Power Electronic Riddle No.11 – Gate drive circuits for MOSFET

The input gate characteristics of a MOSFET differ from its bipolar counterparts. The gate is isolated electrically from the source by a layer of silicon dioxide. Ideally, no current flows into the gate when a dc voltage is applied. However, a small leakage current (of the order of 10-10 A) flows to maintain the gate voltage and also during the transition periods (off-to-on and on-to-off). Therefore, a small current is enough to charge and discharge the device capacitances. However, the device capacitances and gate-drive source impedance determine the device switching speed. There are three capacitances and they are nonlinear and voltage-dependent (Fig. below). If the gate voltage is reduced to zero for turn-off operation, VDS attains the supply voltage magnitude (VDD), the potential at D swings from 0 to VDD and Cgd is charged up to to VDD (say, 100 V) through a low gate source impedance (Rg ) as shown in Figure below. Now if a gate drive signal is applied (higher than the threshold value), the device will be on. Ideally, the voltage across D and S reduces to zero (neglecting small VDS) and terminal D will swing to ground potential (which earlier was at VDD . 100 V). The decreasing VDS produces a feedback current (i =Cgd . dVDS/dt) through Cgd to the gate circuit as shown in Figure below. This feedback mechanism is called the

This site has been made for technical discussion about various branches of power systems, so please say your problem for information sharing and more discussion.