11

IXDD504 / IXDE504

APPLICATIONS INFORMATION

Short Circuit di/dt Limit

A short circuit in a high-power MOSFET such as the IXFN100N20,

(20A, 1000V), as shown in Figure 32, can cause the current

through the module to flow in excess of 60A 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 IXDD504 and IXDE504 have the unique capability, with

additionalcircuitry,tosoftlyswitchoffthehigh-powerMOSFET

module, significantly reducing these Ldi/dt transients.

Thus, the IXDD504 & IXDE504 help to prevent device destruc-

tion from both dangers; over-current, and avalanche break-

down due to di/dt induced over-voltage transients.

The IXDD504 & IXDE504 are designed to not only provide ±4A

per output under normal conditions, but also to allow their

outputs to go into a high impedance state. This permits the

IXDD504 or IXDE504 outputs to control a separate weak pull-

down circuit during detected overcurrent shutdown conditions

to limit and separately control d

is shown in Figure 33.

Referring to Figure 33, the protection circuitry should include

a comparator, whose positive input is connected to the source

of the IXFN100N20. 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 2N7002, in series with a

resistor, will enable the IXFN100N20 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 IXFN100N20.

For resuming normal operation, a Reset signal is needed at

the SRFF's input to enable the IXDD504 again. This Reset can

be generated by connecting a One Shot circuit between the

IXDD504 Input signal and the SRFF restart input. The One Shot

will create a pulse on the rise of the IXDD504 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 IXDD504 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 IXDD504, preventing its destruction.

10uH

Ld

0.1

Rd

Rs

20nH

Ls

1

Rg

10k

R+

IXFN100N20

5k

Rcomp

100pF

C+

+

-

V+

V-

Comp

LM339

1600

Rsh

Ccomp

1pF

VCC

IN

EN

DGND

OUT

IXDD504

+

-

VIN

+

-

VCC

+

-

REF

+

-

VB

CD4001A

NOR2

1M

Ros

NOT2

CD4049A

CD4011A

NAND

CD4049A

NOT1

CD4001A

NOR1

CD4049A

NOT3

Low_Power

2N7000

1pF

Cos

S

R

EN

Q

One Shot Circuit

SR Flip-Flop

Figure 33 - Application Test Diagram