The consequences range from unnoticed or merely annoying in a casual application such as turning a light on or off, all the way to likely damage in the widely used pulse-width modulation (PWM) fast- switching circuits of power supplies, motor drives, and similar subsystems. It can cause short circuits and even permanent damage, in the standard half and full-bridge topologies where the load is placed between an upper and lower MOSFET pair if both MOSFETs on the same side of the bridge are turned on simultaneously even for an instant. This phenomenon is known as “shoot-through” (Figure 5).
voltage, typically between -5 and -10 volts, enables rapid switching controlled by a gate resistor. An appropriate negative drive ensures that the gate-emitter off-voltage is always actually zero or less. This is critical because any emitter inductance (L) (at point ‘x’ in Figure 6) between a switch and the driver reference, causes an opposing gate-emitter voltage when the switch is turning off. While the inductance may be small, even a very small inductance of 5 nanohenries (nH) (a few millimeters of wired connection) will produce 5 volts at a di/dt slew rate of 1000 A per microsecond (A/μs).
Gate-drive details In order to drive current into the gate, the positive rail’s voltage should be high enough to ensure full saturation/enhancement of the power switch, but without exceeding the absolute maximum voltage for its gate. While this voltage value is a function of the specific device type and model, IGBTs and standard MOSFETS will generally be fully on with a 15-volt drive, while typical SiC MOSFETS may need closer to 20 volts for a full on-state. The negative gate-drive voltage situation is a little more complicated. In principle, for the off-state, 0 volts on the gate is adequate. However, a negative
Figure 5: In contrast to the normal MOSFET turn on of Q1 and Q4 (left), or Q2 and Q3 (right), if Q1 and Q2, or Q3 and Q4 of the bridge are turned on simultaneously due to driver issues or other causes, an unacceptable and possibly damaging short- circuit condition called shoot through will occur between the power rail and ground. (Image source: Quora)
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