IL33153
OPERATING DESCRIPTION
GATE DRIVE
Controlling Switching Times
The most important design aspect of an IGBT gate
drive is optimization of the switching characteristics.
The 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 op-
timize turn−on, turn−off and off−state impedance. A
single resistor may be used to control both turn−on and
turn−off as shown in Figure 31. However, the resistor
value selected must be a compromise in turn−on abrupt-
ness and turn−off losses. Using a single resistor is nor-
mally suitable only for very low frequency PWM. An
optimized gate drive output stage is shown in Figure 32.
This circuit allows turn−on and turn−off to be optimized
separately. The turn−on resistor, Ron, provides control
over the IGBT 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 free−wheeling diode determines the turn−on
dv/dt. Excessive turn−on dv/dt is a common problem in
half−bridge circuits. The turn−off resistor, Roff, controls
the turn−off speed and ensures that the IGBT remains
off under commutation stresses. Turn−off is critical to
obtain low switching losses. While IGBTs exhibit a
fixed minimum loss due to minority carrier recombina-
tion, a slow gate drive will dominate the turn−off losses.
This is particularly true for fast IGBTs. It is also possi-
ble 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.
Figure 31. Using a Single Gate Resistor
Figure 32. Using Separate Resistors
for Turn−On and Turn−Off
A negative bias voltage can be used to drive the
IGBT into the off−state. This is a practice carried over
from bipolar Darlington drives and is generally not re-
quired for IGBTs. However, a negative bias will reduce
the possibility of shoot−through. The IL33153 has sepa-
rate pins for VEE and Kelvin Ground. This permits op-
eration using a +15/−5.0 V supply.
The IL33153 contains a bipolar totem pole output
stage that is capable of sourcing 1.0 amp and sinking 2.0
amps peak. This output also contains a pull down resis-
tor to ensure that the IGBT is off whenever there is in-
sufficient VCC to the IL33153.
INTERFACING WITH OPTOISOLATORS
Isolated Input
In a PWM inverter, IGBTs are used in a half−bridge
configuration. Thus, at least one device is always off.
Whilethe 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.
The IL33153 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 HCPL4053. 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 HCPL4053 has an active
low open−collector output. Thus, when the LED is on,
the output will be low. The IL33153 has an inverting
input pin to interface directly with an optoisolator using
a pullup resistor. The input may also be interfaced di-
rectly to 5.0 V CMOS logic or a microcontroller.
When the lower device is turned on, clearing the up-
per 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 pro-
vide a low sink impedance to the device that is in the
off−state. In most applications the turn−off resistor can
be made small enough to hold off the device that is un-
der commutation without causing excessively fast
turn−off speeds.
Rev. 02
IKSEMICON [ IK SEMICON CO., LTD ]
