AN1944

Active Miller clamp

7/21

4

Active Miller clamp

The TD350 offers an alternative solution to the problem of the Miller current in IGBT

switching applications. Instead of driving the IGBT gate to a negative voltage to increase the

safety margin, the TD350 uses a dedicated CLAMP pin to control the Miller current. When

the IGBT is off, a low impedance path is established between IGBT gate and emitter to carry

the Miller current, and the voltage spike on the IGBT gate is greatly reduced (see Figure 7).

The CLAMP switch is opened when the input is activated and is closed when the actual gate

voltage goes close to the ground level. In this way, the CLAMP function doesn’t affect the

turn-off characteristic, but only keeps the gate to the low level throughout the off time. The

main benefit is that negative voltage can be avoided in many cases, allowing a bootstrap

technique for the high side driver supply.

The waveform shown in Figure 8 proves how using the Active Miller clamp provides a

consistent reduction of the voltage spike on IGBT gate.

Figure 7.

Active Miller clamp: principle of operation

high dV/dt !

voltage spike on IGBT gate !

TD350

high dV/dt !

reduced voltage spike

Miller current

Miller current

active clamp