21
Typical Application/Operation
Introduction to Fault Detection and Protection
The power stage of a typical three phase inverter is sus-
ceptible 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 com-
putational 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 cur-
rent 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 overcurrents during a fault condition.
A circuit providing fast local fault detection and shut-
down is an ideal solution, but the number of required
components, board space consumed, cost, and complex-
ity 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.
Applications Information
The ACPL-516x satises these criteria by combining a
high speed, high output current driver, high voltage op-
tical isolation between the input and output, local IGBT
desaturation detection and shut down, and an optically
isolated fault status feedback signal into a single 16-pin
DIP package.
The fault detection method, which is adopted in the
ACPL-516x, is to monitor the saturation (collector) volt-
age of the IGBT and to trigger a local fault shutdown se-
quence 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 dis-
abled 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-516x 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 dissi-
pation 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 conserva-
tive overcurrent threshold is not needed to protect the
IGBT.
Recommended Application Circuit
The ACPL-516x has both inverting and non-inverting
gate control inputs, an active low reset input, and an
open collector fault output suitable for wired ‘OR’ appli-
cations. The recommended application circuit shown in
Figure 62 illustrates a typical gate drive implementation
using the ACPL-516x.
The four supply bypass capacitors (0.1 µF) provide the
large transient currents necessary during a switching
transition. Because of the transient nature of the charg-
ing currents, a low current (5 mA) power supply suces.
The desat diode and 100 pF capacitor are the necessary
external components for the fault detection circuitry.
The gate resistor (10 Ω) serves to limit gate charge cur-
rent and indirectly control the IGBT collector voltage
rise and fall times. The open collector fault output has
a passive 3.3 kΩ pull-up resistor and a 330 pF ltering
capacitor. A 47 kΩ pulldown resistor on VOUT provides a
more predictable high level output voltage (VOH). In this
application, the IGBT gate driver will shut down when a
fault is detected and will not resume switching until the
microcontroller applies a reset signal.