ACPL-38JT Datasheet, PDF (22/33 Pages) AVAGO TECHNOLOGIES LIMITED – 2.5 A maximum peak output current

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 compu-
tational 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 dis-
sipation 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 protec-
tion must be implemented to reduce or turn--off the over-
currents during a fault condition.
A circuit providing fast local fault detection and shutdown
is an ideal solution, but the number of required compo-
nents, 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.
Applications Information
The ACPL-38JT satisfies these criteria by combining a high
speed, high output current driver, high voltage optical
isolation between the input and output, local IGBT de-
saturation detection and shut down, and an optically
isolated fault status feedback signal into a single 16-pin
surface mount package.
The fault detection method, which is adopted in the
ACPL-38JT, 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 de-
structive levels, the IGBT is shut off. During the off 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 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 ACPL-38JT limits the power
dissipation in the IGBT even with insufficient gate drive
voltage. Another more subtle advantage of the desatu-
ration 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-38JT has both inverting and non-inverting gate
control inputs, an active low reset input, and an open
collector fault output suitable for wired ‘OR’ applications.
The recommended application circuit shown in Figure 61
illustrates a typical gate drive implementation using the
ACPL-38JT.
The four supply bypass capacitors (0.1 F) provide the
large transient currents necessary during a switching
transition. Because of the transient nature of the charging
currents, a low current (5 mA) power supply suffices.
The desat diode and 100pF capacitor are the necessary
external components for the fault detection circuitry. The
gate resistor (10 ) serves to limit gate charge current and
indirectly control the IGBT collector voltage rise and fall
times. The open collector fault output has a passive 3.3 k
pull-up resistor and a 330 pF filtering capacitor.
A clamping diode between V_CC1 and RESET will prevent
positive going voltage noises affecting the FAULT status.
A 47 k pulldown resistor on V_OUT provides a more
predictable high level output voltage (V_OH). In this appli-
cation, the IGBT gate driver will shut down when a fault is
detected and will not resume switching until the micro-
controller applies a reset signal.
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