Diodes, Shading, and Reverse Bias: Task Group 4
Task Group 4 is focused on testing diodes, which are essential to protecting the module during partially shaded conditions.
There is increasing evidence that shading or other non-uniformities in modules causes localized stress that can lead to overheating and, in some cases, to fires. Not only is this failure a serious safety issue, but there is some evidence that aging modules show increasing non-uniformity, implying that this may turn out to be a significant wear-out mechanism. The details of the stresses that lead to these failures are not well understood but may be related to shading (and reverse bias operation associated with shading or other situations), high temperatures, and lightning. Thermal cycling may also be important because thermal fatigue failures can cause stress on the diodes.
The Task Group has been studying ESD failures in the United States using a human body model; and using a machine model in Japan. A technical specification/Guideline IEC 62916 was approved early this year 2014 and a committee draft has been prepared and is being reviewed. NWIP draft for "Thermal runaway test for bypass diodes" was submitted by team from Japan to TC82/WG2 and voting was due in September. Preparation of special measuring equipment for establishing Vf-Tj relation to calculate Tj will be undertaken by the team from Japan. The group in the USA is working on a simpler model for predicting thermal runaway in bypass diodes. Additional tests to validate theoretical models related to critical temperature using different diode manufacturers will be performed. This can potentially result in a NWIP for guidance in diode selection. Further test runs such as extended temperature soak tests will be performed, as performance and reliability is known to degrade with exposure to high temperatures for longer periods. Evidence of diode failures in the field has been observed by several groups and the intent is to investigate field diode failures to co-relate relevant tests that can generate a NWIP. Suggestions will be made based on test results to modify qualification test protocols.
The Chinese regional Task Group 4 was started in July 2014, with 22 participant organizations, including diode manufacturers, junction-box manufacturers, module manufacturers, system installers, universities, and testing and auditing organizations.
The Chinese Group has initiated efforts to:
- Define a diode test to be used to confirm that a diode is good, since sometimes the diodes increase in resistance without failing completely in the open or shorted configuration.
- Define a high temperature durability test for diodes.
- Support international efforts already underway on ESD and thermal runaway associated with the transition between the forward and reverse bias conditions.
Vivek Gade — Jabil, representing the Americas
Paul Robusto — Miasole, representing the Americas
Yasunori Uchida — JET, representing Japan
Xian Dong — Zhongshan University, representing China
Chandler Zhang — Hohai University, representing China
Volunteers representing other parts of the world are welcomed.