The measurement of electric current strength is not
always easy, especially when the measured signal requires further
electronic conditioning. Simply connecting an ammeter to an electrical circuit
and reading out the value is no longer enough. The current signal must be fed
into a computer in which sensors convert current into a proportional voltage
with minimal influence on the measured circuit. The basic sensor requirements
are galvanic isolation and a high bandwidth, usually from DC up to at least 100
kHz. Conventional current measurement systems therefore tend to be physically large
and technically complex.
Early Solutions
The oldest technique is to measure the voltage drop across a resistor placed in the current path. To minimize energy losses the resistor is kept very small, so the measured voltage must be highly amplified. The amplifier’s offset voltage must be as small as possible and its supply voltage must be at the potential of the circuit, often 110 V mains (230 V in Europe) with high parasitic peaks from which its output must be isolated. This requirement increases overall system cost.
Another widespread principle is the transformer.
Its construction is much simpler, but it doesn’t
allow the measurement of DC signals. Isolation between primary and secondary
sides is implicitly given. A problem is the limited frequency range.
Hall Current Sensors
Hall sensors also measure the magnetic field
surrounding the conductor but, unlike current transformers, they also sense DC
currents. A circular core of soft magnetic material is placed around the
conductor to concentrate the field. The Hall element, which is placed in a
small air gap, delivers a voltage that is proportional to the measured current.
This sensor also offers a galvanic isolation.
The very small output voltage of the Hall element
must be highly amplified, and the sensitivity is temperature dependent and
requires adequate compensation. There is an inevitable offset, i.e., a small DC
voltage at zero current; the offset amplitude and temperature coefficient are
subject to significant fluctuations. The smaller the current to be measured,
the higher the offset-induced relative error. Also of note is sensitivity to
short current peaks in the circuit: according to the hysteresis properties of
the core material, these peaks can cause a static magnetization in the core
that results in a permanent remanence, and finally to an offset alteration of
the Hall element.
The two types of Hall effect current sensor are
open loop and closed loop. In the former, the amplified output signal of the
Hall element is directly used as the measurement value. The linearity depends
on that of the magnetic core. Offset and drift are determined by the Hall
element and the amplifier. The price of these sensors is low, but so is their
sensitivity.
Closed-loop Hall sensors are much more precise. The
Hall voltage is first highly amplified, and the amplifier’s output current then
flows through a compensation coil on the magnetic core (see Figure 1). It
generates a magnetization whose amplitude is the same but whose direction is
opposite to that of the primary current conductor.
The result is that the magnetic flux in the core is
compensated to zero.The nonlinearity and temperature dependence of the Hall
element are thus compensated but the offset remains. Closed-loop current sensors
work up to frequencies of ~150 kHz. They are not cheap, though, and for high
currents they become very bulky.
Magnetoresistive Field Sensors
Practical magnetic field sensors based on the
magnetoresistive effect are easily fabricated by means of thin film
technologies with widths and lengths in the micrometer range. They have been in
production for years in many different executions . To reduce the temperature
dependence, they are usually configured as a half or a full bridge. In one arm
of the bridge, the barber poles are placed in opposite directions above the two
magnetoresistors, so that in the presence of a magnetic field the value of the
first resistor increases and the value of the second decreases .
For best performance, these sensors must have a
very good linearity between the measured quantity (magnetic field) and the
output signal. Even when improved by the barber poles, the linearity of
magnetoresistive (MR) sensors is not very high, so the compensation principle
used on Hall sensors is also applied here. An electrically isolated aluminum
compensation conductor is integrated on the same substrate above the permalloy
resistors .
The current flowing through this conductor generates a magnetic
field that exactly compensates that of the conductor to be measured. In this
way the MR elements always work at the same operating point; their nonlinearity
therefore becomes irrelevant. The temperature dependence is also almost
completely eliminated. The current in the compensation conductor is strictly
proportional to the measured amplitude of the field; the voltage drop across a
resistor forms the electrical output signal.
Magnetoresistive sensors, as are Hall elements, are
very well suited for the measurement of electric currents. In such applications
it is important that external magnetic fields do not distort the measurement.
This is achieved by forming a full bridge made of four MR resistors, where the
two arms of the bridge are spatially separated. The barber poles have the same
orientation in the two arms, so that only a field difference between the two
positions is sensed. The sensors have been in production for several years.
The size of the actual MR sensor chip with the four
permalloy strips on silicon is 1 mm by 2 mm2, the distance between
the two arms of the bridge being 1.6 mm . It is mounted with the electronics on
a 23 mm by 35 mm2 single in-line hybrid circuit designed for
through-hole PCB mounting with very low profile.
Among the advantages of these sensors are:
• Significantly smaller volume and weight compared
to conventional current sensors, permitting greater flexibility in
application-specific design
• No remanence in the event of overload
• Measurement of DC and AC currents without
additional loss
• Wide frequency range due to low inductive design
• No auxiliary supply necessary on the level of
current to be measured
The ASIC allows digital calibration of the current
measurement system even after installation in an application. The calibration
data are stored on the chip. The MR sensor is located on a second chip. A
co-integration of sensor and interface electronics on the same silicon
substrate is possible but is not yet cost effective.
In the standard sensor types described thus far,
the nominal current is set by the geometry of the primary conductor that is
part of the system and by the distance between the MR chip and the conductor.
Recent investigations, however, have demonstrated that the high-current bus bar
need not be interrupted or guided through a hole, as required with Hall
transducers. Instead, it can simply be shaped in the form of a bus bar plate. A
sensor module realized as a dual in-line surface mount device component can be
mounted on the power PCB board placed above the bus bar plate so that the
current flow can be directly measured.
The result is a differential field measurement
system that is insensitive to homogeneous external magnetic perturbations. The
sensors and the ASIC are mounted on an appropriate substrate and encapsulated
in a plastic package With this sensor, a broad range of currents can be
measured simply by adapting the geometry of the conductor. Potentially
heterogeneous perturbation fields can easily be shielded.The advantages are
obvious: these current sensors are not only small, compact, and light, but also
cheap and easy to mount. The primary current conductor can be part of the
application and need not be mounted separately. This opens the way to completely
new construction possibilities for developers of power electronic modules and
devices such as for mains receivers and frequency inverters.
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