MC33153
10
MOTOROLA ANALOG IC DEVICE DATA
PROTECTION CIRCUITRY
Desaturation Protection
Bipolar Power circuits have commonly used what is known
as “Desaturation Detection”. This involves monitoring the
collector voltage and turning off the device if this voltage rises
above a certain limit. A bipolar transistor will only conduct a
certain amount of current for a given base drive. When the
base is overdriven, the device is in 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 33. Desaturation Detection
V
CC
V
EE
V
CC
8
270
µ
A
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 mΩ) 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.
