9020 Datasheet, PDF (16/25 Pages) Fairchild Semiconductor – IGBT Basic II

4. Short circuit protection
We have discussed types of short circuits, and several ways to prevent short circuits have
been reviewed. However, methods mentioned above are not fundamental ways to deal with
short circuit, so there has to be a way to safely turn off the device when it short circuits.
A. Protecting against over-current condition
Over-current is when more than the rated current flows through the system, and it can be clas-
sified into over-load, short-circuit, turn-on over-current. In traditional applications, over-current
is possible in several cases. Generally, over-current from over-load comes from inrush current,
filter inrush and a rapid change in load during beginning of operation of electrical devices. In
this case, we can only rely on short circuit capability of the device. Over-load, in general, lasts
much longer than the IGBT’s short circuit endurance time. As such, other methods must be
sought to remove the overload. Closed loop control moderates the timing signal of the gate
drive pulse, to modifies the time of switching, and this is used to keep the current output at a
determined level. Response control of the control loop would have to be set to the rate of
changes in the current and pace of the electrical devices or filter inductance. Protection from
over-current due to short-circuit is different from turn-off over current. In the following sections,
protection from short-circuit would be discussed.
B. Protecting against short circuit current condition
In the overload situations mentioned above, removing the closed loop does not considerably
shorten the life of the IGBT. On the other hand, short circuit provides worse condition for the
life of the device than overload or the over-current at turn-on, and there are ground faults, ter-
minal-to-terminal faults. Such short circuit current bypasses the electrical devices or filter
inductance and increases rapidly for IGBT to flow. Conventional PWM loop controls power
output, but it has no control over this type of fault. At the beginning of the fault, the IGBT must
withstand with its own short circuit capability, and protection mechanism receives the fault sig-
nal to reduce the gate voltage while IGBT withstands the short circuit. However, if the fault dis-
appears while during IGBT’s endurance time, then the IGBT must continue to function and
must not turn off unnecessary devices or turn off the entire system. The most notable is the
IGBT turn-on over-current due to the reverse recovery current of the diode. As such, the pro-
tection circuit must be designed to return the circuit to normal operation if the fault is removed
before the IGBT shuts down the system.
When conduction time increases, the border of SOA (SCSOA) decreases. Junction tempera-
ture increases instantly during on state, and as a result, the maximum possible controllable
current decreases. As such, short circuit, whose current rises rapidly, must be turned off in a
very short period of time, and the time from moment short circuit occurs to the moment it is
turned off is should be within 10us. In addition, soft turn-off, which slowly cuts off the short cir-
cuit current, is necessary as abrupt turn-off leads to a large dv/dt, which increases the possibil-
ity for a latch-up. Principles of short circuit protection circuit design to be added in a gate drive
can be summed up as the following:
Fault must be detected as soon as possible.
Must suppress fault current to secure more time for the protection circuit to respond, and send
the gate off signal as quickly as possible.
Induce soft turn-off to avoid the dangers of turn-off over-voltage.
We shall discuss the short circuit protection scheme in detail in the following sections.
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Rev. A, April 2002