12.3 Measuring current
In power electronics circuits, current can bemeasured at different points in the system.
shows an overview of the usual measurement points, taking a full-bridge
inverter for amotor control asan example.
Usual measuringpoints formeasuring current in power electronics systems
The cheapest variant "a" of current measurement is often used for applications in the
low power range. Ideally, the measurement should take place in the DC connection of
the DC-bus, since this usually represents the reference potential of the microcontroller
and it is therefore not necessary to isolate the signal. The disadvantage of measuring
current in this way is the increase in stray inductance in the commutation path caused
by the insertionof themeasuring probes (shunt resistors, hall sensors, etc.).
Another alternative, found particularly in the low to medium power range, is variant "b".
In this case, the current ismeasured in the emitter section of the bottom IGBT of a half-
bridge. It may be possible to dispensewithmeasurement of the third signal, as this can
be derived by calculation based on the first two signals. The advantage of this
measurement method is similar to that of variant "a", in that the negative section of the
DC-bus can be taken as the common reference potential. However, the disadvantage is
the increased stray inductance.
In highly dynamic drives, applications greater than approximately 5.5kW and/or high-
end applications, current is usually measured in the output phases of the inverter
(variant "c" i
. The third current sensor is not necessary in this case either.
Another variant, not shown in
is measuring the current directly using the
IGBTs. This requires sensor IGBTs instead of conventional IGBTs, which are described
There are two different ways of measuring the current itself: A magnetic and a non-
magnetic method. The measuring instruments used in these two procedures in power
electronics are covered below. Specifically, they are: