MC33154
An active high output, resistor, and small signal diode
provide an excellent LED driver. This circuit is shown in
Figure 28.
Figure 28. Output Fault Optoisolator
Short Circuit
Latch Output
voltage on the desaturation input. The voltage reference is
set to about 6.5 V. This will allow a maximum ON–voltage of
about 5.0 V.
Figure 29. Desaturation Detection Using a Diode
VCC
Desaturation
Comparator
1.0 mA
8
VCC
VCC
D1
Q
7
Kelvin
Gnd
VEE
VEE
Kelvin
Gnd
6.5 V
VEE
UNDER VOLTAGE LOCK OUT
It is desirable to protect an IGBT from insufficient gate
voltage. IGBTs require 15 V on the gate to guarantee device
saturation. At gate voltages below 13 V, the “on” state voltage
increases dramatically, especially at higher currents. At very
lower gate voltages, below 10 V, the IGBT may operate in the
linear region and quickly overheat. Many PWM motor drives
use a bootstrap supply for the upper gate drive. The UVLO
provides protection for the IGBT in case the bootstrap
capacitor discharges.
The MC33154 will typically start up at about 12 V. The
UVLO circuit has about 1.0 volt of hysteresis. The UVLO will
disable the output if the supply voltage falls below about 11 V.
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 the collector
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 hfe 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, not current. The maximum current depends on
the gate voltage and the device. 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 MC33154
has a desaturation comparator which senses the collector
voltage and provides an output indicating when the device is
not full 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, diode D1 will pull down the
voltage on the desaturation input. When the IGBT is OFF or
pulls out of saturation, the current source will pull up the
MOTOROLA ANALOG IC DEVICE DATA
A fault exists when the gate input is high and VCE of the
IGBT is greater than the maximum allowable VCE(sat).The
output of the desaturation comparator is ANDed with the gate
input signal and fed into the Short Circuit (SC) latch. The SC
latch will turn–off the IGBT for the remainder of the cycle
when a fault is detected. When the input is toggled low, the
latch will reset. The reference voltage is tied to the Kelvin
Ground instead of the V EE to make the threshold
independent of negative gate bias.
The MC33154 also features a programmable turn–on
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
VCE(sat) of the device. Following turn–on there is normally
considerable ringing on the collector due to the Coss of the
IGBTs and the parasitic wiring inductance.
The error signal from the desaturation signal must be
blanked out 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, and the
current source will charge up the blanking capacitor. The time
required for blanking capacitor to charge up from the
on–voltage of the clamp FET 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
VCE(sat) to the trip voltage of the comparator.
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 concerning collector current.
B e c a u s e I G B Ts h a v e a v e r y l o w i n c r e m e n t a l
on–resistance, sense IGBTs behave much like low–on
resistance current sense MOSFETs. The output voltage of a
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