Overview
IEC 62979:2017 - "Photovoltaic modules – Bypass diode – Thermal runaway test" - defines a standardized method to determine whether bypass diodes, as mounted in PV modules or junction boxes, are susceptible to thermal runaway when switching from forward-bias (conducting during shading) back to reverse-bias operation. The standard is particularly targeted at Schottky barrier diodes, whose reverse leakage rises with temperature, and therefore have higher thermal runaway risk. Modules using P/N diodes are exempted.
Key topics and requirements
- Purpose: Verify that bypass diode and junction-box thermal design prevent overheating and catastrophic breakdown when diode transitions from forward to reverse bias.
- Test conditions:
- Initial module temperature: (90 ± 2) °C (reduced to 75 ± 2 °C for modules labeled “For use in open rack mount only”).
- Forward current applied to diode: 1.25 × short-circuit current (Isc) at STC for the affected PV module string.
- Reverse bias voltage applied: open-circuit voltage (Voc) of the cell string protected by the diode.
- Specimen preparation: Use the actual module or a special sample reproducing the module/junction-box construction. Bypass diode must be mounted identically to production. Cell strings in full modules can be electrically disconnected for testing.
- Measurements & instrumentation: Thermocouples mounted on the cathode lead-wire as close as possible to the diode body to record lead temperature (Tlead). Required test equipment includes a temperature chamber capable of (90 ± 2) °C, temperature monitoring with specified accuracy, and circuitry for applying forward current and reverse bias while recording voltage and temperature.
- Procedure & criteria: The standard provides the test procedure, typical thermal patterns (runaway vs. non-runaway), pass/fail criteria and reporting requirements to document outcomes and measured parameters.
Applications and users
IEC 62979 is intended for:
- PV module manufacturers validating junction-box and diode thermal designs.
- Junction-box and diode suppliers (especially Schottky diode vendors) ensuring product suitability.
- Test laboratories and certification bodies performing type and qualification tests.
- R&D and reliability engineers assessing shading-fault behavior and thermal safety.
Use cases include design verification, failure-mode analysis, and supporting module certification efforts.
Related standards
- IEC 61215 (module design qualification tests) - referenced for bypass diode thermal testing context (see IEC 61215-2:2016, clause 4.18.1).
- IEC TS 61836 - terms, definitions and symbols for PV systems.
Keywords: IEC 62979, thermal runaway test, bypass diode, photovoltaic modules, Schottky barrier diode, PV module testing, junction box, bypass diode thermal.