FI69228C - SKYDDSFOERFARANDE FOER EN SOM VAEXELSPAENNINGSKOPPLING FUNGERANDE EFFEKTTRANSISTOR - Google Patents

Description

1 69228

Method of protection of a power transistor acting as an alternating voltage switch. - Isolation of the power supply to the effect transistor.

The use of power transistors in mains voltage switch elements or in general as AC voltage switches has been limited by the relatively low surge current resistance of the transistors. Thus, it has not been possible to protect the transistors in short-circuit situations, for example with a fuse, using a reasonable dimensioning.

It is generally known that a transistor can be protected from excessive current by measuring the collector current of the transistors and disconnecting the control from the transistor when the current exceeds a permissible value. When this happens fast enough and there is enough inductance in the main circuit, the transistor will not have time to be destroyed.

Another commonly known method has been to measure the collector-emitter voltage of a transistor when the transistor has been driven to a conductive state. As the collector current of the transistor increases, the collector emitter voltage of the transistor also tends to increase. When the voltage has reached a certain value, with the transistor in the conducting state, the control is cut off from the transistor, making it possible to prevent the transistor from being destroyed in the short-circuit situation. Usually, said methods involve manual or automatic acknowledgment of the overcurrent information, whereby the transistor can be returned to the conducting state again. However, reconnection can occur at a time when there may be a high voltage across the transistor. In this case, in a short-circuit situation, the current peak of the transistor can become quite large at each switching moment, which can substantially shorten the life of the transistor.

This prior art is based on a method of protecting a power transistor acting as an alternating voltage switch of the type defined in the preamble of appended claim 1, which is based on the measurement of a collector emitter voltage.

The object of the invention is to improve this method in such a way that, when the 2 69228 transistors are switched on again, the generation of large current spikes which shorten its life can be avoided. This object is achieved by the method set out in the appended claims.

In the method according to the invention, the overcurrent information is automatically reset at the moment when the voltage across the transistor is close to zero. In this case, in the short-circuit situation, the peak current of the transistor is limited to a reasonable value, because the rate of voltage rise across the transistor is limited. The protection method also works in such a way that the transistor cannot be switched to a conductive state even under normal load, if its collector emitter voltage is not close to zero. In addition, filtering has generally been used in conventional protection circuits so that the transistor is switched on at all and thus the collector-emitter voltage would drop so low under normal load that the protection information would not become active. This filtering increases the slowness of the cut-off event in a short-circuit situation and, accordingly, the magnitude of the current peak passing through the transistor, especially when there may be a considerable voltage across the transistor when the transistor is conductive in a short-circuit situation.

The accompanying drawing shows a functional block diagram illustrating the method according to the invention.

In the block diagram, the transistor 1 functions as a switching element controlled by the control logic circuit 4 in the mains voltage line. The collector-emitter voltage of the transistor 1 is sensed by a collector-emitter voltage sensor 2 arranged to control the base control circuit 3 so that the base control of the transistor 1 is cut off when the collector-emitter voltage exceeds a certain value.

According to the invention, the sensor 2 and the base control circuit 3 are arranged to operate in such a way that the transistor 1 cannot be switched back to the conductive state unless the collector emitter voltage of the transistor is close to zero, i.e. below a given relatively small reference value.

In addition, the base control circuit 3 is monitored by a negative operating voltage sensor 5 so that if the negative operating voltage is not close enough to the desired value or is completely absent, the transistor 1 cannot be controlled to a conductive state.

Claims (3)

A method of protecting a power transistor acting as an alternating voltage switch, wherein when the transistor is conductive the collector-emitter voltage of the transistor is measured, above which the transistor base control is cut off . Method according to Claim 1, characterized in that if the negative operating voltage used for the base control of the transistor is not close enough to the desired value, the switching of the transistor to the conductive state is completely prevented. Method according to Claim 1 or 2, characterized in that if the negative operating voltage used for the base control of the transistor is completely absent, the transistor cannot be controlled.