30 A Low-Side RF MOSFET Driver

IXRFD630

8

Applications Information

Introduction

Circuits capable of very high switching speeds and

high frequency operation require close attention to sev-

eral important issues. Key elements include circuit loop

inductance, Vcc bypassing, and grounding.

Circuit Loop Inductance

The Vcc to Vcc Ground current path defines the loop

that generates the inductive term. This loop must

be kept as short as possible. The output lead must be

no further than 0.375 inches (9.5 mm) from the gate of

the MOSFET. Furthermore, the output ground leads

must provide a balanced symmetric coplanar ground

return for optimum operation.

Vcc Bypassing

In order to turn a MOSFET on properly, the IXRFD630

must be able to draw up to 30 A of current from the Vcc

power supply in 2-6 ns (depending upon the input ca-

pacitance of the MOSFET being driven). Good per-

formance requires 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

much larger than the load capacitance. Usually, this is

achieved by placing two or three different types of by-

passing capacitors, with complementary impedance

curves, very close to the driver itself. (These capacitors

should be carefully selected for low inductance, low

resistance, and high pulse current service.) Care

should be taken to keep the lengths of the leads be-

tween these bypass capacitors and the IXRFD630 to

an absolute minimum.

The bypassing should be comprised of several values

of MLC (Multi-Layer Ceramic) capacitors symmetrically

placed on either side of the IC. Recommended values

are 0.01uF and 0.47uF for bypass and at least two

4.7uF tantalums for bulk storage.

Grounding

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

IXRFD630 must be able to drain 30 A of current into

an adequate grounding system. There are two paths

for returning current that need to be considered: Path

one is between the IXRFD630 and its load, and path

two is between the IXRFD630 and its power supply.

Both of these paths should be as low in resistance and

inductance as possible, and thus as short as practical.

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 its load as short

and wide as possible, and treated as coplanar

transmission lines. In configurations where the

optimum configuration of circuit layout and by-

passing cannot be used, a series resistance of a

few ohms in the gate lead may be necessary to

dampen ringing.

Heat Sinking

For high power operation, the bottom side metal-

ized substrate should be placed in compression

against an appropriate heat sink. The substrate

is metalized for improved heat dissipation, and is

not electrically connected to the device or to

ground. See the technical note “DE-Series MOS-

FET and IC Mounting Instructions” on the IXYS

Colorado website at www.ixyscolorado.com for

detailed mounting instructions.

An

IXYS Company

1609 Oakridge Dr., Suite 100

Fort Collins, CO USA 80525

970-493-1901 Fax: 970-232-3025

Email: sales@ixyscolorado.com

Web: http://www.ixyscolorado.com

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