HCPL316J datasheet(21/33 Pages) HP

21 / 33 page

Typical Application/

Operation

Introduction to Fault

Detection and Protection

The power stage of a typical three

phase inverter is susceptible to

several types of failures, most of

which are potentially destructive

to the power IGBTs. These failure

modes can be grouped into four

basic categories: phase and/or

rail supply short circuits due to

user misconnect or bad wiring,

control signal failures due to

noise or computational errors,

overload conditions induced by

the load, and component failures

in the gate drive circuitry. Under

any of these fault conditions, the

current through the IGBTs can

increase rapidly, causing

excessive power dissipation and

heating. The IGBTs become

damaged when the current load

approaches the saturation current

of the device, and the collector to

emitter voltage rises above the

saturation voltage level. The

drastically increased power

dissipation very quickly overheats

the power device and destroys it.

To prevent damage to the drive,

fault protection must be

implemented to reduce or

turn-off the overcurrents during a

fault condition.

A circuit providing fast local fault

detection and shutdown is an

ideal solution, but the number of

required components, board

space consumed, cost, and

complexity have until now limited

its use to high performance

drives. The features which this

circuit must have are high speed,

low cost, low resolution, low

power dissipation, and small size.

The HCPL-316J satisfies these

criteria by combining a high

speed, high output current driver,

high voltage optical isolation

between the input and output,

local IGBT desaturation detection

and shut down, and an optically

isolated fault status feedback

signal into a single 16-pin surface

mount package.

The fault detection method,

which is adopted in the

HCPL-316J, is to monitor the

saturation (collector) voltage of

the IGBT and to trigger a local

fault shutdown sequence if the

collector voltage exceeds a

predetermined threshold. A small

gate discharge device slowly

reduces the high short circuit

IGBT current to prevent

damaging voltage spikes. Before

the dissipated energy can reach

destructive levels, the IGBT is

shut off. During the off state of

the IGBT, the fault detect

circuitry is simply disabled to

prevent false ‘fault’ signals.

The alternative protection

scheme of measuring IGBT

current to prevent desaturation is

effective if the short circuit

capability of the power device is

known, but this method will fail if

the gate drive voltage decreases

enough to only partially turn on

the IGBT. By directly measuring

the collector voltage, the

HCPL-316J limits the power

dissipation in the IGBT even with

insufficient gate drive voltage.

Another more subtle advantage of

the desaturation detection

method is that power dissipation

in the IGBT is monitored, while

the current sense method relies

on a preset current threshold to

predict the safe limit of

operation. Therefore, an overly-

conservative overcurrent

threshold is not needed to protect

the IGBT.

Recommended Application

Circuit

The HCPL-316J has both

inverting and non-inverting gate

control inputs, an active low reset

input, and an open collector fault

output suitable for wired ‘OR’

applications. The recommended

application circuit shown in

Figure 62 illustrates a typical

gate drive implementation using

the HCPL-316J.

The four supply bypass

capacitors (0.1

µF) provide the

large transient currents necessary

during a switching transition.

Because of the transient nature of

the charging currents, a low

current (5 mA) power supply

suffices. The desat diode and 100

pF capacitor are the necessary

external components for the fault

detection circuitry. The gate

resistor (10

Ω) serves to limit

gate charge current and indirectly

control the IGBT collector

voltage rise and fall times. The

open collector fault output has a

passive 3.3 k

Ω pull-up resistor

and a 330 pF filtering capacitor.

A 47 k

Ω pulldown resistor on

VOUT provides a more predictable

high level output voltage (VOH).

In this application, the IGBT gate

driver will shut down when a fault

is detected and will not resume

switching until the

microcontroller applies a reset

signal.

Applications Information

HCPL316J Datasheet(PDF) 21 Page - Agilent(Hewlett-Packard)

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