Hitachi IGBT Module Application Manual
5.8
Short Circuit Protection
5.8.1 Short Circuit Pattern
The short circuit pattern in the inverter can occur in two ways:
(1)
In the inverter side (due to IGBT module destruction, control circuit trouble,
equipment ground fault, etc.)
(2)
In the load side (due to connection error, load trouble, ground fault, etc.)
5.8.2 IGBT Operation during Short Circuit
When the short circuit occurred, in the case where an IGBT is maintained in either the ON steady state
or turn-ON state, the short-circuit current increases up to the IGBT’s saturation current. (This current
increases up to approximately six (6) times the rated current.) Also, almost the entire circuit voltage is
applied to the IGBT. If such a status occurs continuously, the IGBT will be destroyed. To prevent this
phenomenon, the short-circuit current must be cut off before the onset of IGBT destruction is approached.
5.8.3 Short-circuit Current Cut-off
To prevent IGBT destruction, whatever protection method is chosen must cut off the current within 10
microseconds after short-circuit current has begun to flow. See Figure 50. Two cut-off protection methods
and their respective features are described below.
5.8.3.1
Hard Cut-off
Method: IGBT normally turns OFF after detecting an over-current. Such monitoring of
over-current would be performed using a Current Transformer (CT), etc.
Feature: Only current detecting. Large turn-OFF overshoot voltage.
Note:
The circuitry needs to control overshoot voltage. In cases where the
line impedance is large, precautions should be taken to prevent the
snubber circuit voltage from increasing.
5.8.3.2
Soft Cut-off
Method: The gate voltage is controlled after detecting the overcurrent. The driver circuit
monitors V
CE
(sat) increasing and controls gate voltage.
Feature: Small overshoot voltage. Driver needs the monitoring function of V
CE
of one
arm in order to be able to see the troubled arm.
Note:
It is necessary to properly set up detection delay in order to be able
to observe this behavior during a normal IGBT turn ON operation.
Figure 50. Short-circuit Protection Waveforms
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