Breakdown Characteristics of Schottky Barrier Diodes Used as Bypass Diodes in Photovoltaic Modules under Lightning Surges

Damage to photovoltaic power-generation systems by lightning causes the failure of bypass diodes (BPDs) in solar cell modules. Bypass diodes damaged by lightning experience high-resistance open- or short-circuit failures. When a bypass diode experiences short-circuit failure due to indirect lightning, the damage may not be immediately visible. When solar radiation is subsequently received, the current circulating in the closed circuit formed by the cell string and short-circuited bypass diode flows, resulting in overheating and burnout of the short-circuited bypass diode. The authors’ research group previously reported that when a bypass diode fails within a range of approximately 10 −1 Ω to 10 Ω, the heat generated by the failed bypass diode is high, posing the risk of burnout. However, the detailed failure characteristics of the bypass diode that fail because of indirect lightning surges are not clear. In this study, we performed indirect lightning fracture tests and clarified the dielectric breakdown characteristics of Schottky barrier diodes (SBDs) contained in the bypass diodes of photovoltaic solar cell modules, which are subjected to indirect lightning surges. Furthermore, we attempted to determine the conditions of indirect lightning that resulted in a higher risk of heat and ignition. As a result, short-circuit failures occurred in all the Schottky barrier diodes that were destroyed in the forward or reverse direction because of the indirect lightning surges. Moreover, the fault resistance decreased as the indirect lightning surge charge increased. These results indicate that the risks of heat generation and burnout increase when the Schottky barrier diode fails with a relatively low electric charge from an indirect lightning surge. In addition, we observed that for a forward breakdown of the Schottky barrier diode, the range of the indirect lightning surge that results in a fault condition with a higher risk of heat generation and burnout is wider than that for a reverse breakdown.