Some Aspects of Power Transistor Inverter Design

The safe operation of large power transistors in motor-drive applications often requires the use of snubber networks. The design of these networks depends upon the type of device being used and upon the circuit configuration. Some operating experience with a monolithic Darlington device is reported. The importance of controlling the switching losses in these devices by the use of snubber capacitors is indicated. It is shown that these capacitors produce side effects in a variable-voltage, variable-frequency inverter which can lead to overcurrent conditions in the transistors. These currents can be controlled by the use of snubber chokes. But the chokes, which are usually included primarily to limit reverse recovery currents in the free-wheel diodes, must be designed according to a different criterion to achieve this. It is suggested that `thermal feedback¿¿ should be provided for power transistors in order that they may operate at a safe and stable temperature.