MC33154
OPERATING DESCRIPTION
While IGBTs exhibit a fixed minimum loss due to minority
GATE DRIVE
carrier recombination, a slow gate drive will dominate the
turn–off losses. This is particularly true for fast IGBTs. It is
also possible to turn–off an IGBT too fast. Excessive turn–off
speed will result in large overshoot voltages. Normally the
turn–off resistor is a small fraction of the turn–on resistor.
The MC33154 has a bipolar totem pole output. The output
stage is capable of sourcing 4.0 amps and sinking 2.0 amps
peak. The output stage also contains a pull down resistor to
ensure that the IGBT is off when the gate drive power is not
applied.
In a PWM inverter, IGBTs are used in a half–bridge
configuration. Thus, at least one device is always off. While
the IGBT is in the off–state it will be subjected to changes in
voltage caused by the other devices. This is particularly a
problem when the opposite transistor turns on.
When the lower device is turned on clearing the upper
diode, the turn–on dv/dt of the lower device appears across
the collector emitter of the upper device. To eliminate
shoot–through currents it is necessary to provide a low sink
impedance to the device in the off–state. Fortunately, the
turn–off resistor can be made small enough to hold off the
device under commutation without causing excessively fast
turn–off speeds.
Controlling Switching Times
The most important design aspect of an IGBT gate drive is
optimization of the switching characteristics. Switching
characteristics are especially important in motor control
applications in which PWM transistors are used in a bridge
configuration. In these applications, the gate drive circuit
components should be selected to optimize turn–on, turn–off,
and off–state impedance.
A single resistor may be used to control both turn–on and
turn–off and shown in Figure 26. However, the resistor value
selected must be a compromise in turn–on abruptness and
turn–off losses. Using a single resistor is normally suitable
only for very low frequency PWM.
Figure 26. Using a Single Gate Resistor
V
CC
IGBT
R
g
Output
5
Sometimes a negative bias voltage is used in the
off–state. This is a practice carried over from bipolar
Darlington drives. A negative bias is generally not required
for IGBTs. However, a negative bias will reduce the possibility
Kelvin Gnd
V
EE
2
of shoot–through. The MC33154 has separate pins for V
and Kelvin Gnd. This permits operation using a +15/–5 volt
supply.
EE
AnoptimizedgatedriveoutputstageisshowninFigure27.
This circuit allows turn–on and turn–off to be optimized
separately.
INTERFACING WITH OPTOISOLATORS
Isolated Input
Figure 27. Using Separate Resistors
for Turn–On and Turn–Off
The MC33154 may be used with an optically isolated
input. The optoisolator can be used to provide level shifting
and if desired, isolation from AC line voltages. An optoisolator
with a very high dv/dt capability should be used, such as the
Hewlett–Packard HCPL0453. The IGBT gate turn–on
resistor should be set large enough to ensure that the opto’s
dv/dt capability is not exceeded. Like most optoisolators, the
HCPL0453 has an active low open–collector output. Thus,
when the LED is ON, the output will be low. The MC33154
has a non–inverting input pin to interface directly with an
optoisolator using a pull up resistor.
V
CC
IGBT
R
on
Output
D
R
off
off
5
Kelvin Gnd
V
EE
2
Optoisolator Output Fault
The turn–on resistor R provides control over the IGBT
on
The MC33154 has an active high fault output. The fault
output may be easily interfaced to an optoisolator. While it is
important that all faults are properly reported, it is equally
important that no false signals are propagated. Again a high
dv/dt optoisolator should be used.
turn–on speed. In motor control circuits, the resistor sets the
turn–on di/dt that controls how fast the free–wheel diode is
cleared. The interaction of the IGBT and freewheeling diode
determines the turn–on dv/dt.
Excessive turn–on dv/dt is a common problem in
half–bridge circuits.
The LED drive provides a resistor programmable current
of 10 to 20 mA when on and provides a low impedance path
when off.
The turn–off resistor R controls the turn–off speed and
off
ensures that the IGBT remains off under commutation
stresses. Turn–off is critical to obtain low switching losses.
8
MOTOROLA ANALOG IC DEVICE DATA
MOTOROLA [ MOTOROLA ]
