Bypass diode

In a string, every cell carries the same current. If one cell is shaded, it cannot produce that current and is forced into reverse bias by the others, dissipating their power as heat. Temperatures in such a hot spot can exceed 100–150 °C, damaging encapsulant and backsheet and, in extreme cases, causing fire.

Bypass diodes prevent this. A typical module has three diodes in the junction box, each protecting a substring of 20–24 cells (or a pair of substrings in half-cut layouts). When a cell in a substring is shaded enough that the substring voltage turns negative, the diode becomes forward-biased and current flows around it.

• The bypassed substring contributes nothing, so the module loses about one-third of its power and voltage, but the rest of the string keeps producing.
• Schottky diodes are used for their low forward voltage of about 0.4–0.5 V, limiting heating in the junction box.
• Diodes can fail open (losing protection) or short (permanently removing a substring), visible in thermal imaging and I-V curve tests.

Activated diodes create multiple local maxima on the array power-voltage curve, so inverters need a global-scan MPPT routine to find the true peak. Half-cut cell modules split into upper and lower halves that work in parallel, reducing the impact of row-edge shading. Module-level electronics such as optimisers complement but do not replace bypass diodes.