Isolated Current Sensor for High Performance Power Electronics Applications
A planar current sensor, comprised of a magnetic current transformer and a Hall-effect element, is presented. The sensor has a broad frequency bandwidth from DC up to 30 MHz, a high current rating (40 A DC), superior linearity, high EMI immunity, small size, robustness and low realization cost. The main design formulations are given analytically; simulations and finite element results are presented for verification. Experimental results of current step response and dv=dt immunity are included.
Industrial electronic equipments for measurement [1], control [2],
protection [3] and diagnostics [4] often require high-performance
current sensors with features such as:
1) wide frequency bandwidth;
2) high current rating;
3) high dv=dt immunity;
4) linearity and stability with temperature variations;
5) compatibility with integration processes;
6) low realization effort and low cost.
The types of current sensors that have appeared in recent publications
were based on numerous different physical effects, for instance mag-
netic coupling magneto resistance Faraday induction
Hall effect and zero. However, as some recent
comparative studies have shown , none of these sensors have
really met all the requirements above listed in one device. A large
bandwidth from DC to several MHz has been the most difficult
objective, but is also one of the most crucial characteristics for
modern current controlled converters having switching frequencies
up to 1 MHz and fast dynamics . Accuracy and low-cost of
the sensing device are important requirements for current controlled
converters. They are particularly crucial for direct pulsewidth mod-
ulation, such as hysteresis and direct power control, or for current
sensor-based active ripple filters . To circumvent these issues,
many sensorless control techniques for converters and drives have
been developed over recent years . Another important aspect is
the immunity of the sensor against external fields. For instance, a
current sensor that is physically located near a SiC J-FET, can be
affected severely by dv=dt transients up to several tens of kV/s
[22]. The parasitic coupling associated with the sensor itself can
couple these transients into the sensing part causing distortion in the
output. The sensor presented herein uses a planar current transformer
(CT) as the main sensing device. For a high upper bandwidth limit the
parasitic capacitance and leakage inductance must be small . The
number of turns on the sensing coil can therefore be reduced, which
also decreases the value of the magnetizing inductance (cf. Lm1 in
Current sensors technologies based on magnetic core: (a) open-loop,
(b) closed loop, (c) current transformer and (d) the proposed current sensor.
Fig. 5), thus providing some advantages [20]. The realization cost
is lower for a planar transformer rather than a wire wound device.
The drawback of this solution is that the lower corner frequency
increases with fewer turns. This also happens when core materials
with low permeability, a gapped or core-less transformers are used.
Therefore, in order to extend the bandwidth to DC while keeping the
advantages at high frequency, another device able to sense the DC and
low frequency part of the measured signal can be combined with the
CT. The technique of matching the frequency characteristics of two
sensing devices for broad band current sensing from DC to several
MHz has already been proposed in literature . The
proposed sensor features a Hall-effect based element for performing
the low frequency current sensing. There are at least three well known
architectures for combining a Hall-effect sensor and a magnetic core:
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