Applying the equations in Figure 3, a 0.4 mm
(0.0157”) via through a 1.6 mm (0.063”) PCB has ˜ 1.2
nH inductance; even the clearance hole can contribute
some parasitic capacitance.
Since the length of a via is typically very small relative
to the rest of the trace, its contribution is often ignored.
This can cause issues, though with very high frequency
(>100MHz) signals, or in situations where the high-frequency harmonics contain significant energy.
To save you ploughing through the math, help
is available online. The Vehicular Electronics
Laboratory at Clemson University, for example,
offers online calculators for several transmission
methods including surface microstrips, embedded
microstrips, and striplines.
Some Effects Of PCB Parasitics
PCB parasitics cause problems in a variety of areas.
In a switching power supply—a synchronous buck
converter, for example—parasitic inductance can slow
down the turn-on and turn-off time of the power FET
and increase switching losses.
Figure 4: In a synchronous buck converter, the internal
and external inductances in the source path of the HS FET
combine to form the common-source inductance (LCSI).
As Figure 4 illustrates, the inductances of both the
PCB trace and the FET’s internal bond wires contribute
to the total inductance between the HS FET source and
the gate driver return path.
Both the (high) drain-source current and (low) gate-charging current flow through the common-source
inductance. A change in IDS will induce an opposing
voltage VCSI in LCSI, and modify the gate-source
voltage VGS seen by the power MOSFET. The effect is
an increase in FET turn-on and turn-off times, resulting
in increased switching losses.
Parasitics can also change the performance of high-
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