Introduction

A LEM current transducer delivers its specified accuracy only when it is integrated with care. The device measures a magnetic field, so the primary conductor geometry, the surrounding magnetic environment and the signal routing all influence the result. This application note explains the practical rules for integrating a LEM current transducer into a power stage, covering the primary conductor, the magnetic environment, the secondary signal and the thermal design.

The Primary Conductor Is Part of the Sensor

In many LEM transducers the primary conductor geometry is part of the factory calibration, which means the accuracy depends on how the current is routed through the aperture. Keep the conductor centered in the opening, make it symmetric so the field is uniform, and follow the recommended copper width and thickness from the datasheet. A conductor that hugs one side of the aperture, or that is too thin, produces gain and offset errors that no amount of software calibration can remove reliably. Where the transducer uses multiple primary turns to set the range, count the turns carefully and keep them identical between phases in a three-phase bridge.

Primary and Secondary Separation

Maintain the recommended creepage and clearance between the primary conductor and the secondary circuit, because that spacing is what provides the galvanic isolation. Do not route primary copper under or over the secondary components, and keep the isolation barrier continuous across the board. This is both a safety and a performance requirement.

The Magnetic Environment

Because a Hall transducer measures a magnetic field, a strong external field adds an error. Keep the sensor away from transformers, inductors, relay coils and high-current traces that carry a different current. Where a compact layout forces the sensor close to such sources, orient it so the external field is not aligned with the sensitive axis, and prefer sensors with good stray-field immunity. In a three-phase bridge, keep the three phase sensors identical in layout and orientation so their errors are consistent and can be compensated together.

Secondary Signal Routing

The secondary output carries the measurement to the controller, and it is vulnerable to noise from the switching nodes nearby. Keep the connection short, screen it where the environment is noisy, and bring it to the ADC with a clean ground return. Place the measuring resistor close to the ADC rather than at the sensor, so the current-mode signal is least affected by interference along the way. If the signal still carries switching noise, add an RC filter at the ADC input sized for the control-loop bandwidth, not for the switching frequency, so the filter does not add phase lag that destabilizes the loop.

Supply and Reference

Provide a clean, stable supply for the transducer and decouple it close to the device. Where the sensor offers a reference output, use it to compensate for supply variation in the firmware, because the output is ratiometric to the supply in many models. A stable supply improves both offset and gain accuracy.

Thermal Design

The primary conductor carries the full load current and self-heats, so size the copper to keep its temperature within the datasheet limit, which is often 120 °C for the primary bar or jumper. Keep the sensor clear of the hottest components on the board, and verify the conductor temperature under worst-case continuous load, not only at nominal. Because the output drifts with temperature, a cooler sensor is also a more accurate sensor.

From Layout to Validation

After layout, validate the design on the bench. Apply known currents, check the gain and offset against the datasheet, and measure the noise and bandwidth with the power stage switching. Confirm the conductor temperature under load and the isolation with a hipot test where relevant. Our FAE team can review your layout and help you interpret the measurements, so the accuracy you designed for is the accuracy you ship.