Overview
Switch-mode power supplies, uninterruptible power supplies and active power-factor-correction front ends convert and condition large amounts of electrical power, and the quality of the current feedback determines both efficiency and reliability. These converters switch at high frequency and must measure current accurately for control, for current-mode regulation and for protection. Because the measuring point sits on the high-voltage side, isolation is essential. LEM current transducers provide that isolation, and BeiLuo supplies them with genuine traceability and FAE support.
Active Power-Factor Correction
Modern supplies draw near-sinusoidal current to meet harmonic limits, using an active PFC front end that switches at high frequency and handles the full input power. The control loop shapes the input current to follow the input voltage, so it needs a fast, accurate, low-noise current signal. A closed-loop transducer such as the LTS25-NP is well suited because it combines accuracy and low drift with a single +5 V supply that matches a digital control board, and its isolation keeps the controller safe. Because the current is measured continuously through the line cycle, low offset drift is important for a clean, compliant current waveform.
Harmonics and Control
The PFC control loop must reproduce the input current waveform precisely, so the current sensor becomes part of the control law. Any offset appears as a distortion of the current waveform, and any phase lag reduces the achievable loop bandwidth. Choosing a sensor with low offset, low drift and adequate bandwidth makes the compliant waveform easier to achieve and reduces the filtering that would otherwise be needed.
Switch-Mode Power Supplies
A switch-mode supply regulates its output by controlling the current in the transformer or inductor, and it protects its switches by limiting that current. Compact sensors such as the HLSR20-P fit the dense supply board with a single +5 V supply and a direct voltage output, giving the controller the fast feedback it needs for current-mode control and cycle-by-cycle protection. Because a supply may see large inrush current at start-up and overload current during a fault, the sensor's measuring range must cover those transients, not only the steady-state value, so that the control and protection logic keep valid data.
Uninterruptible Power Supplies
A UPS rectifies mains power, stores it in a battery and inverts it to a clean output, and it must switch between modes seamlessly. Measuring the load current accurately across a wide range supports both regulation and overload protection, so a transducer with generous headroom such as the HAS50-S is a good fit. The open-loop design keeps the part compact and economical, and its direct voltage output simplifies the interface to the control board.
Designing the Sensing Stage
Getting the best performance from a current transducer in a supply is as much about layout as about the part. Keep the primary conductor centered in the aperture and follow the recommended geometry, because the conductor is part of the factory calibration. Keep the secondary output away from the switching nodes and the transformer, use a short screened connection to the ADC, and choose the measuring resistor to match the ADC input range. For a supply that also needs voltage feedback, add an isolated voltage transducer such as the LV25-P so the controller sees both quantities on the same isolated basis.
Selection and Supply
Selection starts with the current levels of the PFC stage, the output and the load, then narrows by isolation, accuracy, supply and mounting. Match the measuring range to the worst-case transient current, and choose single-supply parts where the control board runs from a logic rail. BeiLuo keeps mainstream LEM current transducers in regional stock, ships them with import declaration, certificate of origin and RoHS documents, and supports selection, resistor sizing and layout with an in-house FAE team. From a compact PFC front end to a large UPS power stage, our engineers help you choose the sensing that keeps the converter accurate, efficient and protected.