FEATURE SENSORS
EXERCISING CORELESS STRENGTH Robust and precise current measurement in drives and inverters
In addition, differential measurement
The system cycle of a PV inverter
Jutta Heinzelmann, manager of product marketing, and Theodor Kranz, application engineer, Infineon, argue for the benefits of coreless sensor technology
D
emand for industrial current sensors is steadily increasing as detailed information about continuous currents is required in a wide range of applications – from solar inverters and electric motors to smart meters, power supplies and lighting management systems. Each application has different criteria, but in general, the sensors should take up as little space as possible, have low losses, be flexible and cost-effective, and remain precise, robust and safe in operation over their entire service life. The XENSIV TLI4971 Hall-effect current sensor hopes to meet these requirements, the first member of Infineon’s new “coreless” current sensor family.
HOW DOES CORELESS HELP? Numerous sensors using various sensing methods are available on the market. With the resistive method (shunt), the voltage drop is measured across a resistor, through which the measured current flows. With the magnetic method, there is a distinction between direct imaging (open-loop) and compensation (closed-loop) sensors: there are versions with and without a ring core. In open-loop systems with a core, the primary current in the ring core generates a magnetic field that is converted into a measurement voltage by a magnetic sensor. The small output voltage is proportional to the field strength, and is then amplified to constitute the sensor’s output signal.
36 FEBRUARY 2020 | ELECTRONICS
In closed-loop systems, the magnetic flux generated by the primary current is compensated by a secondary winding. A magnetic sensor with associated electronics is used to control and measure the compensation current. The main limitations of closed-loop transducers are the higher current draw of the secondary side power supply, the larger dimensions, and a more expensive design compared to the simpler open-loop type. Compared to core-based methods and solutions using shunts, the coreless technology offers various benefits. Coreless magnetic current sensors do not require a magnetic core to concentrate the magnetic field. As a result, these sensors can be made very small. Shunt-based current sensors do not have a magnetic core either, but they are resistance-based and, therefore, suffer from higher power losses. Coreless technology offers further advantages, such as galvanic isolation and low losses. Also, there is approximately zero magnetic hysteresis.
KEY FEATURES OF THE TLI4971 The XENSIV TLI4971 covers the current measurement range from ± 25A up to ± 120A, providing the measurement results as analogue voltage signals. It uses Infineon’s compensation technology for temperature and mechanical stress; this results in a sensitivity error less than three per cent above temperature.
with two Hall cells ensures high accuracy, even in noisy environments with crosstalk from adjacent power lines or magnetic interference fields. Due to the coreless, open-loop concept, the TLI4971 fits into a small, leadless TISON-8 package (8mm x 8mm x 1mm). The special layout of the busbar ensures thermal performance for alternating currents up to 120A. The lightweight (2.0g) and compact package offers galvanic isolation up to 1150V (maximum repetitive insulation voltage VIORM) and a creepage and clearance distance of 4mm for high robustness. At maximum sensitivity, the power losses are reduced to only 0.55W at 50Aeff. The digital protection functions are routed via a separate signal path with low delay and thus enable overcurrent detection within typically 700ns. The XENSIV TLI4971 also lessens temperature dependencies because it comprises separate structures for measuring temperature and mechanical stress. By measuring both variables separately during operation, permanent, effective compensation can be achieved.
FLEXIBLE FOR A WIDE RANGE OF APPLICATIONS Typical applications are phase current measurements in drives, or current measurements in photovoltaic (PV) inverters. In grid-connected PV systems, the DC voltage generated by the solar modules is converted by means of an inverter into the standard AC voltage (115 or 230V) and fed into the grid. The typical power range of small inverters is 1.5 to 6kW peak power. Current sensors in solar inverters monitor, for example, the output alternating current fed into the power grid and its DC component. Precise current measurement is particularly important for the operator of a solar power system, as this is the basis for the feed-in tariff. With the aid of the new current sensors, not only is the current correctly determined when feeding power into the grid, but technical faults in solar modules, such as those caused by cell breakage or lightning strikes, can also be detected more quickly and easily.
Infineon
www.infineon.com / ELECTRONICS
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