MC33154

9

MOTOROLA ANALOG IC DEVICE DATA

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

7

VEE

VCC

VEE

Q

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

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

VEE

VCC

8

1.0 mA

6.5 V

Desaturation

Comparator

Kelvin

Gnd

D1

Kelvin

Gnd

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 VEE 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.

Because IGBTs have a very low incremental

on–resistance, sense IGBTs behave much like low–on

resistance current sense MOSFETs. The output voltage of a