An MOV at the input of an AC-DC power supply can limit short-duration overvoltage, but its effectiveness depends on placement, wiring inductance, fuse coordination, thermal protection and the actual surge test. It should be designed as part of the complete input network rather than selected from mains voltage alone.
Typical Input Protection Chain
A common sequence is connector, fuse, inrush-current limiter, EMI filter, MOV and rectifier, but the exact order depends on safety, EMC and surge objectives. The MOV should be placed across the conductors it protects with a short, low-inductance current loop. Long PCB traces increase residual voltage during a fast surge.
Select Maximum Continuous Voltage First
Start with the highest continuous line voltage, not the nominal voltage. Include mains tolerance and the system’s credible abnormal conditions. Check the manufacturer’s maximum continuous AC voltage (MCOV) separately from V1mA and clamping voltage.
Translate the Equipment Test into MOV Stress
For IEC 61000-4-5 testing, record the generator level, coupling mode, source impedance, polarity and pulse sequence. Estimate the current through the MOV, then verify the residual voltage at the protected node. The open-circuit test level is not an MOV voltage rating.
Coordinate the Fuse and Thermal Protection
The line fuse protects against excessive circuit current. A degrading MOV can become hot while drawing less than the fuse opening current. Mains-connected designs may therefore require a thermally coupled disconnect or a thermally protected MOV, depending on the safety analysis and applicable standard.
Check the EMI Filter Interaction
The common-mode choke and X/Y capacitors change surge propagation. Decide whether the MOV sits before or after parts of the filter by evaluating insulation stress, conducted emissions and the intended current path. Do not place the MOV where a surge return path bypasses the protective conductor strategy.
Design Checklist
- Maximum normal AC voltage and temporary overvoltage.
- Line-to-line and line-to-ground coupling paths.
- Peak MOV current, waveform, pulse count and interval.
- Maximum residual voltage versus rectifier and bulk-capacitor withstand.
- Fuse clearing behavior and available fault current.
- Thermal disconnect, spacing and flame requirements.
- PCB loop length and distance from heat-sensitive parts.
- Post-test leakage, V1mA shift and functional checks.
Common Mistakes
Typical errors include using V1mA as MCOV, selecting only by joule rating, assuming a line fuse protects every MOV thermal failure, and testing only one polarity or coupling path. Another mistake is lowering the MOV voltage to improve clamping without checking continuous leakage and TOV behavior.
Related Guides
Reference
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