Bypass diode anomaly
A bypass diode behaving abnormally — shorted, leaking or overheating — rather than cleanly switching a shaded substring. Failed diodes cost energy and remove the module's hotspot protection.
Thermal signature
Either a substring held permanently warm regardless of shading conditions (shorted diode), or a concentrated warm point at the junction-box position where a leaking diode dissipates continuously. The tell against ordinary bypassing is persistence: the pattern is identical across frames, times and inspections.
Typical ΔT
2–6 °C over the affected substring for a shorted diode; a continuously dissipating diode can read 10–20 °C hotter at the junction box (typical industry interpretation aligned with IEC TS 62446-3).
What causes it
- Diode shorted by surge or lightning transients
- Thermal runaway from sustained conduction under chronic shading
- Manufacturing defects in the diode or its mounting
- Corrosion or moisture in the junction box degrading the diode's connections
Power impact
A shorted diode permanently removes its substring — about a third of the module — from production. An open-failed diode is quieter but riskier: the substring loses its bypass protection, so the next partial shading event can drive severe cell heating instead of a benign bypass.
Recommended action
Fix: next maintenance window
Test at the next maintenance window: verify each diode's forward and reverse behaviour at the junction box, replace failed diodes where the box design allows, and replace the module where it does not. Prioritise modules whose diode also shows junction-box heating.
Frequently asked questions
What does a bypass diode actually do?
It gives string current a path around a substring whose cells cannot carry it — because they are shaded, soiled or damaged. Without it, the weakest cell would be forced into reverse bias and heat severely. The diode trades a third of the module's output for protection of the cells.
Which is worse: a shorted diode or an open diode?
A shorted diode costs more energy — its substring never produces. An open diode costs nothing day-to-day but removes hotspot protection, so the first sustained partial shading can cook a cell. Shorted diodes show up clearly in thermography; open diodes are why flagged modules should have all diodes tested, not just the obvious one.
Related anomalies
Suspect bypass diode anomaly on your site?
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ΔT figures and abnormality classes on this page describe the typical industry interpretation aligned with IEC TS 62446-3 — indicative engineering guidance, not normative text from the specification. Temperature differentials are only meaningful when captured at ≥600 W/m² irradiance (per the TS) and verified on site before repair work.