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What is “drive circuit”? Circuits that are constructed to turn on/off power devices/switches Control circuit Gate driver • Provides an interface between the control signal (small signal electronics) and power circuits • Sometimes called ‘gate drivers’
Why do we need drive circuit? To amplify control signals to level required to drive the power switches and also to ensure fast turn-on and turn-off • MOSFET/IGBT typically require voltages between 10V - 20V to ON and initial high current to charge parasitic input capacitance. The capacitor need to be discharged during turn off • BJT require continuous current to ON – initial high current is needed to reduce turn on time. Provides electrical isolation between control circuit (small signal) and power circuit • Require for safety reasons and elevated emitter (or source) of the transistor in bridge configuration
Some examples of drive circuits Gate drive with totem-pole configuration
Some examples of drive circuits Electrical isolation methods • Transformer isolation • Opto-coupler isolation • Isolated dc power supplies for drive circuits
Some examples of drive circuits • Negative half cycle of vs(t) - positive dc rail near safety ground potential. T-emitter potential large and negative with respect to safety and logic ground • Postive half cycle of vs(t) - negative dc rail near safety ground potential. T+ emitter substantially positive with espect to safety ground if T- is off • Variation in emitter potentials with respect to safety and logic ground means that electrical isolation of emitters from logic ground is needed.