MC33154
properly terminated sense IGBT is very low, normally less
than 100 mV.
connected to Gnd. The input optoisolator, however, should
be referenced to V
.
EE
The sense IGBT approach requires a blanking time 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 normally produces large transient
voltages during both turn–on and turn–off due to the collector
to mirror capacitance.
A low resistance current shunt may also be used to sense
the emitter current. A very low resistance shunt (5.0 mΩ to
50 mΩ) must be used with high current IGBTs. The output
voltage of a current shunt is also very low.
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 may
be very high. A short circuit discern function may be
implemented using a second comparator with a higher trip
voltage.
Figure 31. Dual Supply Application
15 V
6
V
8
5
CC
Desat/
Blank
7
4
Fault
Output
MC33154
1
2
Sense
Gnd
Input
V
EE
3
This circuit can distinguish between an overcurrent and a
shorted output condition. Under an actual short circuit the die
temperature may get very hot. When a short circuit is
detected the transistor should be turned–off for several
milliseconds to cool down before the device is turned back
on.
The sense circuit is very similar to the Desaturation circuit.
The MC33154 uses a combination circuit that provides
protection for both Short Circuit capable IGBTs and Sense
IGBTs.
–5.0 V
If Desaturation protection is desired as shown in Figure
32, a high voltage diode is connected to the
Desaturation/Blanking pin. The blanking capacitor should be
connected from the Desaturation pin to the V
supply is used the blanking capacitor should be connected to
the Kelvin Gnd.
pin. If a dual
EE
Because desaturation protection is used in this example,
the sense input should be tied high. The MC33154 design
ANDs the output of the overcurrent comparators with the
output of the desaturation comparator, allowing the circuit
designer to choose either type of protection.
Although the reverse voltage on collector of the IGBT is
clamped to the emitter by the free wheeling diode, there is
normally considerable inductance within the package itself. A
small resistor in series with the diode may be used to protect
the IC from reverse voltage transients.
APPLICATION EXAMPLES
The simplest gate drive circuit using the MC33154 is
shown in Figure 30. The optoisolator requires a pull up
resistor. This resistor value should be set to bias the output
transistor at the desired current. A decoupling capacitor
should be placed close to the IC to minimize switching noise.
A bootstrap diode may be used to for a floating supply. If
the protection features are not used, then both the
desaturation input and the current sense input should be
grounded.
Figure 32. Desaturation Application
When used with a single supply the Kelvin Gnd and V
pins should be connected. Separate resistors are
recommended for turn–on and turn–off.
EE
18 V
6
Figure 30. Basic Application
8
5
V
Desat/
Blank
7
4
CC
Fault
18 V
MC33154
Output
6
1
2
V
8
5
Sense
Gnd
CC
Desat/
Blank
7
4
Input
V
EE
3
Fault
Output
MC33154
1
2
Sense
Gnd
Input
V
When using sense IGBTs or a sense resistor, as shown in
Figure 33, the sense voltage is applied to the current sense
input. The sense trip voltages are referenced to the Kelvin
Gnd pin. The sense voltage is very small, typically about 65
mV, and sensitive to noise.
Therefore, the sense and ground return conductors should
be routed as a differential pair. An RC filter is useful in filtering
any high frequency noise. A blanking capacitor is connected
EE
3
When used with a dual supply as shown in Figure 31, the
Gnd pin should be Kelvin connected to the emitter of the
IGBT. If the protection features are not used, then both the
desaturation input and the current sense input should be
10
MOTOROLA ANALOG IC DEVICE DATA
MOTOROLA [ MOTOROLA ]
