AN2123

Active Miller clamp

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5 Active Miller clamp

The TD351 offers an alternative solution to the problem of Miller current in IGBT switching applications.

Traditional solutions to the Miller current problem are:

hand schematic in Figure 8)

The solution proposed by the TD351 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 the right-hand schematic in

Figure 8). The CLAMP switch is open 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

characteristics, but simply keeps the gate at a low level during the entire off-time.

The main benefit is that negative supply voltage can be avoided in most cases, allowing for the use of a

bootstrap technique for the high-side driver supply, and a consistent cost reduction for the application.

In addition, the use of the active Miller clamp feature avoids the need to implement any additional

capacitors between the IGBT gate and the collector. Such capacitors would negatively affect the ability of

the driver to control turn-on and turn-off.

The test results shown in Figure 9 prove how the active Miller clamp results in a consistent reduction of

the voltage spike on IGBT gate.

The left-hand waveform shows the result of a 400 V switching with a 10 nF additional Gate to Emitter

capacitor to control the voltage spike on gate.

Figure 8. Active Miller Clamp: principles of operation

high dV/dt !

voltage spike on IGBT gate !

High-side

TD351

high dV/dt !

reduced voltage spike

Miller current

Miller current

active clamp

Low-side

TD351

High-side

driver

Low-side

driver

10R

10R

10nF

optional capacitor

implemented to

reduce voltage spike

no need for

additional

capacitor