AN2123 STMicroelectronics, AN2123 Datasheet - Page 5

• Manufacturer Part Number: AN2123
• Description: TD351 Advanced IGBT Driver Principles of operation and application
• Manufacturer: STMicroelectronics

AN2123
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:
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
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.
Figure 8. Active Miller Clamp: principles of operation
The test results shown in
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.
to drive the IGBT gate to a negative voltage in OFF-state in order to increase the safety margin
or, to implement an additional capacitor between the IGBT gate and collector as described in the left-
hand schematic in
8). The CLAMP switch is open when the input is activated and is closed when the actual gate
High-side
Low-side
driver
driver
reduce voltage spike
10R
optional capacitor
Miller current
implemented to
Figure
10nF
Figure 9
8)
prove how the active Miller clamp results in a consistent reduction of
voltage spike on IGBT gate !
high dV/dt !
active clamp
High-side
Low-side
TD351
TD351
10R
Miller current
no need for
additional
capacitor
Active Miller clamp
reduced voltage spike
high dV/dt !
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