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Copyright © 2007 IXYS CORPORATION All rights reserved

IXDD504 / IXDE504

When designing a circuit to drive a high speed MOSFET

utilizing the IXDD504 or IXDE504, 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 IXDD504 to charge a

2500pF capacitive load from 0 to 25 volts in 25ns.

Using the formula: I

and

∆t=25ns we can determine that to charge 2500pF to 25

volts in 25ns will take a constant current of 2.5A. (In reality, the

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

around 4A).

SUPPLYBYPASSING

In order for our design to turn the load on properly, the IXDD504

must be able to draw this 2.5A 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 IXDD504

to an absolute minimum.

GROUNDING

In order for the design to turn the load off properly, the IXDD504

must be able to drain this 2.5A of current into an adequate

grounding system. There are three paths for returning current

that need to be considered: Path #1 is between the IXDD504

and it’s load. Path #2 is between the IXDD504 and it’s power

supply. Path #3 is between the IXDD504 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 IXDD504.

OUTPUTLEADINDUCTANCE

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 0.2” 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