Datasheet

BLK DSTH
BLK
CHG
C V 100 pF 7.2 V
t = = = 2.7 μs
I 270 μA
´ ´
+
-
V
C
V
CC2
Gate
Drive
15V
15V
7.2V Q1a
Q1b
Q2aQ2b
Q3
V
OUT
V
EE-P
V
E
Off
On
DESAT
Slow
Off
DESAT
C
BLK
ISO5500
Fault
V
CE
V
IN+
Fault
V
DESAT
V
OUT
7.2V
V
EE-L
Q4
Q4
Dschg
ISO5500
SLLSE64C –SEPTEMBER 2011–REVISED JUNE 2013 www.ti.com
Figure 59. DESAT Fault Detection and Protection
The DESAT fault detection involves a comparator that monitors the IGBT’s V
CE
and compares it to an internal 7.2
V reference. If V
CE
exceeds this reference voltage, the comparator causes the gate-drive and fault-logic to initiate
a fault shutdown sequence. This sequence starts with the immediate generation of a fault signal, which is
transmitted across the isolation barrier towards the Fault indicator circuit at the input side of the ISO5500.
At the same time the fault logic turns off the power-pair Q1 and turns on the small discharge MOSFETs, Q3 and
Q4. Q3 slowly discharges the IGBT gate voltage which causes the high short-circuit current through the IGBT to
gradually decrease, thereby preventing large di/dt induced voltage transients. Q4 discharges the blanking
capacitor. Once V
OUT
is sufficiently close to V
EE-P
potential (at approximately 2 V), the large Q2-pair turns on in
addition to Q3 to clamp the IGBT gate to V
EE-P
.
NOTE
The DESAT detection circuit is only active when the IGBT is turned on. When the IGBT is
turned off, and its V
CE
is at maximum, the fault detection is simply disabled to prevent
false triggering of fault signals.
DESAT BLANKING TIME
The DESAT fault detection must remain disabled for a short time period following the turn-on of the IGBT to allow
its collector voltage to drop below the 7.2 V DESAT threshold. This time period, called the DESAT blanking time,
t
BLK
, is controlled by an internal charge current of I
CHG
= 270 μA, the 7.2 V DESAT threshold, V
DSTH
, and an
external blanking capacitor, C
BLK
.
The nominal blanking time with a recommended capacitor value of C
BLK
= 100 pF is calculated with:
(1)
The capacitor value can be scaled slightly to adjust the blanking time. However, because the blanking capacitor
and the DESAT diode capacitance build a voltage divider that attenuates large voltage transients at DESAT,
C
BLK
values smaller than 100 pF are not recommended. The nominal blanking time also represents the ISO5500
maximum response time to a DESAT fault condition.
If a short circuit condition exists prior to the turn-on of the IGBT, (causing the IGBT switching into a short) the soft
shutdown sequence begins after approximately 3 μs. However, if a short circuit condition occurs while the IGBT
is already on, the response time is significantly shorter due to the parasitic parallel capacitance of the DESAT
diode. The recommended value of 100 pF however, provides sufficient blanking and fault response times for
most applications.
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