8

30 A Low-Side RF MOSFET Driver

IXRFD631

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 IXRFD631

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 IXRFD631 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 0.01 uF and 0.47 uF for bypass and at least two

4.7 uF tantalums for bulk storage.

Grounding

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

IXRFD631 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 IXRFD631 and its load, and path

two is between the IXRFD631 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 in-

ductance. 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 trans-

mission lines. In configurations where the optimum

configuration of circuit layout and bypassing 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 MOSFET and

IC Mounting Instructions” on the IXYS Colorado

website at www.ixyscolorado.com for detailed

mounting instructions.

IXYS Colorado

1609 Oakridge Dr. Suite 100

Fort Collins, CO

Phone: 970-493-1901

Fax: 970-232-3025

Dec. 2012