2.5 Desaturation Detection

The Si828x provides sufficient voltage and current to drive and keep the IGBT in saturation during on time to minimize power dissipa-

tion and maintain high efficiency operation. However, abnormal load conditions can force the IGBT out of saturation and cause perma-

nent damage to the IGBT.

To protect the IGBT during abnormal load conditions, the Si828x detects an IGBT desaturation condition, shuts down the driver upon

detecting a fault, and provides a fault indication to the controller. These integrated features provide desaturation protection with mini-

mum external BOM cost. The figure below illustrates the Si828x desaturation circuit. When the Si828x driver output is high, the internal

current source is on, and this current flows from the DSAT pin to charge the CBL capacitor. The voltage on the DSAT pin is monitored

certain time after turning on the IGBT (blanking period) the block will generate a fault signal. The Si828x desaturation hysteresis is fixed

at 220 mV and threshold is nominally 7 V.

VMID

DSAT

DESAT Sense

Driver ControlSignal

Fault Signal

Driver Disable

7 V

Figure 2.3. Desaturation Circuit

As an additional feature, the block supports a blanking timer function to mask the turn-on transient of the external switching device and

IGBT’s body diode freewheeling period (with possible large collector’s negative voltage, relative to IGBT’s emitter).

2.6 Soft Shutdown

To avoid excessive dV/dt on the IGBT’s collector during fault shut down, the Si828x implements a soft shut down feature to discharge

the IGBT’s gate slowly. When soft shut down is activated, the high power driver goes inactive, and a weak pull down via VH and exter-

nal RH discharges the gate until the gate voltage level is reduced to the VSSB + 2 V level. The high power driver is then turned on to

clamp the IGBT gate voltage to VMID.

After the soft shut down, the Si828x driver output voltage is clamped low to keep the IGBT in the off state.

Si8285/86 Data Sheet

System Overview

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