4
Typical Application/Operation
Introduction to Fault Detection and Protection
The power stage of a typical three phase inverter is susceptible to several types of failures, most of which are potentially
destructive to the power IGBTs. These failure modes can be grouped into four basic categories: phase and/or rail supply
short circuits due to user misconnect or bad wiring, control signal failures due to noise or computational errors, overload
conditions induced by the load, and component failures in the gate drive circuitry. Under any of these fault conditions,
the current through the IGBTs can increase rapidly, causing excessive power dissipation and heating. The IGBTs become
damaged when the current load approaches the saturation current of the device, and the collector to emitter voltage
rises above the saturation voltage level. The drastically increased power dissipation very quickly overheats the power
device and destroys it. To prevent damage to the drive, fault protection must be implemented to reduce or turn-o the
overcurrent during a fault condition.
A circuit providing fast local fault detection and shutdown is an ideal solution, but the number of required components,
board space consumed, cost, and complexity have until now limited its use to high performance drives. The features
which this circuit must have are high speed, low cost, low resolution, low power dissipation, and small size.
The ACPL-34JT satises these criteria by combining a high speed, high output current driver, high voltage optical isola-
tion between the input and output, local IGBT desaturation detection and shut down, and optically isolated fault and
UVLO status feedback signal into a single 16-pin surface mount package.
The fault detection method, which is adopted in the ACPL-34JT, is to monitor the saturation (collector) voltage of the
IGBT and to trigger a local fault shutdown sequence if the collector voltage exceeds a predetermined threshold. A small
gate discharge device slowly reduces the high short circuit IGBT current to prevent damaging voltage spikes. Before
the dissipated energy can reach destructive levels, the IGBT is shut o. During the o state of the IGBT, the fault detect
circuitry is simply disabled to prevent false ‘fault’ signals.
The alternative protection scheme of measuring IGBT current to prevent desaturation is eective if the short circuit
capability of the power device is known, but this method will fail if the gate drive voltage decreases enough to only
partially turn on the IGBT. By directly measuring the collector voltage, the ACPL-34JT limits the power dissipation in the
IGBT even with insucient gate drive voltage. Another more subtle advantage of the desaturation detection method
is that power dissipation in the IGBT is monitored, while the current sense method relies on a preset current threshold
to predict the safe limit of operation. Therefore, an overly- conservative overcurrent threshold is not needed to protect
the IGBT.
Recommended Application Circuit
The ACPL-34JT has non-inverting gate control inputs, and an open collector fault and UVLO outputs suitable for wired
‘OR’ applications.
The recommended application circuit shown in Figure 3 illustrates a typical gate drive implementation using the ACPL-
34JT.
The two supply bypass capacitors (0.1 μF) provide the large transient currents necessary during a switching transition.
The desat diode and 220pF blanking capacitor are the necessary external components for the fault detection circuitry.
The gate resistor (10Ω) serves to limit gate charge current and indirectly control the IGBT collector voltage rise and fall
times. The open collector fault and UVLO outputs have a passive 10kΩ pull-up resistor and a 330 pF ltering capacitor.
