New Gate Drive Circuits characteristics. Carrier lifetime determines the rate at which

5.6.2 New Gate Drive Circuits characteristics. Carrier lifetime determines the rate at which

the minority carriers stored in the drift region recombine. In order to reduce the delay time required for the gate voltage The charge removed from the gate during turn-off has small to increase from V gg − to V ge (th), the external gate capacitor influence on minority carrier recombination. The tail current can be introduced in the circuit only after V ge reaches V ge (th) and di/dt during turn-off, which determine the turn-off losses, as is shown in Fig. 5.14, where the collector current rise occurs. depend mostly on the amount of stored charge and the minor- The voltage tail during turn-on transient is not affected by ity carriers lifetime. Therefore, the gate drive circuit has a this method. In order to prevent shoot through caused by minor influence on turn-off losses of the IGBT, while it affects accidental turn-on of IGBT due to noise, a negative gate voltage the turn-on switching losses. is required during off-state. Low gate impedance reduces the

The turn-on transient is improved by use of the circuit effect of noise on gate.

shown in Fig. 5.15. The additional current source increases the During the first slope of the gate voltage turn-on transient, gate current during the tail voltage time, and therefore reduces the rate of charge supply to the gate determines the collector the turn-on loss. The initial gate current is determined by V gg + current slope. During the miller effect zone of the turn-on and R gon , which are chosen to satisfy device electrical spec- transient the rate of charge supply to the gate determines the ifications and EMI requirements. After the collector current collector voltage slope. Therefore, the slope of the collector reaches its maximum value, the miller effect occurs and the current, which is controlled by the gate resistance, strongly controlled current source is enabled to increase the gate cur- affects the turn-on power loss. Reduction in switching power rent to increase the rate of collector voltage fall. This reduces loss requires low gate resistance. But the collector current slope the turn-on switching loss. Turn-off losses can only be reduced also determines the amplitude of the conducted electromag- during the miller effect and MOS turn-off portion of the turn- netic interference (EMI) during turn-on switching transient. off transient, by reducing the gate resistance. But this increases Lower EMI generation requires higher values of gate resis- the rate of change of collector voltage, which strongly affects tance. Therefore, in conventional gate drive circuits by selecting the IGBT latching current and RBSOA. During the turn-off

R goff

A circuit for reducing the turn-on delay.

FIGURE 5.15 Schematic circuit of an IGBT gate drive circuit.

84 S. Abedinpour and K. Shenai period, the turn-off gate resistor R goff determines the maxi-

Special sense IGBTs have been introduced at low power mum rate of collector voltage change. After the device turns levels with a sense terminal to provide a current signal pro- off, turning on transistor T 1 prevents the spurious turn-on of portional to the IGBT collector current. A few active device IGBT by preventing the gate voltage to reach the threshold cells are used to mirror the current carried by the other cells. voltage.

But unfortunately, sense IGBTs are not available at high power levels and there are problems related to the higher conduc-