JPH02168862A - Switching power supply circuit - Google Patents

Abstract

PURPOSE:To stop the working of a switching transistor perfectly when the tertiary winding of the switching transistor is short-circuited, by connecting a capacitor to a start resistor at the ON-time of a power source, in series, to make current flow to the start resistor only at the ON-time of the power source. CONSTITUTION:When a power source is turned ON, then voltage on a 130V line rises. When the 130 line rises completely, then only DC component is left on the 130V line, and due to a DC cut capacitor 8, current does not flow to a start resistor 3 at all. Accordingly, when the tertiary winding L3 of a switching transformer 2 is short-circuited, then a self-oscillation circuit is not to be worked, and the base current of a switching transistor 1 is not fed from any place, and so the oscillation of the switching transistor 1 is perfectly stopped.

Description

[Detailed description of the invention] Industrial applications The present invention relates to a switching power supply circuit.

2. Description of the Related Art A conventional switching power supply circuit will be described below with reference to the drawings.

FIG. 2 is a basic circuit diagram of a conventional switching power supply circuit.

In the figure, 1 is a switching transistor, 2 is a switching transformer, 3 is a starting resistor when the power is turned on,
4 is a resistor for controlling the base current of the switching transistor 1, 5 is a speed-up capacitor for making the rise and fall of the base current of the switching transistor 1 steep, and 6 and 7 are rectifier diodes on the load side.

Note that the base current control resistor 4, the speed-up capacitor 5, and the tertiary winding L3 of the switching transformer 2 constitute an automatic oscillation circuit.

The operation of the switching power supply circuit configured as described above will be described below.

When the power is turned on, the voltage on the 130■ line rises,
A base current flows into the switching transistor 1 through the starting resistor 3, and the switching transistor 1 is turned on.

When the switching transistor 1 turns on, (VIN
V port 1fsATl l LVIN
130V life 0) Voltage VQ+ +3ATl '---
Collector voltage L of switching transistor 1 A voltage is generated in the inductance of the primary winding of switching transformer 2, and a current begins to flow.

At this time, a voltage corresponding to the turns ratio with respect to the primary winding L1 is also generated in the tertiary winding L3 of the switching transformer 2, and is applied to the switching transistor 1 through the speed-up capacitor 5 of the automatic oscillation circuit and the base current control resistor 4. It supplies the base current and maintains the ON state.

Since the collector current [, is a simple increase, the initial base current 1. may be small, but since the collector current increases with time, the base current also becomes larger, and the base current of the switching transistor 1 flows, and the switching transistor 1 is turned ON again.

Thereafter, the above-mentioned operations are repeated to operate as a switching power supply circuit.

Problem to be Solved by the Invention However, in such a configuration, the switching transformer 2
If the tertiary winding, l, of the tertiary winding of Oscillation continues. (However, the oscillation frequency changes and a normal voltage is not generated on the load side.) Therefore, an excessive short circuit current flows through the tertiary winding L3 of the switching transformer 2, and if left in this state for a long time, the tertiary winding There was a problem in that the wires generated abnormal heat, which resulted in dielectric breakdown of the switching transformer 2, making it impossible to maintain safety.

A large current is required.

However, the base current is limited by the resistance value of the base current control resistor 4, and cannot exceed a certain value. Therefore, when IP<hf, IB, the base current becomes insufficient and the switching transistor 1 cannot maintain a saturated state.

Then ■ of switching transistor 1. , increases, so the voltage across the primary winding of the switching transformer 2 decreases, and the voltage across the tertiary winding L3 also decreases in proportion to this, promoting a lack of base current, and the switching transistor l suddenly decreases. becomes OFF.

At the moment when the switching transistor 1 is turned off, a back electromotive force is generated in the secondary winding L2, and the rectifier diode 6.7 starts conducting, supplying power to the load.

The back electromotive voltage generated in the secondary winding L2 simply decreases with time, and eventually becomes zero. At this time, the tertiary winding of switching transformer 2.

A back electromotive force is induced in the automatic oscillation circuit, and the present invention solves these conventional problems, and has a simple configuration that can reduce the tertiary voltage of the switching transformer. This improves safety by completely stopping the operation of the switching transistor when the windings are short-circuited.

Means for Solving the Problems The switching power supply circuit of the present invention connects a capacitor in series with the starting resistor when the power is turned on, so that current flows through the starting resistor only when the power is turned on. It is.

Function The switching power supply circuit of the present invention allows current to flow through the starting resistor only when the power is turned on, so that even if an abnormality occurs in the automatic oscillation circuit, the operation of the switching transistor can be completely stopped. It can be stopped and safety can be improved.

Embodiment Hereinafter, a switching power supply circuit according to an embodiment of the present invention will be explained with reference to the drawings.

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

In the figure, l is a switching transistor, 2 is a switching transformer, 3 is a starting resistor when the power is turned on,
8 is a DC cut capacitor for allowing current to flow through the starting resistor 3 only when the power is turned on, 4 is a resistor for controlling the base current of the switching transistor 1, and 5 is a resistor for controlling the base current of the switching transistor 1 to steeply rise and fall. Speed-up capacitors 6 and 7 are rectifier diodes on the load side.

The operation of the switching power supply circuit configured as described above will be described below.

When the power is turned on, the voltage on the 130* line rises, but this rise is steep, and this rise period is a kind of alternating current. Therefore, the switching transistor 1 is connected through the starting resistor 3 and the DC cut capacitor 8.
The base current flows through the switching transistor 1, and the switching transistor 1 becomes O.
It becomes N state.

The subsequent operations are the same as those explained in the section of the prior art.

When the 130v line is fully activated, 130■
The line has only DC components, and the DC cut capacitor 4
Therefore, no current flows through the starting resistor 3 at all. In this state, the switching transistor 1 receives base current only from the automatic oscillation circuit and continues normal oscillation operation.

Therefore, the tertiary winding of switching transformer 2.

When the automatic oscillation circuit is short-circuited, the automatic oscillation circuit ceases to operate, and the base current of the switching transistor 1 is no longer supplied from anywhere, so that the switching transistor 1 completely stops oscillating.

Effects of the Invention As described above, the switching power supply circuit of the present invention completely stops the operation of the switching transistor when an abnormality occurs in the automatic oscillation circuit, thereby improving safety, and is extremely advantageous in practice. .

[Brief explanation of the drawing]

FIG. 1 is a basic circuit diagram of a switching power supply circuit according to an embodiment of the present invention, and FIG. 2 is a basic circuit diagram of a conventional switching power supply circuit. l...Switching transistor, 2...
... Switching transformer, 3... Resistor for starting, 4... Resistor for base current control, 5...
...Speed-up capacitor, 6.7 Old... Rectifier diode, 8 Old... DC cut capacitor.

Claims (1)

[Claims] A DC circuit consisting of a switching transistor, a switching transformer that transforms the output of the switching transistor and outputs it to the secondary side, and a starting resistor and a DC cut capacitor inserted between the power supply voltage line and the base of the switching transistor. , a switching power supply circuit comprising a series circuit of a base current control resistor and a speed-up capacitor inserted between the base of the switching transistor and the tertiary winding of the switching transformer.