TECHNOLOGYENERGY
functional cells account for the remaining 370 volts. With cell currents approaching 10 amperes, this shaded cell could now dissipate 300 watts. This can be destructive.
The industry’s solution has been to provide a bypass path. Typically, a bypass path is provided around every 12 to 24 cells (see Fig. 2). The choice of 12 to 24 cells for bypass groupings comes from a comparison of the summation of the forward voltages versus the expected breakdown voltage of the weakest cell in that grouping. For example, in a group of 24 cells, each with a forward voltage of 0.5 volts, an overall voltage of 12 volts will be produced. Each of these same cells hopefully has a reverse breakdown voltage in excess of 25 to 30 volts. If the bypass is activated, the protected local loop’s voltage will be lower than the members’ reverse breakdown voltages.
Initially, the industry used PN diodes to provide the bypass path. These bypass diodes had a forward voltage of 0.7 to 1.0 volts and reverse breakdown rating of 600 volts. With low amperage, the diode’s heat was acceptable. But as the cell efficiency improved and the wafer size increased, the string currents increased to 5, 6, 8 and even 10 amperes. This drove the industry to adopt Schottky diodes. With Schottky forward voltages of 0.4 to 0.5 volts, power dissipated during bypass mode was cut in half. From a heat-management perspective, this helped. But unlike classical PN diodes, Schottky diodes typically have reverse breakdown voltages of 40 to 60 volts. This introduced new problems. Schottky diodes are leaky at high temperatures and they are easily permanently damaged by transient energy. If they fail “open,” this can leave the corresponding cells in the grouping vulnerable to a destructive “hot spot” event during the next occurrence of shading or soiling. If they fail “shorted,” this will (at a minimum) steal produced energy.
There is one more subtle benefit to the bypass diode. With solar arrays, DC voltages are present and arcing can be disastrous. Unlike an AC system where the arc might be able to self-clear at the “zero-crossing” of the 50/60Hz waveform, a DC-generated arc will not self-extinguish. Bypass diodes provide some (by no means complete) protection against “series” arcs within the module itself, because they limit the local arcing voltage to 10 to 20 volts. This is very important.
The bypass function may be ready for a new technology. There are a few companies that have
Fig: 5 Measured results
Fig: 4 High temperature significantly shortens packaging lifetime
recently developed a new category of diode. This new technology is promoted as a lossless diode because it might have a 40-50 mV forward voltage rather than the Schottky’s 0.4 volt forward voltage (the definition of “lossless” varies widely and should be examined closely with respect to operating conditions and lifetime expectations). Under reverse bias, these “lossless” diodes have high temperature leakages measured in micro- amperes rather than the milli-amperes of Schottky diodes. The evolution of bypass diode technology can perhaps be better understood with the help of fig 3. This brings the interesting question of whether diodes are failing in the field and, if so, what are the causes of failure.
Cause and effect
All modules must pass IEC 61215 and part of this test relates to bypass diodes. This test is not a measure of reliability. It is merely a qualification test that looks at the survivability/performance of the module (and diode) against its rating under controlled conditions. This test produces virtually no information as to the expected time-to-failure rates of the diodes in the field.
Even though IEC 61215 is just a “spot” test, an interesting result was recently presented by TamizhMani, et al. (“Failure Analysis of Design
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www.solar-pv-management.com Issue VI 2010
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