MC33153
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10
saturation. When the collector current rises above the knee,
the device pulls out of saturation. The maximum current the
device will conduct in the linear region is a function of the
base current and the dc current gain (h
FE
) of the transistor.
The output characteristics of an IGBT are similar to a
Bipolar device. However, the output current is a function of
gate voltage instead of current. The maximum current
depends on the gate voltage and the device type. IGBTs tend
to have a very high transconductance and a much higher
current density under a short circuit than a bipolar device.
Motor control IGBTs are designed for a lower current
density under shorted conditions and a longer short circuit
survival time.
The best method for detecting desaturation is the use of a
high voltage clamp diode and a comparator. The MC33153
has a Fault Blanking/Desaturation Comparator which
senses the collector voltage and provides an output
indicating when the device is not fully saturated. Diode D1
is an external high voltage diode with a rated voltage
comparable to the power device. When the IGBT is “on” and
saturated, D1 will pull down the voltage on the Fault
Blanking/Desaturation Input. When the IGBT pulls out of
saturation or is “off”, the current source will pull up the input
and trip the comparator. The comparator threshold is 6.5 V,
allowing a maximum on−voltage of about 5.8 V.
A fault exists when the gate input is high and V
CE
is
greater than the maximum allowable V
CE(sat)
. The output of
the Desaturation Comparator is ANDed with the gate input
signal and fed into the Short Circuit and Overcurrent
Latches. The Overcurrent Latch will turn−off the IGBT for
the remainder of the cycle when a fault is detected. When
input goes high, both latches are reset. The reference voltage
is tied to the Kelvin Ground instead of the V
EE
to make the
threshold independent of negative gate bias. Note that for
proper operation of the Desaturation Comparator and the
Fault Output, the Current Sense Input must be biased above
the Overcurrent and Short Circuit Comparator thresholds.
This can be accomplished by connecting Pin 1 to V
CC
.
Figure 34. Desaturation Detection
V
CC
V
EE
V
CC
8
270 mA
V
ref
6.5 V
Desaturation
Comparator
Kelvin
GND
D1
The MC33153 also features a programmable fault
blanking time. During turn−on, the IGBT must clear the
opposing free−wheeling diode. The collector voltage will
remain high until the diode is cleared. Once the diode has
been cleared, the voltage will come down quickly to the
V
CE(sat)
of the device. Following turn−on, there is normally
considerable ringing on the collector due to the C
OSS
capacitance of the IGBTs and the parasitic wiring
inductance. The fault signal from the Desaturation
Comparator must be blanked sufficiently to allow the diode
to be cleared and the ringing to settle out.
The blanking function uses an NPN transistor to clamp the
comparator input when the gate input is low. When the input
is switched high, the clamp transistor will turn “off”,
allowing the internal current source to charge the blanking
capacitor. The time required for the blanking capacitor to
charge up from the on−voltage of the internal NPN transistor
to the trip voltage of the comparator is the blanking time.
If a short circuit occurs after the IGBT is turned on and
saturated, the delay time will be the time required for the
current source to charge up the blanking capacitor from the
V
CE(sat)
level of the IGBT to the trip voltage of the
comparator. Fault blanking can be disabled by leaving Pin 8
unconnected.
Sense IGBT Protection
Another approach to protecting the IGBTs is to sense the
emitter current using a current shunt or Sense IGBTs. This
method has the advantage of being able to use high gain
IGBTs which do not have any inherent short circuit
capability. Current sense IGBTs work as well as current
sense MOSFETs in most circumstances. However, the basic
problem of working with very low sense voltages still exists.
Sense IGBTs sense current through the channel and are
therefore linear with respect to the collector current.
Because IGBTs have a very low incremental on−resistance,
sense IGBTs behave much like low−on resistance current
sense MOSFETs. The output voltage of a properly
terminated sense IGBT is very low, normally less than
100 mV.
The sense IGBT approach requires fault blanking to
prevent false tripping during turn−on. The sense IGBT also
requires that the sense signal is ignored while the gate is low.
This is because the mirror output normally produces large
transient voltages during both turn−on and turn−off due to
the collector to mirror capacitance. With non−sensing types
of IGBTs, a low resistance current shunt (5.0 to 50 mW) can
be used to sense the emitter current. When the output is an
actual short circuit, the inductance will be very low. Since
the blanking circuit provides a fixed minimum on−time, the
peak current under a short circuit can be very high. A short
circuit discern function is implemented by the second
comparator which has a higher trip voltage. The short circuit
signal is latched and appears at the Fault Output. When a
short circuit is detected, the IGBT should be turned−off for
several milliseconds allowing it to cool down before it is
turned back on. The sense circuit is very similar to the
desaturation circuit. It is possible to build a combination
circuit that provides protection for both Short Circuit
capable IGBTs and Sense IGBTs.
