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Unformatted text preview: turning ground
current from the load to affect the logic line driving
the driver IC. A good method is to dedicate one
copper plane in a multilayered PCB to provide a
ground surface. All ground points in the circuit
should return to the same physical point to avoid
generating differential ground potentials.
With desired rise and fall times in the range of 25 to
50 ns, extreme care is required to keep lengths of
current carrying conductors to the bare minimum.
Since every inch of length adds approximately 20 nH
of inductance, a di/dt of 240 Amps/microsecond
(used in the example calculation for Eq. 5.1)
generates a transient LdI/dt voltage of 4.8 volts per
inch of wire length, which subtracts from the driver’s
output. The real effect will be a significant increase
in rise time for every tiny increase in conductor
length from output pin of driver to the Gate lead of
MOSFET/IGBT. For example, one extra inch of
conductor length could increase rise time from 20 ns
to 70 ns, in an ultra h
igh-speed gate drive circuit.
Another detrimental effect of longer conductor length
is transmission line effect and resultant RFI/EMI.
This inductance could also resonate with parasitic
capacitances of MOSFET/IGBT, making it difficult to
obtain clean current waveforms at rise and fall.
It is important to also keep in mind the fact that
every MOSFET/IGBT has some inductance
depending on the package style and design. The
lower this value, the better is the switching
performance, as this inductance is, in effect, in
series with the source/emitter and the resulting
negative feedback increases switching times.
While applying driver IC for any application, it is also
necessary to compute power dissipated in the driver
for a worst-case scenario. The total power dissipated
in the driver IC is a sum of the following:
1. Capacitive load power dissipation;
2. Transition power dissipation; IXAN0009 3. Quiescent power dissipation.
For all IXD_series of drivers, transition power
dissipation is absent due to a unique method
(Patent pending) to drive t...
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- Winter '08