JPH0690559A - Power circuit - Google Patents

Abstract

PURPOSE:To make short-circuit current of an oscillating circuit small after the stoppage of oscillation by detecting the output of a feedback winding which induces an electromotive force which changes the same as the output voltage of a transformer does and then controlling an oscillating transistor. CONSTITUTION:When the current which flows in a load 30 becomes too large, the output voltage of a secondary winding 24 of a transformer 22 drops and the output of a feedback winding 20 drops, too. Therefore, the voltage of a variable resistor 32 drops. When the voltage of the variable resistor 32 becomes a certain level or lower, a FET 38 is turned on and then current flows into a gate of a thyristor 42 and the thyristor 42 is turned on and so, an oscillation controlling transistor 44 is turned on. Therefore, the base current of an oscillation transistor 16 is dragged into the oscillation controlling transistor 44 and then an oscillating circuit 12 stops oscillation.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power supply circuit, and more particularly to, for example, a power supply circuit for converting an input from an oscillation circuit into an output according to a winding ratio by a transformer and supplying the output to a load.

[0002]

2. Description of the Related Art Referring to FIG. 3, in a conventional power supply circuit 1, a part of a current flowing through a main winding 3 of a transformer 2 and an oscillation transistor 4 is input to a base of an oscillation transistor 5 to control oscillation. The transistor 5 is turned on. Power supply circuit 1
Becomes abnormal and the base current of the oscillation control transistor 5 exceeds a certain value, it is considered that an excessive current is flowing in the load 6, and the base current of the oscillation transistor 4 is drawn into the oscillation control transistor 5 to I was trying to stop the oscillation by turning off.

[0003]

The power supply circuit 1 as described above is provided.
Then, the base current of the oscillation transistor 4 cannot be entirely drawn into the oscillation control transistor 5, and the oscillation transistor 4 slightly oscillates. Therefore, when the short circuit current Is flows in the secondary circuit of the transformer 2, the load characteristic Is as shown in FIG. Therefore, there is a problem that a large collector current continues to flow in the oscillation transistor 4 and the oscillation transistor 4 is destroyed.

Therefore, a main object of the present invention is to provide a power supply circuit in which a large short circuit current does not flow even when an abnormality occurs in the power supply circuit.

[0005]

According to the present invention, in a power supply circuit for converting an output from an oscillation circuit into an output according to a turns ratio by a transformer and supplying it to a load, a feedback winding is provided in the transformer, and A power supply circuit comprising an oscillation stopping means for stopping the oscillation of the oscillation circuit when the output of the feedback winding is below a certain level.

[0006]

When the load current becomes large, the output voltage of the transformer drops and the output of the feedback winding also drops. When the output of the feedback winding falls below a certain value, the oscillation stopping means operates and the oscillation of the oscillation circuit stops. Specifically, the oscillation stopping means includes a thyristor that is fired when the output of the feedback winding is below a certain level, and an oscillation control transistor that draws the input current of the oscillation transistor when the thyristor fires. Therefore, even if the output of the feedback winding exceeds a certain value, the oscillation stop state of the oscillator circuit is maintained.

[0007]

According to the present invention, the output of the feedback winding that induces an electromotive force corresponding to the output voltage is detected to stop the oscillation, so that the short-circuit current can be reduced as compared with the prior art. . The above-mentioned objects, other objects, features and advantages of the present invention will become more apparent from the following detailed description of the embodiments with reference to the drawings.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, a power supply circuit 10 of this embodiment.
Includes an oscillator circuit 12. In this oscillator circuit 12,
DC current is generated by the oscillation transistor 16 via the starting resistor 14.
Is input to the base of the oscillation transistor 16 and the oscillation transistor 16 is turned on. Therefore, a collector current flows in the oscillation transistor 16 via the collector winding 18. After that, the oscillation transistor 16 is maintained in the ON state by the electromotive force induced in the feedback winding 20, but when the collector current reaches a constant value, the electromotive force of the feedback winding 20 disappears, so the oscillation transistor 16 is Turned off. After that, start resistance 14
The base current again flows through the oscillation transistor 16 via the, and the above operation is repeated.

Collector winding 18 included in oscillator circuit 12
Is also used as the primary winding of the transformer 22, this transformer 2
AC is output from the secondary winding 24 of No. 2, but this AC output is converted to DC by the diode 26 and the smoothing capacitor 28, so that the load 30 is supplied with DC output. On the other hand, the feedback winding 20 has a secondary winding 2 of the transformer 22.
An AC voltage having a magnitude corresponding to the output voltage of 4 is induced, and when the AC voltage becomes negative, a current flows through the variable resistor 32, the resistor 34, and the diode 36. FET38
Since the gate of the FE receives the voltage from the variable resistor 34,
T38 is turned on when the voltage generated in the variable resistor 34 drops to a certain value, and the base current of the transistor 40 is changed to FE.
Pull in to T38. As a result, the current that should be input to the collector of the transistor 40 is input to the gate of the thyristor 42, and the thyristor 42 is ignited. Therefore, a current flows from the power source Vc through the thyristor 42,
The current is input to the base of the oscillation control transistor 44, and when the oscillation control transistor 44 turns on, the base current of the oscillation transistor 16 changes to the oscillation transistor 44.
Be drawn into.

In a steady state, the output voltage of the secondary winding 24, that is, the output voltage of the feedback winding 20 is equal to or higher than a certain value, so that the FET 38, the transistor 40, and the thyristor 4 are provided.
2 and the oscillation control transistor 44 remain off. When an excessive current flows through the load 30, the output voltage of the secondary winding 24 decreases, the voltage induced in the feedback winding 20 decreases accordingly, and the terminal voltage of the variable resistor 32 decreases. Turns on. As a result, the thyristor 42 is turned on and the oscillation control transistor 44 is turned on, so that the base current of the oscillation transistor 16 is drawn into the oscillation transistor 44 and the oscillation is stopped.
Therefore, the load characteristic of this embodiment has a substantially V-shaped characteristic as shown in FIG. 2, so that a short-circuit current does not flow and the oscillation transistor 16 is not destroyed.

Even if the load current returns to the normal value, the thyristor 42 continues to be turned on, so that the oscillation continues to stop.
Further, by changing the resistance value of the variable resistor 32, the detection point of the excessive current, that is, the current value at which the oscillation is stopped can be arbitrarily set.

[Brief description of drawings]

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

FIG. 2 is a graph showing load characteristics of the embodiment of FIG.

FIG. 3 is a circuit diagram showing a conventional technique.

FIG. 4 is a graph showing a load characteristic of a conventional technique.

[Explanation of symbols]

 10 ... Power supply circuit 12 ... Oscillation circuit 16 ... Oscillation transistor 20 ... Feedback winding 32 ... Variable resistance 38 ... FET 40 ... Transistor 42 ... Thyristor 44 ... Oscillation control transistor

Claims (2)

[Claims] 1. A power supply circuit for converting an output from an oscillation circuit into an output according to a winding ratio by a transformer and supplying the output to a load, wherein a feedback winding is provided in the transformer, and an output of the feedback winding is further provided. A power supply circuit comprising an oscillation stopping means for stopping the oscillation of the oscillation circuit when the value is equal to or less than a certain value. 2. The power supply circuit according to claim 1, further comprising state holding means for holding a stopped state of the oscillation circuit.