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.
