Metal Oxide Varistor for IEC 61000-4-5 Surge Compliance
Power systems that pass functional testing often fail during IEC 61000-4-5 surge validation. The failure mode is typically controller reset, communication loss, or power latch-up, indicating insufficient surge energy handling or incorrect MOV placement. IEC 61000-4-5 defines a combination wave with a 1.2/50 µs open circuit voltage and an 8/20 µs short circuit current. This waveform delivers significantly higher energy than ESD and requires surge protection devices to be selected based on peak current and absorbed energy rather than clamping voltage alone. TSV and TSVG metal oxide varistors provide nonlinear voltage-dependent resistance and support varistor voltages from 18 V to 1800 V with peak current capability scaling with disc diameter. Undersized MOV discs experience thermal stress during repetitive surge. MOV voltage ratings too close to AC RMS cause continuous leakage and aging. Single-stage protection without impedance coordination increases residual stress. Long trace placement increases overshoot due to parasitic inductance. Larger disc diameter increases peak current and energy capability but also increases capacitance and footprint. Step 1 Determine maximum AC RMS voltage and select MOV AC rating above steady-state voltage. Step 2 Identify surge level such as line-to-line 1 kV and line-to-ground 2 kV. Step 3 Calculate peak current using the 8/20 µs waveform. Step 4 Select disc size with sufficient energy margin. Step 5 Verify clamping voltage against downstream component limits. MOV degradation is cumulative. Each surge increases leakage current and shifts varistor voltage. Design must account for repetitive surge, thermal spacing, and avoidance of operation near maximum continuous voltage. Place the MOV at the power entry point with short traces. Install across line and neutral for differential mode protection. Add line-to-ground MOV when required by surge category. MOV handles high surge energy but does not always reduce residual voltage to IC-safe levels. A coordinated network may include a common mode choke and a secondary clamp at the DC rail to reduce stress on control ICs. Input AC 230 V with surge requirement of 1 kV line-to-line and 2 kV line-to-ground. Use a 14D MOV across line and neutral with series impedance before the bridge rectifier. Verify DC bus clamping voltage and system recovery after surge. Measure DC bus clamping voltage during surge. Monitor MOV temperature under repetitive pulses. Check leakage current after stress testing. Validate functional recovery time. AC RMS voltage and tolerance Surge level and coupling mode Peak current waveform Number of pulses Maximum allowable clamping voltage Available PCB space IEC 61000-4-5 surge compliance requires MOV selection based on energy, peak current, disc diameter, and placement. Coordinated protection reduces residual stress and improves system reliability.
Surge Stress Characteristics
Common MOV Design Failure Modes
Disc Size and Surge Energy Relationship
Disc Size Typical Surge Capability Use Case 5D Low energy Secondary DC protection 7D Moderate energy Adapters and small SMPS 10D Industrial low power Control boards 14D High surge Motor drives 20D Very high surge Mains distribution Parameter-Driven MOV Selection
Thermal and Lifetime Considerations
Placement Strategy
Layered Protection Architecture
Application Example AC-DC Industrial Power Supply
Verification Method
Design Parameters Required
Conclusion
