JPH0739343Y2 - Transistor power converter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an overload protection circuit for a transistor power converter.

Conventionally, this type of transistor power conversion device has a simple circuit, but often has no overload or load short-circuit protection circuit. Further, even when an overload protection circuit is provided, a method has been taken in which the load current is detected by a current transformer (CT) and the relay or the like is operated by this to directly cut off the power input. However, the input interruption by the relay is especially high voltage,
The more the overcurrent is, the more severe the deterioration of the contact is, which is not a preferable method from the viewpoint of reliability.

The present invention proposes an overload protection circuit which solves the above-mentioned drawbacks, in which the temperature rise of a transistor caused by an overload, especially a load short circuit is detected by a positive temperature coefficient thermistor, and the base current of the transistor is limited. It is intended to limit the collector current of a transistor, and to provide a novel load short circuit protection circuit by simply replacing the base resistance of the transistor with a positive temperature coefficient thermistor.

The operation of the conventional transistor power converter and the overload protection circuit according to the present invention will be described below with reference to FIG. 1 is a DC power source, 2 and 3 are transistors, 4 is a transformer, 5 and 5'are 4 primary windings, 6 and 6'are 4 secondary windings, 7 and 7'are 4 base windings, Reference numeral 8 denotes a resistor for starting and adjusting the base current, which is a positive temperature coefficient thermistor in the present invention. 9 is a capacitor for speeding up, 10
And 10 'are rectifying diodes, 11 is a smoothing capacitor,
12 is the load. In this circuit, for example, when a constant voltage is applied from the DC power supply 1 between the base and emitter of the transistor 2 via the starting / base current adjusting resistor 8, the DC power supply 1 → the resistor 8 → the base winding 7 ′ → A current flows through the base-emitter of the transistor 2, which causes the collector current of the transistor 2 to flow in the primary winding 5 of the transformer 4. This collector current is fed back to the base winding 7'and increases until the magnetic core of the transformer 4 is saturated. During this time, a reverse bias voltage is applied between the base and emitter of the transistor 3, and the transistor 3 is in a cutoff state. Next transformer 4
Is saturated, a back electromotive force is generated in the base winding 7, 7 ',
As a result, the transistor 3 is turned on and the transistor 2 is turned off. Further, the speed-up capacitor 9 connected in series with the resistor 15 is charged with the polarity shown in the figure, and when the voltage of the transistor 2 is switched from conduction to cutoff, the base-emitter of the transistor 2 is reverse-biased. Shut off rapidly. In this way, the transistors 2 and 3 are alternately turned on, the alternating current can be taken out from the secondary windings 6 and 6 ', further rectified by the rectifying diodes 10 and 10', smoothed by the co-capacitor 11, and then applied to the load 12. Get DC output. When the load 12 is short-circuited or overloaded, the transistor 2 or 3 rapidly generates heat, and when the temperature exceeds the junction temperature, the transistor is damaged. In order to prevent this, the present invention replaces the conventional circuit start-up and base current adjusting resistor 8 with a positive temperature coefficient thermistor, and contacts or closes to either or both of the transistors 2 and 3 in a state of good thermal conductivity. Resin coating And the like, which are thermally coupled. If alumina powder is mixed with the resin coating material, an adhesive having better heat conduction can be obtained.

The positive temperature coefficient thermistor shows the characteristics shown in Fig. 2. When the temperature exceeds the Curie point, the temperature changes by about 10 ° C and the resistance value becomes 2
It is a resistance element that changes by an order of magnitude. Assuming that the junction temperature of the transistor is 150 ° C., if the Curie point of the positive temperature coefficient thermistor 8 is 100 ° C. and the resistance of the thermistor 8 at this temperature is 500Ω, then 100 ° C.
In the following normal temperature state, the thermistor 8 supplies the base current to the transistor and the transistors 2 and 3 operate normally as described above. Here, if the load is overloaded or short-circuited, the transistors 2 and 3 will rapidly rise in temperature, and the positive temperature coefficient thermistor 8 thermally coupled to this will have a resistance value of about 50 KΩ at about 110 ° C. 3 has its base current limited and limits the supply of load current.

In this way, by replacing the conventional circuit starter / base current adjusting resistor with a positive temperature coefficient thermistor and thermally coupling it to the transistor, the overload or load short-circuit protection circuit configuration can be realized without changing the circuit configuration. A compact and inexpensive transistor power converter with an overload protection circuit can be obtained, and the practical effect is extremely large.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a transistor power converter, and FIG. 2 is a characteristic diagram of a positive temperature coefficient thermistor. In FIG. 1, 1 is a DC power supply, 2 and 3 are transistors, 4 converters, 5 and 5 '.
Is 4 primary windings, 6 and 6'is 4 secondary windings, 7 and 7'is 4 base windings, 8 is a resistance of a conventional circuit, a positive temperature coefficient thermistor according to the present invention, 9 is a capacitor, 10 , 10 'is a rectifying diode, 11 is a smoothing capacitor, and 12 is a load.

Claims (1)

[Scope of utility model registration request] 1. At least a direct current power source and alternating currents 2
A transistor power converter comprising a set of transistors and a transformer, the transformer provided with two sets of base windings for driving the transistors, arranged to be thermally coupled to one or two of the transistors The positive characteristic thermistor is provided, the positive characteristic thermistor and the capacitor are connected in series and connected in parallel with the DC power source, and the connection point of the positive characteristic thermistor and the capacitor is connected between the connection points of the two base windings. A transistor power conversion device characterized by the above.