Predicting thermal runaway in bypass diodes in photovoltaic modules

Abstract

Bypass diodes in photovoltaic (PV) modules can undergo thermal runaway while transitioning from forward bias state to reverse bias. Theoretical framework has been developed for predicting the susceptibility of bypass diodes to thermal runaway. The operating temperature of diode in forward bias is dependent on thermal resistance of diode-junction box system and the forward current through the diode. A new parameter -`critical temperature' (junction temperature at which forward power dissipation in diode equals reverse power dissipation for given forward current and reverse voltage) is introduced. Critical temperature is only dependent on the forward current through the diode and reverse voltage that would get applied to the diode in reverse bias. Critical temperature is shown to be independent of external factors such as thermal resistance of diode/junction box and ambient temperature. It is shown that the diode undergoes thermal runaway while transitioning from forward bias to reverse bias only if the operating diode junction temperature is greater than the critical temperature. Based on this understanding, a model is developed to predict vulnerability of various Schottky bypass diodes for thermal runaway as a function of thermal resistance and forward current. The results are experimentally verified using a specially developed setup for thermal runaway testing of bypass diodes.