ACPL-5161 datasheet(21/33 Pages) AVAGO

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Typical Application/Operation

Introduction to Fault Detection and Protection

The power stage of a typical three phase inverter is sus-

ceptible 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 com-

putational 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 cur-

rent 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 shut-

down is an ideal solution, but the number of required

components, board space consumed, cost, and complex-

ity 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.

Applications Information

The ACPL-516x satisfies these criteria by combining a

high speed, high output current driver, high voltage op-

tical 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

DIP package.

The fault detection method, which is adopted in the

ACPL-516x, is to monitor the saturation (collector) volt-

age of the IGBT and to trigger a local fault shutdown se-

quence 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 dis-

abled 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 ACPL-516x 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 dissi-

pation 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 conserva-

tive overcurrent threshold is not needed to protect the

IGBT.

Recommended Application Circuit

The ACPL-516x has both inverting and non-inverting

gate control inputs, an active low reset input, and an

open collector fault output suitable for wired ‘OR’ appli-

cations. The recommended application circuit shown in

Figure 62 illustrates a typical gate drive implementation

using the ACPL-516x.

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 charg-

ing 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 cur-

rent 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.

ACPL-5161 Datasheet(PDF) 21 Page - AVAGO TECHNOLOGIES LIMITED

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