Bypass diode

A bypass diode is built into the PV module, connected in parallel and with reverse polarity across a group of cells, and sits inside the junction box. It limits the power lost to partial shading and stops single cells from being driven into reverse bias beyond their allowed voltage. In normal operation the diode is reverse biased and idle; when cells are mismatched or shaded it becomes forward biased, carries the string current and runs hotter than inactive diodes. Schottky diodes are the usual type, and they are easily damaged by static discharge and mechanical stress, so handle them only when grounded. Bypass diode failure shows up as an open or short circuit: a shorted diode keeps module voltage and power low, while an open diode leaves cells exposed to reverse bias, hotspots, burn marks and possibly fire. Cell interconnection failure can keep the diode switched on and age it early, and junction box failure can damage the diodes; in turn, a failing diode can deform or burn the junction box. Check diodes with a multimeter in diode or resistance mode, and back this up with open-circuit voltage, IV curves and infrared thermography; open diodes are hard to find unless the module is strongly mismatched. IEC 62979:2017 tests whether a diode mounted in a module can go into thermal runaway, and IEC TS 62916:2017 covers electrostatic discharge testing of bypass diodes.