This page contains a Flash digital edition of a book.
TECHNOLOGYENERGY


Svante Arrhenius’s equation is studied, it would suggest the answer is “no.”


The same effect applies to the diode packaging. An interesting study relating to temperature effects on plastic encapsulated devices was published in 1997 by John Devaney, et al. (“Thermal wear-out of plastic encapsulated devices”) showing that high temperature significantly shortens the lifetime of the packaging. (See Fig. 4).


Failed diode 3


into the 125ºC oven for accelerated testing. Under that condition, the self-heating of the diodes would behave more realistically, and the results of the experiment would be more pronounced. However, the material cost of accelerated-testing on diodes is far cheaper than that of fully assembled junction boxes, so tradeoffs were made.


There is evidence that prolonged over-heating is a culprit for failures, even though the controlled 125 ºC burn-in experiment only showed a few percent of the diodes failing. Diodes that failed in the field within two years were received from a well- respected, well-established module manufacturer. Failure analysis was performed on the 10 units that were received. Most of them had a failure signature that they were failing due to sustained over-heating over a long period of time. Some of the more interesting photos are shown below.


It


was not clear if this over-heating was due to forward or reverse operation.


All engineers are familiar with Arrhenius’s Equation. Assuming standard activation energies (“Ea”) for silicon devices, a good rule of thumb is that for every additional 10ºC rise in temperature, the expected time to intrinsic failure is cut in half. IEC 61215 tests the module at 85 ºC, and as a by- product drives the bypass diode temperature up to 150-200 ºC during the bypass diode test. Many diode manufacturers optimistically rate their datasheets based on survivability of 1000 diodes in a controlled-temperature oven for 2000 hours. This test condition might be 125 or 150 ºC with good air flow to maintain the overall oven temperature. The presence of this air flow, unfortunately, strips away excess heat due to self- heating. A typical solar module warranty might be for 30 years. Is there really enough guard-band in the operating temperature of these diodes? If


Reverse Bias Thermal Runaway An unexpected result occurred when testing 50 of the same 12-amp/40-volt solar bypass diodes at 105 degrees with -15 volts statically applied. At approximately 500 hours, about 20 percent of the diodes self-destructed. In a well-designed installation the diodes are in standby mode for the majority of the time with perhaps -10 volts bias. A test condition of 105 ºC and -15 volts is not that much acceleration. It is hard to believe that the thermal runaway observed after 500 hours of mild acceleration would not occur at some point within 30 years given normal mission profiles.


After observing this thermal runaway, a closer look was given to reverse leakage. A common mistake that manufacturers make when rating their diodes is to only consider the leakage at +85 or +95 ºC. It will be shown why this is a bad design choice. For the axial-leaded 12A/40V Schottky diodes used during the 500 hour thermal runaway events, the leakage doubles every 10 ºC. This is typical of all classical diodes (but not including “lossless” diodes). The measured results are in Fig. 5.


Note that the leakage at 105 ºC was only about 5mA. Further, notice that at 150 ºC, the diode leakage is now above 100mA and can only be applied for a very short time (1-2 seconds) before


25


Failed diode 4


www.solar-pv-management.com Issue VI 2010


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64
Produced with Yudu - www.yudu.com