Magnetics Business & Technology - November/December 2019 - 16

FEATURE ARTICLE

Using Precision Anistropic Magnetoresistive Current Sensing to Optimize System Performance
By Khagendra Thapa, Aceinna

There is a tremendous
amount of pressure on
the embedded electronics community for higher
efficiencies and better
system
performance.
This is due to multiple
factors from thermal management issues, to increased power densities,
to battery life and more.
This pressure to improve
is driving development in
many areas. Much of the
focus today is on advanced materials, topologies, and advanced
software-defined architectures.
There are other factors to consider in the pursuit of increased efficiency, like good old-fashioned feedback. The more you know
about the performance of your system, the better you can manage it. You can have the best semiconductors, in the latest circuit
designs, driven by the most advanced software, and unless you
know exactly what the system is doing, you cannot optimize it.
There is no precision without feedback, and the more accurate the
information, the more precise your system can be.
One of the things to look at in high-performance computing is how
you manage the power to it, to either optimize performance, or
monitor processor workloads. One can measure the current flow
into a system as a proxy, to see whether it's running at optimal
level, or you can load more computations, or workload, onto the
processor. In addition to optimization, accurate current sensing
also empowers revenue generation from servers and data centers, charging clients based on the workload or computational
power used.
Current sensing
Current sensing in a circuit is an important way to get precise performance feedback, giving you an insight into how efficiently the
system is operating. Used in control, protection, and measurement
circuits, current sensors measure the power flow within the system, and is often used to dynamically control switching frequencies to minimize losses. Accurate and fast current measurement
is key to reducing loss in zero-current and zero-voltage switching
systems, as any current or voltage across a switching transistor
during the switch phase is wasted energy.
AMR technology
Anisotropic Magnetoresistive (AMR) isolated current sensors offer
high accuracy and bandwidth in a small surface-mount technology
package. In comparison to sense resistors, Hall-effect devices,
and current transformers, an AMR-based current sensor like the
ones from our company are drop-in devices made from an NiFe
thin film that exhibits a very high-sensitivity and high-bandwidth
response to magnetic fields.
Another approach is to use a current transformer, but that solution
can be bulky. In addition, current transformers only work with AC.

Compared to sense resistors, Hall-effect devices, and current transformers, AMR-based current sensors exhibit a very high sensitivity and
high-bandwidth response to magnetic fields.
A current transformer also has a saturation effect. A third way is
to put a Hall-effect sensor in the gap and use it to measure the
current through the wire. As there are numerous ways to measure
current, the kind of sensor you specify in your power design, will
impact the cost, size and effectiveness of your efforts.
The AMR technology that we develop is a compact, single-chip
solution. Compared to a shunt register, AMR technology uses an
insulating substrate, with 4.8KV isolation, and does not require
additional components other than a decoupling capacitor. Compared to a transformer, it's not only the size, AMR technology can
respond to both DC and AC bi-directional current. Compared to
Hall-effect-based solutions, AMR tech offers a bandwidth of 1.5
megahertz, and has a lower offset and noise, which leads to better
accuracy and lower phase shift. That, combined with AMR tech's
very fast output step response, provides an accurate and compact
solution that can perform critical measurements for protection and
control of power systems.
How it works
Within the sensor, four pins take the current in from one side, and
on the same side, it returns through another four pins. As the current flows through the lead frame it flows in a U bend through the
device, with the current going in one direction and returning on the
bend. As the current flows through the lead frame it generates the
field to be measured. When the current reverses, it has a reverse
field. There are two separate AMR current sensors in the device,
that measure the field from both current directions, canceling out
external fields and offsets which might be present.
This dual-sensor configuration gives an AMR sensor the ability to
ignore external fields perpendicular to the current flow, thereby the
ability to ignore other components on the board. An AMR sensor
is only sensitive to horizontal fields in the silicon. Whereas, if you
were using a Hall-effect sensor, they also sense fields which are
perpendicular to the silicon. This AMR based design's resistance
to stray magnetic fields gives developers much more flexibility in
component placement in an AMR-based system.
The dual-sensor construction of our AMR sensor, the high level
of integration and the materials used provide not only a high ac-

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