Page 40 - EE Times Europe Magazine | February 2020

38 EE|Times EUROPE — Boards & Solutions Insert

SENSORS
Miniaturize Current Sensing for Effi cient

Power-Conversion Systems

By Maurizio Di Paolo Emilio

he future of power electronics requires the evolution of mod-
ern energy-conversion systems to make them more effi cient,
cheaper, and smaller than their predecessors. Such systems
T require accurate current measurement. Open-loop all-
effect sensors are often used for this purpose: A conductor produces a
magnetic fi eld comparable to the current that is then concentrated by a
magnetic core and measured by the all sensor.
Recent introductions of custom ASIC solutions have helped to
increase measurement accuracy. The evolution of ASIC technology has
paved the way for the development of open-loop all-effect sensors
that match the performance of closed-loop technology. Figure 3: Layout of a backup power supply system monitor with
open-loop technology (Image: Honeywell)
OPEN- VS. CLOSED-LOOP SENSORS
Open-loop current sensors (Figures 1 and 2) consist of a all sensor because it feeds an opposite current into a secondary coil, wound on
mounted in the air space of a magnetic core. They measure both AC and the magnetic core, to zero the fl ux produced in the magnetic core by
DC and provide electrical isolation between the input and output sec- the primary current.
tions, with the galvanic isolation ensuring non-contact measurement. The output current of a closed-loop sensor is converted into a
The amplifi ed all signal represents the sensor output. Open-loop voltage value by connecting a resistor to the sensor output and ground.
sensors are usually less expensive than other options, and their low Selecting the resistor value may result in the output being resized.
operating-power requirements and small footprint suit them for use in The choice of technology, whether open- or closed-loop all-effect
battery-powered circuits. Their disadvantage is that they can be prone or some other type, will largely depend on the particular constraints of
to saturation and temperature drift. the application (Figure 3).
Figure 1 illustrates the principles behind the all current sensor.
The magnetic fl ux produced in proportion to the primary current, LEM’S HMSR SERIES
I f , induced in the magnetic circuit, passes through the all element LEM’s new MSR series of miniature integrated-circuit sensors for AC
inserted in the gap of the magnetic circuit, resulting in a potential and DC isolated-current measurement can handle overload current
difference, h , expressed by the formula shown in the fi gure. bursts of up to 20 kA. LEM tailored the series to meet the market demand
Closed-loop current sensors offer fast response, high linearity, and for cost reduction, performance improvements, and miniaturization.
low temperature drift, with low output noise, suiting them for applica- The sensors include a low-resistance primary conductor to minimize
tions requiring exact measurements. The closed-loop sensor takes the power losses and ensure ease of use. The sensor ASIC and ferrite
concepts of the open-loop version and adds a secondary winding to the magnetic element enable direct current
output. The closed-loop device is sometimes called a “zero fl ux” sensor measurements and maintain insulation Giant steps.
performance.
The MSR series uses a proprietary
open-circuit all-effect ASIC combined
with a single low-resistance primary With us.
conductor to minimize power loss,
allowing measurement of direct current
and high-transient-overload current
without damage. Manufactured as SO16
surface-mount devices, they measure Figure 4: LEM HMSR
Figure 1: The physics of the Hall effect (Image: Tamura) 6 mm high and can be mounted directly 20-SMS (Image: LEM) Hall 1 | Booth 578 | 25 - 27 February 2020
onto the PCB using the same process
used for other board-level components, saving cost and space. An inte-
grated E PROM is used for internal temperature compensation and to TQ-Embedded and our strong alliances.
2
counteract offset and gain drift through compensation.
The use of ferrite for the magnetic element enables a high-frequency Experience the wide array of our newest embedded
bandwidth of 2 0 k z ( dB) and provides excellent rejection against modules and solutions.
external fi elds, according to LEM. The mechanical design of the sensor
results in dispersion and clearance of 8 mm when using materials with Including the latest processors from our outstanding
a comparative tracking index (CTI) of 600, allowing for reinforced insu- technology partners.
lation according to IEC 60 0-1, the company said.
MSR sensors’ unique primary conductor allows an overload of pri-
mary currents and a high level of insulation. The ferrite-based circuit
provides the requisite immunity against the inhomogeneous fi elds
Figure 2: Hall-effect open-loop current sensor (Image: LEM) present in various power electronics applications, such as solar panels.
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