MC33153

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9

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 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 under commutation without

causing excessively fast turn−off speeds.

Figure 31. Using a Single Gate Resistor

Output

5

3

IGBT

Figure 32. Using Separate Resistors

for Turn−On and Turn−Off

Output

5

3

IGBT

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 required for IGBTs.

However, a negative bias will reduce the possibility of

Kelvin Ground. This permits operation using a +15/−5.0 V

supply.

INTERFACING WITH OPTOISOLATORS

Isolated Input

The MC33153 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 MC33153 has an inverting input pin to

interface directly with an optoisolator using a pullup

resistor. The input may also be interfaced directly to 5.0 V

CMOS logic or a microcontroller.

Optoisolator Output Fault

The MC33153 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.

The LED drive provides a resistor programmable current

of 10 to 20 mA when on, and provides a low impedance path

when off. An active high output, resistor, and small signal

diode provide an excellent LED driver. This circuit is shown

in Figure 33.

Figure 33. Output Fault Optoisolator

Short Circuit

Latch Output

7

Q

UNDERVOLTAGE LOCKOUT

It is desirable to protect an IGBT from insufficient gate

voltage. IGBTs require 15 V on the gate to achieve the rated

on−voltage. At gate voltages below 13 V, the on−voltage

increases dramatically, especially at higher currents. At very

low 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 MC33153 will typically start up at about 12 V. The

UVLO circuit has about 1.0 V of hysteresis and 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 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