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Copyright © 2006 IXYS CORPORATION All rights reserved

IXDD514 / IXDE514

When designing a circuit to drive a high speed MOSFET utilizing the IXDD514/IXDE514, 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 IXDD514 to charge a 5000pF capacitive load from 0 to 25 volts in 25ns…

Using the formula: I=

∆V C / ∆t, where ∆V=25V C=5000pF & ∆t=25ns we can determine that to charge 5000pF to 25 volts in 25ns will

take a constant current of 5A. (In reality, the charging current won’t be constant, and will peak somewhere around 8A).

SUPPLY BYPASSING

In order for our design to turn the load on properly, the IXDD514 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 IXDD514 to an absolute minimum.

GROUNDING

In order for the design to turn the load off properly, the IXDD514 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 IXDD514 and it’s load. Path #2 is between

the IXDD514 and it’s power supply. Path #3 is between the IXDD514 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 IXDD514.

OUTPUT LEAD INDUCTANCE

Of equal importance to Supply Bypassing and Grounding are issues related to the Output Lead Inductance. Every effort should be made

to keep the leads between the driver and it’s load as short and wide as possible. If the driver must be placed farther than 2” from the

load, then the output leads should be treated as transmission lines. In this case, a twisted-pair should be considered, and the return

line of each twisted pair should be placed as close as possible to the ground pin of the driver, and connect directly to the ground terminal

of the load.

Supply Bypassing and Grounding Practices, Output Lead inductance