722 Power Transistors

The power bipolar junction transistor (BJT) is a three-terminal controlled switch. The circuit symbol and the i-v characteristics of a BJT are shown in Figure 7.5. When sufficient positive base current iB flows through the base of a npn BJT, the transistor action allows large positive collector current ic to flow through the junctions of the device with a small positive collector to emitter voltage VCE (saturation) drop. The amplitude of the base current determines the amplification in the collector current. The power transistors are always operated as a switch, at saturation with high enough base current or at cut-off with zero base current. The power BJT is a controllable switch that can be turned on or off with the help of the base current. The device allows forward current or blocking voltage and, hence, is a Quadrant I switch. Steady state operation in Quadrants II and IV is not possible.

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