23
Slow IGBT Gate
Discharge During Fault
Condition
When a desaturation fault is
detected, a weak pull-down
device in the HCPL-316J output
drive stage will turn on to ‘softly’
turn off the IGBT. This device
slowly discharges the IGBT gate
to prevent fast changes in drain
current that could cause
damaging voltage spikes due to
lead and wire inductance. During
the slow turn off, the large output
pull-down device remains off until
value can be scaled slightly to
adjust the blanking time, though
a value smaller than 100 pF is not
recommended. This nominal
blanking time also represents the
longest time it will take for the
HCPL-316J to respond to a
DESAT fault condition. If the
IGBT is turned on while the
collector and emitter are shorted
to the supply rails (switching into
a short), the soft shut-down
sequence will begin after
approximately 3 µsec. If the IGBT
collector and emitter are shorted
to the supply rails after the IGBT
is already on, the response time
will be much quicker due to the
parasitic parallel capacitance of
the DESAT diode. The
recommended 100 pF capacitor
should provide adequate blanking
as well as fault response times for
most applications.
region and quickly overheat. The
UVLO function causes the output
to be clamped whenever
insufficient operating supply
(V
CC2
) is applied. Once V
CC2
exceeds V
(the positive-
UVLO+
going UVLO threshold), the
UVLO clamp is released to allow
the device output to turn on in
response to input signals. As
V
CC2
is increased from 0 V (at
some level below V
), first
UVLO+
the DESAT protection circuitry
becomes active. As V is
CC2
further increased (above
), the UVLO clamp is
V
UVLO+
the output voltage falls below V
+ 2 Volts, at which time the large
pull down device clamps the
EE
released. Before the time the
UVLO clamp is released, the
DESAT protection is already
active. Therefore, the UVLO and
DESAT FAULT DETECTION
features work together to provide
seamless protection regardless of
IGBT gate to V .
EE
DESAT Fault Detection
Blanking Time
The DESAT fault detection
circuitry must remain disabled for
a short time period following the
turn-on of the IGBT to allow the
collector voltage to fall below the
DESAT theshold. This time
period, called the DESAT
supply voltage (V ).
CC2
Under Voltage Lockout
The HCPL-316J Under Voltage
Lockout (UVLO) feature is
designed to prevent the
application of insufficient gate
voltage to the IGBT by forcing
the HCPL-316J output low during
power-up. IGBTs typically require
gate voltages of 15 V to achieve
blanking time, is controlled by
the internal DESAT charge
current, the DESAT voltage
threshold, and the external
DESAT capacitor. The nominal
blanking time is calculated in
terms of external capacitance
their rated V
voltage. At
CE(ON)
gate voltages below 13 V
typically, their on-voltage
(C
), FAULT threshold
BLANK
increases dramatically, especially
at higher currents. At very low
gate voltages (below 10 V), the
IGBT may operate in the linear
voltage (V
charge current (I
), and DESAT
DESAT
) as
CHG
t
= C
x V
/ I
.
BLANK
BLANK
DESAT CHG
The nominal blanking time with
the recommended 100 pF
capacitor is 100 pF * 7 V / 250 µA
= 2.8 µsec. The capacitance
AGILENT [ AGILENT TECHNOLOGIES, LTD. ]
