Functional description

TD351

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Doc ID 10977 Rev 2

5

Functional description

5.1

Input stage

The TD351 input is compatible with optocouplers or pulse transformers. The input is

triggered by the signal edge and allows the use of low-sized, low-cost pulse transformers.

Input is active low and output is driven high when input is driven low. The IN input is

internally clamped at about 5 V to 7 V. When using an open collector optocoupler, the

resistive pull-up resistor can be connected to either VREF or VH. Recommended pull-up

resistor value with VH = 16 V is from 4.7 k

Ω to 22 kΩ. When driven by a pulse transformer,

5.2

Voltage reference

A voltage reference is used to create accurate timing for the turn-on delay with external

resistor and capacitor. The same circuitry is also used for the two-level turn-off delay. A

decoupling capacitor (10 nF to 100 nF) on the VREF pin is required to ensure good noise

rejection.

5.3

Active Miller clamp

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

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

margin, the TD351 uses a dedicated CLAMP pin to control the Miller current. When the

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

the Miller current, and the voltage spike on the IGBT gate is greatly reduced. During turn-off,

the gate voltage is monitored and the clamp output is activated when the gate voltage goes

below 2 V (relative to VL). The clamp voltage is VL+4V max for a Miller current up to 500

mA. The clamp is disabled when the IN input is triggered again.

The CLAMP function does not affect the turn-off characteristic, but only keeps the gate at

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.

5.4

Two-level turn-off

During turn-off, the gate voltage can be reduced to a programmable level in order to reduce

the IGBT current (in the event of overcurrent). This action prevents both dangerous

overvoltages across the IGBT and RBSOA problems, especially at short-circuit turn-off.

accurate timing.