JPH065988B2 - Self-excited switching power supply - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switching power supply, and more particularly to a self-excited switching power supply having an emitter current limiting circuit of a switching transistor.

(Prior Art) As a conventional self-exciting type switching power supply of this type, there is a circuit as shown in FIG.

In this example, a switching transformer 24 having a main primary winding 21, a base driving winding 22, and a secondary winding 23, a switching transistor 25, a base driving circuit 26,
It is composed of an emitter current limiting circuit 27 and a secondary rectifying / smoothing circuit 28, and protects against an excessive output current due to a secondary output short circuit or the like as follows.

When an excessive current flows to the secondary output, the collector current and the emitter current of the switching transistor 25 tend to increase, but the voltage across the current detection resistor 29 in the emitter current limiting circuit 27 increases and the transistor 30 turns off.
Therefore, the emitter current and the collector current are limited to specified values. Therefore, since the energy applied to the main primary winding 21 is limited, the secondary short-circuit current is also limited and protected. The limit value of the emitter current is set to a large value so that the output current during normal times is not limited.

(Problems to be solved by the invention) In the conventional self-excited switching power supply, the limit value of the emitter current remains the same during normal operation and during abnormal conditions such as output short circuit. However, the current cannot be reduced so much, and there is a drawback that the protection is not sufficient for the short-circuited portion.

An object of the present invention is to solve the above-mentioned drawbacks, and to provide a self-exciting switching power supply capable of reliably protecting a short-circuit portion at the time of abnormality by changing a limit value of an emitter current of a switching transistor at the time of normal operation and at the time of abnormality. To do.

(Means for Solving the Problems) In order to achieve the above object, the self-excited switching power supply according to the present invention connects a collector of a switching transistor to a main primary winding and connects a base drive winding through a base drive circuit. The switching transistor is connected between the base and the emitter, the emitter of the transistor is connected to a current detection resistor, and the base and the emitter of the switching transistor are connected to the collector and the base of the first transistor, respectively. A predetermined output is obtained from a secondary rectifying and smoothing circuit connected to a secondary winding by connecting an emitter of the transistor and the other end of the current detection resistor, and the first transistor is output when the output current exceeds a predetermined value. A self-exciting switch configured to turn on and limit the on-operation of the switching transistor. In the switching power supply, a Zener diode is inserted in the emitter of the first transistor, and a series circuit of a voltage dividing resistor and a capacitor is connected between both ends of the base driving winding.
The collector, the emitter and the base of the transistor are respectively connected to the emitter of the first transistor, the anode of the Zener diode and the voltage dividing point of the voltage dividing resistor.

According to the above configuration, the limit value of the emitter current of the switching transistor changes depending on the winding voltage value when the base of the switching transistor base drive winding is not driven, and the limit of the emitter current in the normal state is limited. The value is large and the limit value of the emitter current at the time of abnormality can be made small, and the object of the present invention is completely achieved.

(Example) Hereinafter, the present invention will be described in more detail with reference to the drawings.
FIG. 1 is a circuit diagram showing an embodiment of a self-excited switching power supply according to the present invention.

In this embodiment, a switching transformer 4 having a main primary winding 1, a base driving winding 2 and a secondary winding 3, a switching transistor, a base driving circuit 6, an emitter current limiting circuit 7 and a secondary winding. It is composed of a rectifying and smoothing circuit 8.

The emitter current limiting circuit 7 is configured by adding a Zener diode 16, resistors 14 and 15, a capacitor 13 and a diode to the conventional circuit.

Since the winding voltage of the base drive winding 2 when the base is not driven is sufficiently large under normal conditions, the negative voltage obtained by the diode 12 and the capacitor 13 in the emitter current limiting circuit 7 is large, and when the base drive winding 2 is driven. And the base voltage of the transistor 11 obtained by the voltage dividing resistors 14 and 15 becomes negative. Therefore, since the transistor 11 is off, the emitter current of the switching transistor 5 is limited by the current detection resistor 9, the transistor 10 and the zener diode 16. At this time, the limit value of the emitter current is set large.

In the event of an abnormality such as an output short circuit, the transistor 11 is off, and the emitter current of the switching transistor 5 is limited first. Due to this limitation, the voltage of the base drive winding when the base is not driven is lowered, so that the negative voltage across the capacitor 13 is reduced and the base voltage of the transistor 11 is increased. Therefore, the transistor 11 is turned on, and the emitter current of the switching transistor 5 is the current detection resistor 9, the V _{BE of the} transistor 10 and the transistor 1
Limited by V _{CE of} 1.

When the transistor 11 is on, the current flowing through the current detection resistor 9 turns on at a smaller value than when the transistor 11 is off, so the limit value of the emitter current is sufficiently smaller than in the normal state. Therefore, the short circuit current becomes small.

(Effect of the Invention) As described above, the present invention is configured so that the limit value of the emitter current is large in the normal state and small in the abnormal state such as the output short circuit. Therefore, the output short circuit current value can be made sufficiently small. The effect is that the short-circuited portion can be reliably protected.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of a self-excited switching power supply according to the present invention, and FIG. 2 is a circuit diagram showing an example of a conventional self-excited switching power supply. 1, 21 ... Main primary winding 2, 22 ... Base driving winding 3, 23 ... Secondary winding 4, 24 ... Switching transformer 5, 25 ... Switching transistor 6, 26 ... Base driving circuit 7, 27 ... Emitter Current limiting circuit 8, 28 ... Secondary rectifying / smoothing circuit 9, 29 ... Current detection resistor 10, 11, 30 ... Transistor 12 ... Diode 13 ... Capacitor 14, 15 ... Voltage dividing resistor 16 ... Zener diode

Claims (1)

[Claims] 1. A main primary winding is connected to a collector of a switching transistor, a base driving winding is connected between a base of the switching transistor and an emitter through a base driving circuit, and a current is detected in the emitter of the transistor. A resistor is connected, and the base and the emitter of the switching transistor are connected to the collector and the base of the first transistor, respectively, and the emitter of the first transistor and the other end of the current detection resistor are connected to each other to form a secondary winding. A secondary rectifying / smoothing circuit connected to the output of the switching transistor, and when the output current exceeds a predetermined value, the first transistor is turned on to limit the on-operation of the switching transistor. In the excitation type switching power supply, a Zener diode is inserted in the emitter of the first transistor. Both across the base drive winding, connect a series circuit of voltage dividing resistors and a capacitor, the second
A self-exciting switching power supply characterized in that a collector, an emitter and a base of a transistor are respectively connected to an emitter of the first transistor, an anode of the Zener diode and a voltage dividing point of the voltage dividing resistor.