DEIC421

RF MOSFET DRIVER

Applications Information

Introduction

Circuits capable of very high switching speeds and high

frequency operation require close attention to several

important issues. Key elements include circuit loop in-

ductance, Vcc bypassing, and grounding.

Circuit Loop Inductance

The Vcc to Vcc Ground current path defines the loop

which will generate the inductive term. This loop must

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

no further than 0.375 inches (9.5mm) 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 DEIC421

must be able to draw up to 20A of current from the Vcc

power supply in 2-6ns (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, low inductance, low resis-

tance, high-pulse-current-service capacitors.) Care

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

tween these bypass capacitors and the DEIC421 to an

absolute minimum.

The bypassing should be comprised of several values

of chip capacitors symmetrically placed on either side

of the IC. Recommended values are .01uF and .47uF

chips and at least two 4.7uF tantalums.

Grounding

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

DEIC421 must be able to drain this 20A of current into

an adequate grounding system. There are two paths

for returning current that need to be considered: Path

#1 is between the DEIC421 and its load, and path #2 is

between the DEIC421 and its power supply. Both of

these paths should be as low in resistance and induc-

tance as possible, and thus as short as practical.

The DEI421 has separate ground leads for input and

power which allows the addition of a common mode

choke in the input and input ground leads (see Fig. 2).

The common mode choke will provide a means of pre-

venting ground bounce from affect the input to the

driver. The selection of the common mode choke is

related to the device being driven, the board layout,

and the Vcc bypassing.

Output Lead Inductance

Of equal importance to supply bypassing and ground-

ing 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 bypassing cannot be used, a series

resistance of a few ohms in the gate lead may be nec-

essary to prevent ringing.

Heat Sinking

For high power operation, the bottom side metalized

substrate should be placed in compression against an

appropriate heat sink. The substrate is metalized for

improved heat dissipation, and is not electrically con-

nected to the device or to ground.

See the DEI technical note “DE-Series MOSFET and

IC Mounting Instructions” on the IXYSRF website at

www.ixysrf.com for detailed mounting instructions.