The power transistor should be used like this in the power adapter

　　Applications of Power Transistors

　　Power transistor components are not only used in industrial machines such as servo amplifiers for industrial electromechanical machines or VVVF inverters controlled by AC motors, but also in many household appliances such as inverter air conditioners and induction cookers. Or for OA equipment, chargers. The main circuit structure of AC motor control, general-purpose inverters, industrial inverters, inverters for servo and general industrial equipment, and air conditioners, etc., are almost all based on this basic circuit. Transistor components are developing towards high-power three-terminal power adapters, and it is possible to achieve 500kvA inverter control.

　　First: prevent branch short circuit

　　In bridge circuits such as inverters, it is necessary to pay attention to the short-circuit accident of the upper and lower branches due to the delay of the response time of the power transistor. Especially in the Darlington transistor, the delay is very long, so according to the rating and characteristics of the power transistor, a rest time of several tens of seconds must be set between the base-level signals of each transistor in the upper and lower branches.

　　Second: base drive circuit

　　The idea of providing optimum base drive conditions when using a power transistor as a switch is as follows. The base forward current of the power transistor must be determined according to the saturation voltage characteristics, switching time and short-circuit tolerance. The base reverse current has an effect on the turn-off time. Increasing the base reverse current reduces the turn-off time required for high frequency switching. But pay attention: the safe working area of reverse bias is narrowed, and the drive circuit of power transistors needs to be designed according to various uses, but these base drive circuits have been mixed and integrated recently, and the installation is very convenient.

　　Third: dv/dt resistance

　　In the inverter analog circuit, when current flows in the direction shown by the dotted line, if Td is turned on, dv/dt with a steep slope is added to T. If the reverse base bias is insufficient, the displacement current caused by dv/dt will increase, which will increase the switching loss at turn-off. On the other hand, even if the reverse base bias of T1 is large enough, if the reverse recovery current of the feedback diode of T1 is large, the turn-off loss will increase, so the feedback diode should be a high-speed transistor.

　　Fourth: buffer circuit

　　In order to suppress the surge voltage applied to the power transistor, a snubber circuit is used. Different from the application of bipolar transistors in the power supply industry, the effect of this circuit is to suppress the operating point of the power transistor during the disconnection action within the reverse bias safe working area, and at the same time Westbrook disconnects the loss.

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