ACPL-336J-000E View Datasheet(PDF) - Avago Technologies
Part Name
Description
MFG CO.
ACPL-336J-000E
Avago Technologies
'ACPL-336J-000E' PDF : 18 Pages View PDF
DESAT Diode and DESAT Threshold
The DESAT diode's function is to conduct forward current, allowing sensing of the IGBT's saturated collector-to-emitter
voltage, VCESAT, (when the IGBT is "on") and to block high voltages (when the IGBT is "off").
When the IGBT is switching off and toward the end of the forward conduction of the DESAT diode, a reverse current
will flow for short time. This reverse recovery effect prevents the diode from achieving its blocking capability until the
mobile charge in the junction is depleted. During this time, there is commonly a very high dVCE/dt voltage ramp rate
across the IGBT’s collector-to-emitter. This results in ICHARGE = CD-DESAT x dVCE/dt charging current which will charge
the blanking capacitor, CBLANK. To minimize this charging current and avoid false DESAT triggering, it is best to use fast-
response diodes.
In the recommended application circuit shown in Figure 20, the voltage on pin 14 (DESAT) is VDESAT = VF + VCE, where VF
is the forward ON voltage of DDESAT and VCE is the IGBT collector-to-emitter voltage. The value of VCE that triggers DESAT
to signal a FAULT condition is nominally 7 V – VF. If desired, this DESAT threshold voltage can be decreased by using
multiple DESAT diodes or low-voltage Zener diode in series. If n is the number of DESAT diodes, the nominal threshold
value becomes VCE,FAULT(TH) = 7 V – n × VF. If a Zener diode is used, the nominal threshold value becomes VCE,FAULT(TH) = 7
V – VF – VZ. When using two diodes instead of one, then diodes with half of the total required maximum reverse-voltage
rating may be chosen.
VEE2 16
VEE2 16
VLED 15
DESAT 14
VE 13
1 k Ω D ZENER D DESAT
CBLANK
+
VCE
Q1 -
Figure 25. DESAT diode and DESAT threshold
VLED 15
DESAT 14
VE 13
10 V Zener
1N5925A
1 k Ω D DESAT
CBLANK
Schottky
Diode
MBR0540
+
VCE
Q1 -
Figure 26. False fault prevention diodes
DESAT Pin Protection Resistor
The freewheeling of flyback diodes connected across the IGBTs can have large instantaneous forward voltage transients
that greatly exceed the nominal forward voltage of the diode. This may result in a large negative voltage spike on the
DESAT pin, which will draw substantial current out of the driver if protection is not used. To limit this current to levels that
will not damage the driver IC, make sure a 1 kΩ resistor is inserted in series with the DESAT diode.
False Fault Prevention Diodes
A situation that may cause the driver to generate a false fault signal is if the substrate diode of the driver becomes
forward biased. This can happen if the reverse recovery spikes coming from the IGBT freewheeling diodes bring the
DESAT pin below Ground. Therefore, the DESAT pin voltage will be ‘brought’ above the threshold voltage. This negative
going voltage spikes are typically generated by inductive loads or reverse recovery spikes of the IGBT/MOSFETs free-
wheeling diodes. To prevent a false fault signal, it is highly recommended that you connect a Zener diode and a Schottky
diode across the DESAT pin and VE pin
Figure 26 shows this circuit solution. The Schottky diode will prevent the substrate diode of the gate driver optocoupler
from being forward biased while the Zener diode (10 V) is used to prevent any positive high transient voltage from
affecting the DESAT pin.
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Data subject to change. Copyright © 2005-2014 Avago Technologies. All rights reserved.
AV02-4391EN - May 9, 2014
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