IGBT Modules - Technologies, Driver and Application (Second Edition) - page 433

terminals is kept as small as possible. The smaller this enclosed surface, the less the
influence of parasitic inductance in themeasurement.
Fig. 12.5
Pseudo four-wiremeasurement
Conductor routing is not the only means by which parasitic inductance affects the
measurement. In general, real resistances, unlike ideal ohmic resistances, show
parasitic effects that distort the current/voltage proportionality. Themost serious effect is
caused by the small but ever-present inherent inductance of the resistor, which is
caused by its internal design and the leadsmentioned above. According t
compound value for the voltage at the shunt comprises the voltage drop caused by the
ohmic and inductiveportion.
L )t(iR U
+ ⋅ =
Eq. 12.2
If the current changes over time, signal overshoots occur with rising edges and signal
attenuation having falling edges as shown in
In this example, the current
increases from 0A to 40A within 200ns and, at the end, drops back from 40A to 0A
within 100ns. The shunt maps the current correctly only as long as the current remains
constant. The measuring error is therefore dependent on the gradient of the current to
be determined and the inherent inductance of the shunt. Particularly in rapid switching
contexts, thismay causewrongmeasurements, which can lead to current overshoots at
switch-on and negative current spikes at switch-off that do not actually exist. Depending
on the quality of the shunt, the difference between themeasured and the real valuemay
vary in terms of orders of magnitude that render the measurement useless. It is
therefore important to keep the inherent inductance of the chosen shunt very low.
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