JPH0530740A - Switching power supply - Google Patents

Abstract

PURPOSE:To make it possible to suppress overshoot of output voltage at the time of starting. CONSTITUTION:A control winding 2d is additionally provided on the primary of a transformer 2 and the output of the control winding 2d is fed through a capacitor 28 and a resistor 30 to the base of a control transistor 12 which controls a switching transistor 4.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a self-excited switching power supply device, and more particularly, to a means for suppressing overshoot of an output voltage at the time of startup thereof.

[0002]

2. Description of the Related Art A conventional example of this type of switching power supply device is shown in FIG.

This switching power supply device is an example of a self-excited and flyback type (that is, RCC type).
A switching transistor (switching element) 4 is connected in series to the primary winding 2a of the transformer 2, and the output of the base winding 2c of the transformer 2 is fed back to the base of the switching transistor 4 via a capacitor 6 and a resistor 8 to oscillate. Is configured to let. Reference numeral 10 is a starting resistance. Then, when the switching transistor 4 is turned off, the stored energy in the transformer 2 is taken out as a DC output (for example, a high voltage output) from the secondary winding 2b via the rectifying / smoothing circuit 14.

The constant voltage control of the output voltage V ₂ is performed by the feedback circuit 16 and the control transistor (control element) 12
Is performed using. That is, the feedback circuit 16
It is provided with voltage dividing resistors 18 and 20 for dividing the output voltage V ₂ , a reference voltage source 22, an operational amplifier 24 for obtaining the difference between the inputs, and an insulating photocoupler 26. When the output voltage V ₂ rises above the reference voltage, The output of the operational amplifier 24 increases, and this is given to the base of the control transistor 12 via the photocoupler 26, and the collector current of the control transistor 12 increases. As a result, the base current of the switching transistor 4 is shunted and the switching transistor 4 works in the OFF direction, and as a result, the rise of the output voltage V ₂ is suppressed.

[0005]

However, in the above switching power supply device, the generation of the secondary voltage, the rectifying / smoothing circuit 14, and the voltage dividing resistor 1 in the feedback circuit 16 are used.
8 and 20, the operational amplifier 24, the photocoupler 26, and the like delay the control transistor 1 at the time of startup.
There is a problem in that the control by 2 causes a delay, which causes an overshoot in the output voltage V ₂ as shown in FIG. 3, for example.

By the way, if such an overshoot occurs, the components constituting the rectifying / smoothing circuit 14 and the like are adversely affected in terms of withstand voltage, so that not only the reliability of the switching power supply device is deteriorated, but also the load is similarly affected. It will be adversely affected.

Therefore, the main object of the present invention is to provide a switching power supply device capable of suppressing the output voltage overshoot at the time of startup.

[0008]

In order to achieve the above object, the switching power supply device of the present invention is further provided with a control winding on the primary side of the transformer, and the output of the control winding is provided to the control electrode of the control element. It is characterized in that it is given to.

[0009]

According to the above construction, the ON period of the switching element can be controlled through the control element also by the output of the control winding. Moreover, since the voltage generated in the control winding has no time delay with respect to the voltage generated in the base winding, the above configuration controls the output voltage at the time of start-up without time delay in accordance with the growth of oscillation. As a result, overshoot of the output voltage is suppressed.

[0010]

1 is a circuit diagram showing a switching power supply device according to an embodiment of the present invention. The same or corresponding portions as those of the conventional example in FIG. 2 are denoted by the same reference numerals, and the differences from the conventional example will be mainly described below.

In this embodiment, a control winding 2d is further provided on the primary side of the transformer 2 described above, and the control winding 2d is provided.
In this example, the output of d is the capacitor 2 for adjusting the control characteristic.
8 and the resistor 30 are connected in series to the base of the above-mentioned control transistor 12. The output voltage V ₂ is fed back to the base of the control transistor 12 via the feedback circuit 16 as in the conventional example.

Since a voltage similar to that of the base winding 2c is generated in the control winding 2d, the output of the control winding 2d can control the ON period of the switching transistor 4 via the control transistor 12. Moreover, the voltage generated by the control winding 2d has no time delay with respect to the voltage generated by the base winding 2c, and a capacitor 28 and a resistor 30 are provided between the control winding 2d and the control transistor 12. Since an element that causes a large time delay as in the conventional example is not inserted, the output voltage V ₂ can be controlled at the time of starting the switching power supply device in accordance with the growth of oscillation without time delay. That is, at startup, the switching transistor 4 is driven by the voltage on the base winding 2c side to grow self-excited oscillation to raise the output voltage V _2. At the same time, the voltage on the control winding 2d side is used. By controlling the switching transistor 4 via the control transistor 12, the rise of the output voltage V ₂ can be appropriately suppressed so as not to go too far. The magnitude and timing of suppression by the voltage on the control winding 2d side can be adjusted by the values of the resistor 30 and the capacitor 28 and the like. In this way, overshoot of the output voltage V ₂ can be suppressed.

Further, by suppressing the overshoot of the output voltage V ₂ , the components constituting the rectifying / smoothing circuit 14 and the like are protected, the reliability of the switching power supply device is improved, and the load is adversely affected. Lost.

The control winding 2d and the control transistor 1
The circuit for connecting the base of 2 is not limited to the series circuit of the capacitor 28 and the resistor 30.

As the switching element, FET may be used instead of the switching transistor 4 as described above. Similarly, an FET may be used as the control element instead of the control transistor 12 as described above.

Further, the present invention is not limited to the flyback system as in the above example, but can be naturally applied to the forward system.

[0017]

As described above, according to the present invention, the output of the control winding provided on the primary side of the transformer is used to control the output voltage at the time of start-up in accordance with the growth of oscillation without time delay. Therefore, overshoot of the output voltage can be suppressed. As a result, transient overvoltage is eliminated, so that the reliability of the switching power supply device is improved and the adverse effect on the load is eliminated.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a switching power supply device according to an embodiment of the present invention.

FIG. 2 is a circuit diagram showing an example of a conventional switching power supply device.

FIG. 3 is a schematic diagram showing an example of rising of an output voltage.

[Explanation of symbols]

 2 transformer 2a primary winding 2b secondary winding 2c base winding 2d control winding 4 switching transistor 12 control transistor 16 feedback circuit

Claims (1)

Claim: What is claimed is: 1. A switching element is connected in series to a primary winding of a transformer, and an output of a base winding of the transformer is fed back to a control electrode of the switching element to oscillate. In a switching power supply device comprising a feedback circuit that feeds back the output voltage of the secondary side of the transformer to the primary side, and a control element that forcibly turns off the switching element in response to a signal from this feedback circuit, A switching power supply device characterized in that a control winding is further provided on the primary side of the transformer, and an output of the control winding is given to a control electrode of the control element.