circuit example conventionally used in an active filter. The point of the operation of the circuit of this figure is that a current is forced to flow even in a low input voltage section in which a non-conduction angle is present in the capacitor input type rectifier circuit. In the figure, the capacitor 102 has a relatively large capacity, and the output voltage of the bridge rectifier 118 repeats a half-cycle waveform of a sine wave, whereas it can be regarded as being charged with a substantially constant DC voltage. . Therefore, when the output voltage of the bridge rectifier 118 is lower than the charging voltage of the capacitor 102, the switching current flowing through the primary winding 111A and the switch element 113 of the transformer 111 is covered by the charge charged in the capacitor 102. During the ON period of the switch element 113, the voltage of the capacitor 102 is applied to the primary winding 111A of the transformer 111, and a voltage corresponding to the turn ratio is generated in the auxiliary winding 111C. The polarity of the winding is selected so that the voltage on the auxiliary winding 111C is positive, and the value obtained by adding the output voltage of the bridge rectifier 118 and the voltage of the auxiliary winding 111C is the capacitor 102. The output current of the bridge rectifier 118 flows. That is, an AC input current flows even in a section where the output voltage of the bridge rectifier 118 is lower than the charging pressure of the capacitor 102, whereby a current flows even in a section where the capacitor input type rectifier circuit has a non-conduction angle. As a result, the power factor is improved.
[0003]
[Problems to be solved by the invention]
In the circuit shown in FIG. 5, in order for the output current of the bridge rectifier 118 to flow from a lower section of the output voltage of the bridge rectifier 118, the number of turns of the auxiliary winding 111C must be increased. As the number of turns of 111C increases, the voltage charged in the capacitor 102 increases. As a result, the voltage applied to the switch element 113 during the OFF period of the switch element 113 also increases, and the withstand voltage of a product used in a general switching power supply device is insufficient.
[0004]
Accordingly, an object of the present invention is to provide a circuit in which a current flows from a lower section of the output voltage of the bridge rectifier while preventing the voltage applied to the switch element from becoming higher than that in the case of a general switching power supply.
[0005]
[Means for Solving the Problems]
In order to achieve the above object, according to the present invention, a reactor is connected in series to a primary winding of a transformer, and a voltage generated at a switch element side terminal of a series circuit including the primary winding and the reactor is charged. A diode and a capacitor are added, an auxiliary winding is added to the transformer, and the electric charge charged in the capacitor is configured to flow through the auxiliary winding and discharge through the auxiliary element during the ON period of the switching element, and the switching element In order to prevent a flyback current from flowing through the auxiliary winding during the off period, a second diode for blocking current in this direction is added.
[0006]
[Action]
In the present invention, a capacitor having a certain large capacity is selected, and after a while after the power supply is started, the voltage at both ends thereof is stable even with respect to the input voltage that changes with the cycle of the commercial power supply.
[0007]
When the input voltage is sufficiently lower than the voltage charged in this capacitor, the current flowing through the switch element is mainly provided by the charge charged in the capacitor, but it flows directly to the switch element through the first diode. Since it is blocked, the transformer is excited through the auxiliary winding.
[0008]
In the primary winding, a positive voltage is generated on the side to which the switch element is connected. When the value obtained by adding the input voltage to the voltage exceeds the voltage of the capacitor, a current flows in the primary winding. Excitation current flows through a reactor connected in series with the next winding. That is, the input current flows even if the input voltage is lower than the capacitor voltage.
[0009]
When the switch element is turned off, a flyback voltage proportional to the number of turns is generated in each winding, but a reactor connected in series with the primary winding is generated in the primary winding in order to release excitation energy. A voltage obtained by subtracting the input voltage from the value obtained by adding the capacitor voltage to the flyback voltage is generated, and the flywheel current is supplied. This flywheel current flows even if the input voltage is lower than the capacitor voltage.
[0010]
The exciting current and flywheel current flowing through the reactor shown above are both part of the input current, and this causes the input current to flow even in the section where the capacitor input type rectifier circuit has a non-conduction angle, improving the power factor. Is done.
[0011]
The voltage charged in the capacitor is approximately equal to the peak value of the input voltage, but the high voltage transiently generated by the primary winding and the reactor is also clamped by the capacitor voltage. On the other hand, the voltage applied to the switch element by the flyback voltage generated in the auxiliary winding is the same as that of a general switching power supply, and therefore the breakdown voltage of the switch element may be the same as that used for a general switching power supply.
[0012]
【Example】
FIG. 1 is a circuit diagram showing a switching power supply with a power factor correction function according to an embodiment of the present invention. FIG. 2 is a waveform diagram showing voltage-current waveforms in the main part of the circuit diagram of FIG. 3 and 4 are enlarged views of the time axis of the waveform diagram of FIG.
[0013]
In the circuit of FIG. 1, the output voltage of the bridge rectifier 18 has a waveform in which a half cycle of a sine wave is repeated as shown in FIG. As shown in FIG. 2B, the output current of the bridge rectifier 18 is pulsed, and the current starts to flow from a low voltage, and the peak value of the pulse increases as the voltage increases. Further, when this current passes through a low-pass filter comprising a capacitor 19 and a choke coil 20, a waveform closer to a sine wave is obtained as shown in FIG.
[0014]
The time axis of the waveforms of the current flowing through the bridge rectifier 18, the current flowing through the switch element 13, and the current flowing through the secondary diode 14 in the low output voltage section of the bridge rectifier 18 in FIG. It is shown in FIG. 3 (a), FIG. 3 (b) and FIG. 3 (c), respectively. As shown in FIG. 3A, the current of the bridge rectifier 18 rises while the current flows through the switch element 13, and falls from when the switch element 13 is turned off. The current flowing through the bridge rectifier 18 has a pulse width that increases as the output voltage of the bridge rectifier 18 increases as shown in FIG. On the other hand, the pulse widths of the current of the switch element 13 and the current of the secondary diode 14 are substantially constant as shown in FIGS. 3B and 3C. When the width of the portion where the output current of the bridge rectifier 18 decreases reaches the next turn-on time, the current becomes continuous as shown in FIG.
[0015]
4A is an enlarged view of the waveforms of the current flowing through the bridge rectifier 18, the current flowing through the switch element 13, and the current flowing through the secondary diode 14 when the output voltage of the bridge rectifier 18 shown in FIG. A), FIG. 4B, and FIG. As shown in FIG. 4A, a continuous current flows through the bridge rectifier 18.
[0016]
The voltage of the capacitor 2 is stable against changes in the output voltage of the bridge rectifier 18, and the pulse width of the switch element 2 is also stable. Therefore, the switch element 13 operates in the same manner as that of the capacitor input type rectification switching power supply circuit. Therefore, the conventional method used for the capacitor input rectification type switching power supply can be used as it is for the oscillation control circuit for controlling on / off of the switch element 13.
[0017]
As an embodiment of the invention described in claim 1, the circuit diagram shown in FIG. 1 and the waveform diagrams shown in FIGS. 2, 3, and 4 are based on a flyback converter controlled at a fixed frequency. The present invention can be applied regardless of whether the oscillation system is a separately excited type or a self-excited type. Further, the circuit configuration can be applied to a forward converter.
[0019]
【The invention's effect】
As described above, according to the present invention, it is possible to configure a circuit that improves the power factor without significantly changing the conventional capacitor input type rectification system components. In addition, the problem that the voltage applied to the switch element is higher than that of a general switching power supply can be solved.
[Brief description of the drawings]
FIG. 1 is a circuit diagram showing a switching power supply with a power factor correction function according to an embodiment of the present invention;
FIG. 2 is a waveform diagram showing voltage and current waveforms in the main part of the circuit diagram of FIG. 1;
FIG. 3 is a waveform diagram in which the time axis of the waveform diagram of FIG. 2 is enlarged.
4 is a waveform diagram in which the time axis of the waveform diagram of FIG. 2 is enlarged. FIG.
FIG. 5 is a circuit diagram showing an example of a conventional system.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Capacitor 2 Diode 3 Diode 4 Auxiliary winding 5, 105 Reactor 11, 111 Transformer 13, 113 Switch element 14, 114 Diode 15, 115 Capacitor 16, 116 Load 17, 117 Oscillation control circuit 18, 118 Bridge rectifier 19, 119 Capacitor 20, 120 Choke coils 21, 121 AC power supply 11A, 111A Primary winding 11B, 111B Secondary winding 102 Capacitor 111C Auxiliary winding

Claims (1)

A transformer having a primary winding and a secondary winding; a switch element connected in series to the primary winding of the transformer; a rectifying / smoothing circuit connected to the secondary winding of the transformer; In a switching power supply circuit including an oscillation control circuit that detects a DC output voltage of the circuit and controls oscillation of the switch element, a reactor is inserted in series with the primary winding of the transformer, and the primary winding of the transformer A capacitor that charges a voltage generated at a terminal on the switch element side of the series circuit including the reactor is connected, and the charge charged in the capacitor is discharged directly through the switch element during the ON period of the switch element. In order to prevent the above, a first diode is inserted in series between the capacitor and the series circuit composed of the primary winding of the transformer and the reactor, and the capacitor A second diode is inserted in series between the switch element and a series circuit composed of a primary winding of the lance and the reactor, and an auxiliary winding is wound around the transformer and one terminal thereof is connected to the first diode. A switching power supply with a power factor improving function, wherein the switching power supply is connected to a connection point of the capacitor and another terminal is connected to a connection point of the second diode and the switch element.

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