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-off the overcurrents during a fault condition.
The alternative protection scheme of measuring IGBT
current to prevent desaturation is effective 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 HCPL-316J limits
the power dissipation in the IGBT even with insufficient
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- conser-
vative overcurrent threshold is not needed to protect
the IGBT.
Recommended Application Circuit
The HCPL-316J 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 HCPL-316J.
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.
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 suffices.
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 filtering
Applications InformationThe HCPL-316J satisfies these cri-
teria 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 an optically isolated fault status feedback sig-
nal into a single 16-pin surface mount package.
The fault detection method, which is adopted in the
HCPL-316J, 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 off. During the off
state of the IGBT, the fault detect circuitry is simply dis-
abled to prevent false ‘fault’signals.
capacitor. A 47 kΩ pulldown resistor on V
more predictable high level output voltage (V ). 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.
provides a
OH
OUT
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AVAGO [ AVAGO TECHNOLOGIES LIMITED ]
