ACPL-331J datasheet(20/23 Pages) AVAGO

20 / 23 page

Output Control

The outputs (VOUT and FAULT) of the ACPL-331J are con-

trolled by the combination of IF, UVLO and a detected

IGBT Desat condition. Once UVLO is not active (VCC2 - VE >

VUVLO), VOUT is allowed to go high, and the DESAT (pin 14)

detection feature of the ACPL-331J will be the primary

source of IGBT protection. UVLO is needed to ensure

DESAT is functional. Once VUVLO+ > 10.5V, DESAT will

remain functional until VUVLO- < 11.1V. Thus, the DESAT

detection and UVLO features of the ACPL-331J work in

conjunction to ensure constant IGBT protection.

Desaturation Detection and High Current Protection

The ACPL-331J 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

ACPL-331J is to monitor the saturation (collector)

voltage of the IGBT and to trigger a local fault shutdown

sequence if the collector voltage exceeds a predeter-

mined 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 ACPL-331J 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 over

current threshold is not needed to protect the IGBT.

UVLO (VCC2 – VE)

Desat Condition Detected on Pin 14

Pin 3 (FAULT) Output

VOUT

Active

Low

Yes

Low

OFF

Low

Not Active

High

Slow IGBT Gate Discharge during Fault Condition

When a desaturation fault is detected, a weak pull-down

device in the ACPL-331J output drive stage will turn on

to ‘softly’ turn off the IGBT. This device slowly discharges

the IGBT gate to prevent fast changes in drain current

that could cause damaging voltage spikes due to lead

and wire inductance. During the slow turn off, the large

output pull-down device remains off until the output

voltage falls below VEE + 2 Volts, at which time the large

pull down device clamps the IGBT gate to VEE.

DESAT Fault Detection Blanking Time

The DESAT fault detection circuitry must remain disabled

for a short time period following the turn-on of the IGBT

to allow the collector voltage to fall below the DESAT

threshold. This time period, called the DESAT blanking

time is controlled by the internal DESAT charge current,

the DESAT voltage threshold, and the external DESAT

capacitor.

The nominal blanking time is calculated in terms of

external capacitance (CBLANK), FAULT threshold voltage

(VDESAT), and DESAT charge current (ICHG) as tBLANK =

CBLANK x VDESAT / ICHG. The nominal blanking time with

the recommended 100pF capacitor is 100pF * 6.5 V / 240

µA = 2.7 µsec.

The capacitance value can be scaled slightly to adjust the

blanking time, though a value smaller than 100 pF is not

recommended. This nominal blanking time represents

the longest time it will take for the ACPL-331J to respond

to a DESAT fault condition. If the IGBT is turned on while

the collector and emitter are shorted to the supply rails

(switching into a short), the soft shut-down sequence

will begin after approximately 3 µsec. If the IGBT collector

and emitter are shorted to the supply rails after the IGBT

is already on, the response time will be much quicker due

to the parasitic parallel capacitance of the DESAT diode.

The recommended 100pF capacitor should provide

adequate blanking as well as fault response times for

most applications.

ACPL-331J Datasheet(PDF) 20 Page - AVAGO TECHNOLOGIES LIMITED

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