Tech Briefs Magazine - March 2021 - ST-16

Applications
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Sensor Position

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Magnetic flux in mT

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Magnetic flux in mT

Sensor Position

0
Figure 2. Simulation of a core-based current sensor. (Image courtesy of TDK-Micronas)

80
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Sensor Position

> 10
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Magnetic flux in mT

> 100
Magnetic flux in mT

Sensor Position

Figure 3. Simulation of a U-shaped shielded sensor configuration. (Image courtesy of TDK-Micronas)

ing or applying compensating techniques.
However, compensating for all forms
of stray EMI would demand a deep
understanding of all the various sources
of interference, which would be challenging. The simplest and arguably
more robust approach is to select a contactless current sensor that offers some
inherent immunity to stray fields.
In general, there are three
approaches to contactless current sensing, as shown in Figure 1. These
include the core-based sensor, the Ushaped sensor, and the symmetrical
'sandwich' shielded sensor. While comparing all three is difficult due to the
number of variables presented by
unique applications, it is useful to
measure performance based on a typical scenario. In this instance, the sensors were evaluated using a 20 mm
wide by 2.5 mm thick busbar with a rectangular cross-section, carrying 1000 A.
Core-Based Contactless Current
Sensors
In a core-based sensor, a flux concentrator is positioned around the material carrying the current to be measured. The circular shape of the concentrator is interrupted by a small air gap, into which the
magnetic field sensor is placed. The core
helps focus the flux induced by the current flowing in the busbar to the sensor.

The sensitivity of the sensor to the flux
generated by the current flow is dependent on several factors. The first of these
is the size of the air gap, as a smaller air
gap allows a higher amount of flux to
reach the sensor. It follows, therefore,
that a smaller sensor would allow for a
smaller air gap. As shown in Figure 2, in
this experiment, a current of 1000 A
results in the sensor registering a flux
density of 200 mT. By comparison, if no
core were present, the same sensor
would register a flux density of just 20
mT. Sensors suitable for this configuration include the HAL 24xy from TDK.
To measure this configuration's immunity to stray fields, a simulation was carried out assuming an external field with a
flux density of 5 mT. The results can be
seen in Figure 2, showing how the magnetic field is trained by the shape of the
core to flow through the sensor. With the
external field present, the sensor's ability
to accurately detect the field generated by
the current flow is reduced by a factor of
40. The conclusion here is that the corebased sensor offers a good level of shielding from other sources of EMI, and with
an appropriate level of signal conditioning, its effects can be further mitigated. In
this case, it would be reasonable to expect
an offset error of just 0.06% full scale.
However, the core-based approach has
the disadvantage of being relatively difficult to mount, as the busbar needs to pass

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through the core, while the sensor must
be located in the air gap. Furthermore, to
avoid saturation from large current flows,
the core also needs to be physically large.
In addition to this, the amount of magnetically sensitive material used in the core
itself can result in it becoming a source of
hysteretic error and interference.
The U-shaped current sensor addresses many of these disadvantages.
The U-Shaped Shielded Current
Sensor
As the name suggests, the U-shaped
sensor features a larger air gap, but still
offers a degree of shielding from stray
EMI. The sensor benefits from shielding
on three of its sides thanks to the use of a
soft magnetic material. The shape of the
concentrator makes assembly easier than
a core-based configuration, as the sensor
itself can be positioned above the busbar,
mounted on a small printed circuit board.
This style of shielded sensor will have
lower sensitivity than a core-based
approach, which is one of the trade-offs
engineers must consider when selecting
the most appropriate design for their
application. As shown in Figure 3, with
1000 A flowing through the busbar, the
sensor detected a magnetic flux density
of 50 mT, corresponding to a gain of 2.
The low gain has its benefits, however.
It means almost any sensor can be used,
such as the HAL 24xy, or the CUR 423x
Sensor Technology, March 2021

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Tech Briefs Magazine - March 2021

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