8

IXDD408PI/408SI//408YI/408CI

Short Circuit di/dt Limit

A short circuit in a high-power MOSFET module such as the

VM0580-02F, (580A, 200V), as shown in Figure 26, can cause

the current through the module to flow in excess of 1500A for

10

µs or more prior to self-destruction due to thermal runaway.

For this reason, some protection circuitry is needed to turn off

the MOSFET module. However, if the module is switched off

too fast, there is a danger of voltage transients occuring on the

drain due to Ldi/dt, (where L represents total inductance in

series with drain).

If these voltage transients exceed the

MOSFET's voltage rating, this can cause an avalanche break-

down.

The IXDD408 has the unique capability to softly switch off the

high-power MOSFET module, significantly reducing these

Ldi/dt transients.

Thus, the IXDD408 helps to prevent device destruction from

both dangers; over-current, and avalanche breakdown due to

di/dt induced over-voltage transients.

The IXDD408 is designed to not only provide ±8A under normal

conditions, but also to allow it's output to go into a high

impedance state. This permits the IXDD408 output to control

a separate weak pull-down circuit during detected overcurrent

shutdown conditions to limit and separately control d

turnoff. This circuit is shown in Figure 27.

Referring to Figure 27, the protection circuitry should include

a comparator, whose positive input is connected to the source

of the VM0580-02. A low pass filter should be added to the input

of the comparator to eliminate any glitches in voltage caused

by the inductance of the wire connecting the source resistor to

ground. (Those glitches might cause false triggering of the

comparator).

The comparator's output should be connected to a SRFF(Set

Reset Flip Flop). The flip-flop controls both the Enable signal,

and the low power MOSFET gate. Please note that CMOS 4000-

series devices operate with a V

18 VDC being the maximum allowable limit).

A low power MOSFET, such as the 2N7000, in series with a

resistor, will enable the VMO580-02F gate voltage to drop

gradually. The resistor should be chosen so that the RC time

constant will be 100us, where "C" is the Miller capacitance of

the VMO580-02F.

For resuming normal operation, a Reset signal is needed at

the SRFF's input to enable the IXDD408 again. This Reset can

be generated by connecting a One Shot circuit between the

IXDD408 Input signal and the SRFF restart input. The One Shot

will create a pulse on the rise of the IXDD408 input, and this

pulse will reset the SRFF outputs to normal operation.

When a short circuit occurs, the voltage drop across the low-

value, current-sensing resistor, (Rs=0.005 Ohm), connected

between the MOSFET Source and ground, increases. This

triggers the comparator at a preset level. The SRFF drives a low

input into the Enable pin disabling the IXDD408 output. The

SRFF also turns on the low power MOSFET, (2N7000).

In this way, the high-power MOSFET module is softly turned off

by the IXDD408, preventing its destruction.

10uH

Ld

0.1ohm

Rd

Rs

20nH

Ls

1ohm

Rg

10kohm

R+

VMO580-02F

High_Power

5kohm

Rcomp

100pF

C+

+

-

V+

V-

Comp

LM339

1600ohm

Rsh

Ccomp

1pF

VCC

VCCA

IN

EN

GND

SUB

OUT

IXDD408

+

-

VIN

+

-

VCC

+

-

REF

+

-

VB

CD4001A

NOR2

1Mohm

Ros

NOT2

CD4049A

CD4011A

NAND

CD4049A

NOT1

CD4001A

NOR1

CD4049A

NOT3

Low_Power

2N7002/PLP

1pF

Cos

0

S

R

EN

Q

One ShotCircuit

SR Flip-Flop

APPLICATIONS INFORMATION

Figure 27 - Application Test Diagram