TECHNOLOGYENERGY
temperature observed during shading. But if 31 percent of the modules initially had trouble with the diode test before requiring a redesign, and the diodes are almost at the datasheet rating, these diodes will likely not survive 30 years in the field for all possible mission profiles.
In the course of investigating bypass diode failures, examination of forward operation over heating, reverse bias thermal runaway, “shaded-to- unshaded” transition survivability, surge (lightning) survivability, and packaging fatigue were explored. Diodes that had failed in the field within a couple of years were examined. Controlled, “mildly” accelerated testing was conducted in the lab. Following is a summary of these examinations.
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Forward voltage over heating During shading events, bypass diodes have junction temperatures reaching upwards of 150 to 200 ºC. The junction box on the back of the module might peak at 90 ºC, but the diode has significant self-heating with virtually no air flow.
Failed Diode 1
Qualification Testing: 2007 vs. 2005”) showing that, between 2005 and 2007, 31 percent of the silicon modules submitted for IEC 61215 testing failed the bypass diode test. Comparatively, from 1995 to 2005, only 4 percent of the submitted modules were failing due to the bypass diode test. During the diode shading test, it was commonly observed that the diode operated at 150-200 ºC.
Before a module is sold, it will have passed IEC 61215. The diodes will have a datasheet rating that is at least 1 degree beyond the 150-200 ºC
In an experiment with an oven controlled to +125 ºC (with air flow), 50 axial leaded diodes (rated at 12 amps/40 volts/150 ºC) were statically biased at 10 amps. One does not commonly think of a 125 ºC oven as a being a mechanism for cooling, but in this case, the air flow served to strip away much of the self-heating. Lacking airflow, the junction temperature would have been much hotter. After 1000 hours, 2 of 50 diodes had failed.
Surprisingly, though, this did not seem to be the dominant mode of failure. One shortcoming of this experiment was that the oven had a fan equalizing the chamber temperature. It would have been more realistic to put the diodes in a sealed junction box with no internal airflow, and then put the junction boxes (under 10 amp bias)
Failed diode 2
www.solar-pv-management.com Issue VI 2010
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