Deploying Primary-Side Current Sensors For PSU Output Current Limiting — Protecting Against Low Impedance Overloads

Focus:

Current transformers (CTs) used to implement primary-side current-mode control can also provide the sense signal for PSU output current limiting. However, filtering of the CT signal to remove the spike generated by hard switching—needed for accuracy of current magnitude monitoring—can undermine current limiting when the output is shorted by a very low impedance presented by a failed component or a PCB defect. In this article, this sensor error at very low impedance load is explained, and the short circuit current increase and its sensitivities to the magnitude of the overload and CT sensor time constant are explained. Based on this analysis some guidance is given on selection of the CT sensor filter capacitor value, rating of the power components for the two forms of current limiting (constant current and current foldback), and on activation of the PSU latch-off feature. Benefits of primary-side current sensing and the principles of constant current and current foldback current limiting are also discussed.

What you’ll learn:

• How to protect against low impedance overloads when implementing power supply current limiting using primary-side current sensing
• How to understand the cause of sensor error in primary-side current sensing at very low impedance overloads

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Author & Publication:

Viktor Vogman, Olympia, Wash., How2Power Today, Feb 15 2026

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