Thermal runaway

Thermal runaway in a bypass diode happens when the heat released as the diode switches from one operating mode to another is more than the diode can get rid of. It is listed as a root cause of bypass diode failure during operation, alongside electrostatic discharge and diode aging. Diodes that failed this way show visible damage. The IEC 62979 standard tests whether a diode, as mounted in its module, can get thermal runaway. It checks whether cooling is good enough for the diode to move from forward bias to reverse bias without overheating. Inside the module, thermal runaway can also start when a reverse current heats a spot. The heat pulls more current into that spot, which can leave burn marks. These burn marks usually form where weak spots left by production or handling meet operating conditions such as shading, open-circuited bypass diodes or reverse current flow. Bypass diode failures are more common in hot climates and in diodes that switch on more often, for example when partial shading keeps coming back. Avoiding partial shading makes diodes last longer. If you find hotspots or burn marks, check the bypass diodes to make sure they still switch on, so the damage does not get worse.

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• sentence 2: support check: partial - Thermal runaway is listed as an operation-stage root cause alongside ESD and aging, but the figure caption only shows a diode failed by thermal runaway; visible damage is not stated. -->