TECHNOLOGYENERGY


thermal runaway occurs. For the case of -20 volts, thermal runaway occurred earlier.


From the “Shaded” to “Unshaded” Lacking airflow in the junction box, the forward biased diode during shading can reach 150-200 ºC. When the bypass diode returns to its normal reverse-biased condition, the temperature of the diode will cool down. But this cooling does not happen right away. This is a critical point to understand. During this transition, the diode leakage will be very high due to the residual forward-biased self-heating (as high as 0.1 to 0.5 amps) and, in turn, can easily maintain self- heating due to high leakage current multiplied by the 10 volts of reverse bias.


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With the reverse biased diodes tested at 105 ºC, 20 percent failed at 500 hours. Increase the temperature to 155 ºC and expect 20% failures at 31 hours (by Arrhenius’s equation). In instances of frequent shading, the accumulated effect of the “shaded-to-unshaded” transition periods will degrade the lifetime of the diode.


This was observed in the lab under controlled conditions. The same diode was placed in a pre- heated oven at 85 ºC with no air flow (important: no air flow). A forward current of 4.75 A was applied until reasonable self-heating occurred. When the current was shut off and -10 volt bias was applied, the leakage current immediately increased to 125mA and then entered thermal runaway, and ultimately, tripped the safety clamp of the current supply. With 4.5 A, the result was just short of a thermal runaway event. At the lab- replicated “shaded-to-unshaded” transition, the leakage current shot to 75mA and then slowly decayed back to 2mA approximately following.


Shawn Fahrenbruch is an analogue integrated circuit design engineer employed by Microsemi Corporation. He received is BSEE/MSEE from Montana State University in 1993/1995. Most recently, he designed Microsemi’s LX2400 lossless


“CoolRUN” solar bypass active diode.


Although some heat sinking was used in the above experiment, it apparently was not enough. To the best of this author’s knowledge, this potential failure mode has not been investigated. But considering that it was created fairly easily in the lab, it is very possible that it is occasionally occurring. Two possible solutions are possible for the problem of over-heating. Add an infinite heat sink to prevent the bypass diode’s junction temperature from ever getting above 100 ºC during forward conduction. Alternately, migrate to a new technology node and use lossless diodes with negligible self-heating. A “lossless” diode with a 40-50mV forward voltage at 10 amperes will only generate a 5 to 10 ºC rise over ambient.


Lightning/Surge Current Survivability Solar systems are fully exposed to outdoor conditions and must endure at least some transient energy induced by nearby lightning storms. The frequency of lightning strikes per Megawatt of installed power varies by region, but it is not a negligible number.


A very good study was performed in 2007 by Professor Haeberlin of Berne University (“Damages at Bypass Diodes by Induced Voltages and Currents in PV Modules Caused by Nearby Lightning Currents”). The reported failure mode for the diode was interesting. The majority of the transient energy stressing the bypass diode was not coupled in via the “mains.” It was actually coupled in via a local magnetic loop antenna. This local loop was comprised of the bypass grouping of 12 to 24 cells with a return path through the bypass diode. This magnetic loop antenna area depends on the module’s cell layout. As the lightning surge strikes nearby with 250kA/µs, the magnetic field couples into the local loop antenna. This in turn induces 100’s to 1000’s of amperes of transient current that the bypass diode must endure.


IEC 61000-4-5 gives a standardized IEC method for examining this effect. A capacitor bank is pre- charged to some level (i.e., 600 volts) and then discharged through an inductor into a design under test. Schottky diodes can withstand about 0.05 to 0.1 Joules of this energy when injected into the cathode before permanent damage occurs. By comparison, “Lossless” diodes can survive about 1.4 Joules (or more). Many module warranties today do not cover lightning damage. Lacking a direct strike, however, there will not be a “tell-tale” that lightning made the diode leaky. As a result, the manufacturer may have to honor the warranty. It is difficult for a failure analysis effort to say, definitively, what caused a diode to be leaky once it is removed from the field.


Conclusion


Solar installation owners are chasing every Joule of harvestable energy and will demand higher performance if they detect that their operational profit margins are at risk. Tracking bypass diode failures has traditionally been a challenge, but now, with the advent of Individual Module Monitoring solutions, the incidence of detected failure will likely rise. It may be time for the bypass function to move to a new technology node of “lossless” diodes.


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


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