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
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
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 this circuit must have include high speed, low cost, low resolution, low power dissipation, and small size.
The ACPL-344JT satises these criteria by combining a high-speed, high-output current driver, high-voltage optical
isolation 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 adopted in the ACPL-344JT monitors the saturation (collector) voltage of the IGBT and
triggers 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-344JT 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 required
to protect the IGBT.
Recommended Application Circuit
The ACPL-344JT 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 shows a typical gate-drive implementation using the ACPL-
344JT.
The two supply bypass capacitors (1.0 μF minimum) provide the large transient currents necessary during a switch-
ing transition. The Desat diode and 220 pF 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 10 kΩ pull-up resistor and a 330
pF ltering capacitor.
