Deploying Primary-Side Current Sensors For PSU Output Current Limiting — Protecting Against Low Impedance Overloads
by Viktor Vogman, Olympia, Wash., How2Power Today, Feb 15 2026
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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