IGBT Modules - Technologies, Driver and Application (Second Edition) - page 450

Fig. 12.22
Open loop converter
The open loop converter is the simplest design of integratedHall sensors for measuring
current. Its functional principle is shown in
The electric conductor, with the
current I
which is to be measured, is surrounded by a magnetic conducting core that
concentrates themagnetic field induced by the current. This core has an air gap at one
point, into which the Hall sensor is placed. Integrated electronics provide the gate
current I
, compensationmeasures and amplification of the Hall voltage U
. As long as
the sensor is within the linear range of the B-H magnetisation curve of the core, the
magnetic flux B is proportionate to the current I
to be determined. Accordingly, the Hall
voltage, andwith it the output voltage of the open loop converter, is proportionate to the
current I
. However, despite the integrated compensation measures, an offset voltage
is added to the output voltage. This offset voltage is brought about partly by the
above-mentioned factors, as well as by influences from the electronic circuit
(amplification errors, linearity, noise, attenuation and phase shift when the permissible
bandwidth is reached) and themagnetic saturation of the core.
The advantages of the open loop converter are, essentially, its galvanic isolation from
the signal to bemeasured, the ability tomeasure bothDC and AC, the need of only low
supply current and the low cost. The main disadvantage is that overload currents can
lead to saturation and therefore to residual magnetisation of the core. Further
disadvantages are the comparatively low bandwidth, long response time and relatively
high temperature sensitivity. In particular, the long response time can limit the use of
open loop converters in applications with specific IGBTs, including all applications with
IGBTs with a nominal short circuit time of less than 10µs, in which the short circuit
protection is tobe realisedusing open loop converters.
Open loop converters up to currents of >10kA are available commercially from many
manufacturers, including, to name just a few, ABB, LEMandTamura.
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