JPH08275527A - Self-excited forward converter - Google Patents

Abstract

PURPOSE: To simplify the circuit by reducing the size of a saturable inductor used for the voltage control of a self-excited forward converter, and removing the reset-current control circuit of this saturable inductor. CONSTITUTION: A feedback control circuit 21 is connected to the base of a control transistor 18 constituting an oscillating circuit, and the oscillating period is changed. A saturable inductor 16 is inserted between a secondary winding 11C and a rectifier smoothing circuit 14. A constant delay time is imparted to a pulse outputted to the secondary winding 11C. The duty ratio of the pulse inputted into the rectifier smoothing circuit is controlled.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switching power supply device, and more particularly to an output voltage control circuit for a self-excited forward converter.

[0002]

2. Description of the Related Art Conventionally, a self-exciting forward converter oscillates on the primary side for a constant ON period as in the circuit example shown in FIG. 2, and a saturable inductor on the secondary side and a reset current of this saturable inductor are used. The output voltage was controlled by the control circuit.

[0003]

In the conventional circuit, a large saturable inductor is required because the ON period due to the oscillation on the primary side is constant.

The present invention seeks to provide a low cost circuit that achieves the same effect with a small saturable inductor.

[0005]

To achieve the above object, the self-exciting forward converter of the present invention controls an output voltage by using an oscillation cycle variable circuit instead of the conventional reset current control circuit.

[0006]

The saturable inductor inserted between the secondary winding and the rectifying / smoothing circuit is completely reset every oscillation cycle, so that the forward pulse voltage generated in the secondary winding causes a rise in pulse. High impedance is maintained for a certain period. Therefore, the rectifying / smoothing circuit is supplied with a pulse delayed by that period. If the amplitude of the forward pulse of the secondary winding is V volts, the pulse width is TON seconds, the pulse cycle is T seconds, and the delay width of the saturable inductor is TD seconds, the current smoothing circuit The output voltage can be expressed as V · (TON / T-TD / T) volts.

On the other hand, the duty ratio TON / T of the forward pulse of the secondary winding is determined by the winding number N1 of the primary winding of the transformer and the winding number NR of the reset winding of the transformer.
It can be expressed as N / T = N1 / (N1 + NR).

From the above two equations, it can be seen that the output voltage can be controlled by changing the period T even when the duty ratio of oscillation and the delay width of the saturable inductor are constant.

The self-excited forward converter of the present invention has a cycle determined by the resistance connected between the base of the control transistor and the positive feedback winding of the transformer and the charge / discharge time constant of the capacitor connected between the base and emitter of the control transistor. However, if the current supplied to the base of the control transistor is increased by the feedback control circuit, the capacitor connected between the base and emitter of the control transistor is charged faster and the oscillation cycle is shortened.

When the output voltage rises slightly, the current supplied to the base of the control transistor by the feedback control circuit increases and the oscillation cycle becomes shorter. Then, even if the duty ratio of the pulse generated in the secondary winding is constant, the duty ratio of the pulse supplied to the rectifying and smoothing circuit becomes small due to the delay of a constant width due to the saturable inductor, and the output voltage becomes Go down. In this way, the output voltage is controlled.

[0011]

1 is a circuit diagram showing an embodiment of the present invention.
The same or equivalent parts as those of the circuit of FIG. 2 showing the conventional example are designated by the same reference numerals.

In the circuit shown in FIG. 1, the forward voltage generated in the positive feedback winding 11D of the transformer 11 is a resistor 15C,
The switching transistor 12 is turned on through the circuit including the capacitor 15B and the diode 15A, and at the same time, the capacitor 20 is charged through the resistor 19. Next, the voltage of the capacitor 20 changes to the control transistor 18
When the base-emitter forward voltage is reached, the control transistor 18 turns on and the switching transistor 12 turns off.

Before the output voltage reaches the value detected by the feedback control circuit 21, the time constant of the resistor 19 and the capacitor 20 and the positive feedback winding 11 during the ON period of the switching transistor 12.
It depends on the value of the forward voltage generated in D.

The switching transistor 12 is turned
When turned off, the excitation energy of the transformer 11 is returned to the DC power supply 1 by the reset winding 11B and the diode 13. When the discharge of the excitation energy of the transformer 11 is completed, the voltage charged in the capacitor 15B is first discharged during the off period, so that the switching transistor 12 is turned on, and then the forward voltage generated in the positive feedback winding 11D is turned on. Becomes

During the off period of the switching transistor 12, the primary winding 11A and the reset winding 11B of the transformer 11 are
It is the product of the turn ratio and the ON period.

When the output voltage reaches the detection value of the feedback control circuit 21, a current flows through the photocoupler light receiving portion 21A, the charging speed of the capacitor 20 increases, and the period until the control transistor is turned on is shortened. The ON period of 12 becomes shorter.

On the other hand, since the saturable inductor 16 is reset by the current flowing through the diode 17A and the resistor 17B during the off period, the switching transistor 1
2 does not saturate until a certain period of time elapses after the forward pulse voltage is generated in the secondary winding 11C by turning on.

The time required for the saturable inductor 16 to saturate is

This corresponds to the delay width TD described in the section of "Operation". Also,
There is also a mechanism that the output voltage is controlled by constant voltage.

It is as described in the section of [Operation].

As the output current becomes smaller, the output voltage becomes larger, so that the oscillation cycle becomes shorter. Therefore, the saturable inductor 16 may be one that can obtain the delay required to control the output voltage in the minimum cycle that the feedback control circuit 21 can follow.

In the embodiment shown in FIG. 1, a bipolar transistor is used as the switching element, but a MOSFET can be used instead. Positive feedback winding 11D
From the positive feedback circuit 1 of the switching transistor base
5, resistor 15C, capacitor 15B and diode 15A
However, it is also possible to use only a capacitor or a circuit consisting of a resistor and a capacitor. Further, although the series circuit of the diode 17A and the resistor 17B is used for the reset circuit 17 of the saturable inductor 16, it is also possible to use a circuit consisting of only a capacitor or a capacitor and a resistor instead.

[0021]

As described above, according to the present invention, the size of the saturable inductor used in the self-exciting forward converter can be greatly reduced.

The reason why the self-exciting forward converter is not widely used in comparison with the ringing choke converter in which the self-exciting forward converter is the same self-exciting type is that the output voltage is as low as 5 V or 3.3 V, but the performance is excellent. Magnifier circuit complexity and saturable inductor cost. INDUSTRIAL APPLICABILITY The present invention can broaden the range of applications by eliminating the drawbacks of the conventional self-excited forward converter.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention.

FIG. 2 is a circuit diagram showing a conventional method.

[Explanation of symbols]

 1 DC power supply 2 Load 11 Transformer 12 Switching transistor 13 Diode 14 Rectification smoothing circuit 15 Positive feedback circuit 16 Saturable inductor 17 Reset circuit 18 Control transistor 19 Resistor 20 Capacitor 21 Feedback control circuit 22 Reset current control circuit 23 Resistor 14A Diode 14B Diode 14C Diode 14C Inductor 14D Capacitor 17A Diode 17B Resistor 21A Photocoupler light receiving part 21B Photocoupler light emitting part 21C Voltage detecting IC 21D Resistor 21E Resistor 22A Diode 22B Resistor 22C Op Amp 22D Reference voltage source 22E Resistor 22F Resistor

Claims (1)

[Claims] 1. A transformer having a primary winding, a reset winding, a secondary winding, and a positive feedback winding, a switching element having a control electrode connected in series to the primary winding, and the reset winding. A diode including a diode connected in series to the wire, a rectifying / smoothing circuit connected to the secondary winding, and a positive feedback circuit connected between the positive feedback winding and the control electrode of the switching element. In an exciting forward converter,
A saturable inductor is inserted in series between the secondary winding and the rectifying / smoothing circuit, a circuit that supplies a reset current to the saturable inductor is connected, and the strength of a signal applied to the control electrode of the switching element is connected. , A collector of the control transistor of which is connected to the control electrode of the switching element, a base of which is connected to one terminal of the positive feedback winding through a resistor, and an emitter of which is the other terminal of the positive feedback winding. Self-excited type, characterized in that a feedback control circuit is connected to the base of the control transistor, and a capacitor is connected between the base and emitter of the control transistor, and a current is supplied to the base of the control transistor according to an output voltage detection value. Forward converter.