SI8285 Datasheet, PDF (6/32 Pages) Silicon Laboratories – System Safety Features

Si8285/86 Data Sheet
System Overview
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
by an internal comparator. Since the DSAT pin is connected to the IGBT collector through the D_DSAT and a small R_DSAT, its voltage is
almost the same as the V_CE of the IGBT. If the V_CE of the IGBT does not drop below the Si828x desaturation threshold voltage within a
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.
Driver ControlSignal
Ichg
Fault Signal ^{DESAT Sense}
7V
Driver Disable
DSAT
DDSAT
RDSAT
CBL
VMID
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
avoid unexpected fault signal generation. This function requires an external blanking capacitor, C_BL, of typically 100 pF between DSAT
and VMID pins. The block includes a 1 mA current source (I_Chg) to charge the C_BL. This current source, the value of the external C_BL,
and the programmed fault threshold, determine the blanking time (t_Blanking).
t_Blanking
= C_BI
×
V _DESAT
I_chg
An internal nmos switch is implemented between DSAT and VMID to discharge the external blanking capacitor, C_BL, and reset the
blanking timer. The current limiting R_DSAT resistor protects the DSAT pin from large current flow toward the IGBT collector during the
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.
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