JPH0255576A - Switching power source - Google Patents

Abstract

PURPOSE:To protect a transistor from breakdown by constituting an AND gate with a circuit which is turned ON upon detection of conducting state of a switching transistor and a circuit which is turned ON upon detection of applying voltage higher than e predetermined level onto the collector. CONSTITUTION:A switching power source comprises a switching transistor(Tr)Q 1, a switching transformer T1, diodes D1-2, a capacitor C1 and an anti- breakdown circuit A for the switching transistor. The transformer T1 is provided with a main primary winding P1, an auxiliary primary winding P2 and a secondary output winding S1. The auxiliary winding P2 is connected through the diode D1 with the base of the TrQ1. The anti-breakdown circuit A comprises a conduction detecting circuit B and an application voltage limiting circuit C. By such arrangement, the switching power source operates such that the collector voltage of the TrQ1 does not exceed over a predetermined level when the TrQ1 makes transition from ON state to OFF state while conducting.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention is directed to a switching power supply, particularly to a switching transistor which is a switching element of the switching power supply, by preventing power from being applied transiently to the switching transistor so as not to damage the switching transistor. The present invention relates to a switching power supply having a function of preventing

[Conventional technology]

In conventional switching power supplies, efforts have been made to increase the breakdown resistance of switching transistors to prevent them from being destroyed.

Next, a conventional example will be explained with reference to the drawings.

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

The switching power supply shown in FIG. 3 includes a switching transistor Q1, a switching transformer T1, diodes DI and D2, and a capacitor CI7.

The switching transformer T1 has a primary side main winding P1 whose one end is connected to one side of the input power source, a primary side auxiliary winding P2 whose one end is connected to the other side of the input power source, and an output via a diode D2 and a capacitor C1. It is configured to include a secondary output winding S1 for supplying an output to the circuit.

The other end of the primary main winding P1 is connected to the collector of the switching transistor Q1, and the other end of the auxiliary primary winding P2 is connected to the base of the switching transistor Ql via the diode DI.

Further, the emitter of the switching transistor Q1 is connected to one end of the primary side auxiliary winding P2.

When the switching transistor Q1 transitions from ON to OFF, the switching transistor Q1 becomes conductive and
A transition voltage may occur where the collector voltage of the switching transistor Q1 exceeds a certain value, and as a result, when excessive power is applied to the switching transistor Q1,
will destroy it.

[Problem to be solved by the invention]

However, in order to increase the breakdown resistance of the switching transistor, the above-mentioned conventional switching power supply requires a high-voltage power supply and a high-speed power supply, which inevitably makes it expensive. However, there are disadvantages in that the base drive circuit becomes complicated and a circuit for suppressing the transiently generated high voltage is required.

[Means to solve the problem]

The switching power supply of the present invention serves as a switching transistor destruction prevention circuit: a circuit that detects the conduction state of the switching transistor and turns it on; and a circuit that turns it on when it detects that a voltage of a certain value or more is applied to the collector of the switching transistor. are connected to a switching transistor to form an AND circuit.

〔Example〕

Next, embodiments of the present invention will be described with reference to the drawings.

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

The switching power supply shown in FIG. 1 includes a switching transistor Q1, a switching transformer T1, diodes DI and D2, a capacitor C1, and a switching transistor breakdown prevention circuit A.

The switching transformer T1 has a primary side main winding P1 whose one end is connected to one side of the input power source, a primary side auxiliary winding P2 whose one end is connected to the other side of the input power source, and an output via a diode D2 and a capacitor C1. It is configured to include a secondary output winding S1 for supplying an output to the circuit.

The other end of the primary main winding P1 is connected to the collector of the switching transistor Q1, and the other end of the primary auxiliary winding P2 is connected to the base of the switching transistor Q1 via the diode D1.

The switching transistor destruction prevention circuit A is configured to include a continuity detection circuit B and an applied voltage limiting circuit C.

The continuity detection circuit B includes an emitter current detection resistor R1 connected between the emitter of the switching transistor Q1 and one end of the primary side auxiliary winding P2, and a base and an emitter connected to both ends of the emitter current detection resistor R1. It consists of a transistor Q3, and a switching transistor Q
This circuit detects that 1 is conductive and turns on.

The applied voltage limiting circuit C includes a transistor Q2 whose collector is connected to the collector of the switching transistor Q1 and whose emitter is connected to the collector of the transistor Q3, and a constant voltage regulator connected between the collector of the switching transistor Q1 and the base of the transistor Q1. It is configured with a voltage diode ZDI and can operate while the continuity detection circuit B is ON, and when the collector voltage of the switching transistor Q1 exceeds the voltage of the constant voltage diode ZD1, the transistor Q2 starts constant voltage operation.

Thereby, when the switching transistor Q1 transitions from ON to OFF, it conducts and operates so that the collector voltage of the switching transistor Q1 does not exceed a certain value.

As is clear from Figure 2, this operation maintains the power applied to the switching transistor Q1 at a constant value when it transitions from ON to OFF, and is known as a parameter that indicates the breakdown resistance of the transistor. The operation is performed to prevent entry into the shaded area of the safe operation area shown in FIG. 2, thereby preventing destruction of the switching transistor due to transient applied power.

〔Effect of the invention〕

The switching power supply invented by Kiyoshi is able to maintain the power applied to the switching transistor at a constant value when the switching transistor transitions from ON to OFF, thereby preventing damage to the switching transistor due to transient applied power. effective.

Next output winding, Ql・・Switching transistor,
ZD]... Constant voltage diode, R1... Emitter current detection resistor, Q2. Q3... Transistor, DI
, D2... - Diode, CI... Capacitor.

Ic... Collector current, King. may...Maximum collector current, VBE. 3 Saturation pace emitter voltage, VCE...collector voltage, vcEmax...
...Maximum collector N pressure, VZD...Zener voltage.

Agent Patent Attorney Susumu Uchihara

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram showing one embodiment of the wooden invention, and Fig. 2 is a circuit diagram showing an embodiment of the wooden invention.
FIG. 3 is a circuit diagram showing a conventional example. A: Suisochink transistor destruction prevention circuit,
B: Continuity detection circuit, C: Applied voltage limiting circuit.

Claims (1)

[Claims] A switching transformer connected to an input power source, a switching transistor connected to the switching transformer, a continuity detection circuit that detects that the switching transistor is conductive, and when the continuity detection circuit detects continuity. 1. A switching power supply comprising: a switching transistor breakdown prevention circuit comprising an applied voltage limiting circuit that turns on when voltage applied to the collector of the switching transistor exceeds a certain value.