11

IXDN430 / IXDI430 / IXDD430 / IXDS430

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 27, 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 IXDD430 has the unique capability to softly switch off the

high-power MOSFET module, significantly reducing these

Ldi/dt transients.

Thus, the IXDD430 helps to prevent device destruction from

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

di/dt induced over-voltage transients.

The IXDD430 is designed to not only provide ±30A under

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

impedance state. This permits the IXDD430 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 28.

Referring to Figure 28, 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

(with 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 IXDD430 again. This Reset can

be generated by connecting a One Shot circuit between the

IXDD430 Input signal and the SRFF restart input. The One Shot

will create a pulse on the rise of the IXDD430 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 IXDD430 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 IXDD430, preventing its destruction.

APPLICATIONS INFORMATION

Supply Bypassing and Grounding Practices,

Output Lead inductance

When designing a circuit to drive a high speed MOSFET

utilizing the IXDD430/IXDI430/IXDN430, it is very important to

keep certain design criteria in mind, in order to optimize

performance of the driver. Particular attention needs to be paid

to Supply Bypassing, Grounding, and minimizing the Output

Lead Inductance.

Say, for example, we are using the IXDD430 to charge a 15nF

capacitive load from 0 to 25 volts in 25ns.

Using the formula: I= C

∆V / ∆t, where ∆V=25V C=15nF &

∆t=25ns we can determine that to charge 15nF to 25 volts in

25ns will take a constant current of 15A. (In reality, the charging

current won’t be constant, and will peak somewhere around

30A).

SUPPLYBYPASSING

In order for our design to turn the load on properly, the IXDD430

must be able to draw this 5A of current from the power supply

in the 25ns. This means that there must be very low impedance

between the driver and the power supply. The most common

method of achieving this low impedance is to bypass the power

supply at the driver with a capacitance value that is a magnitude

larger than the load capacitance.

Usually, this would be

achieved by placing two different types of bypassing capacitors,

with complementary impedance curves, very close to the driver

itself.

(These capacitors should be carefully selected, low

inductance, low resistance, high-pulse current-service

capacitors). Lead lengths may radiate at high frequency due

to inductance, so care should be taken to keep the lengths of

the leads between these bypass capacitors and the IXDD430

to an absolute minimum.

GROUNDING

In order for the design to turn the load off properly, the IXDD430

must be able to drain this 5A of current into an adequate

grounding system. There are three paths for returning current

that need to be considered: Path #1 is between the IXDD430

and it’s load. Path #2 is between the IXDD430 and it’s power

supply. Path #3 is between the IXDD430 and whatever logic is

driving it. All three of these paths should be as low in resistance

and inductance as possible, and thus as short as practical. In

addition, every effort should be made to keep these three

ground paths distinctly separate. Otherwise, (for instance), the

returning ground current from the load may develop a voltage

that would have a detrimental effect on the logic line driving the

IXDD430.