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IXDD509 / IXDE509
IXYS reserves the right to change limits, test conditions, and dimensions.
When designing a circuit to drive a high speed MOSFET
utilizing the IXDD509/IXDE509, 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 IXDD509 to charge a
5000pF capacitive load from 0 to 25 volts in 25ns…
Using the formula: I= C(∆V / ∆t), where ∆V=25V C=5000pF
& ∆t=25ns we can determine that to charge 5000pF to 25
volts in 25ns will take a constant current of 5A. (In reality,
the charging current won’t be constant, and will peak
somewhere around 9A).
SUPPLY BYPASSING
In order for our design to turn the load on properly, the
IXDD509 must be able to draw this 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 IXDD509 to an absolute minimum.
GROUNDING
In order for the design to turn the load off properly, the
IXDD509 must be able to drain this 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 IXDD509 and it’s load. Path #2 is between the
IXDD509 and it’s power supply. Path #3 is between the
IXDD509 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
IXDD509.
Supply Bypassing and Grounding Practices, Output Lead inductance
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 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.
