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Diodes - Multi-junction Reverse Recovery Time
Deep-diffused multi-junction diodes ideally address the problem of individual diodes with mismatched reverse recovery times. The question is, how does one prevent the fastest diode from seeing all reverse voltage while the other diodes are recovering? As stated earlier, reverse recovery time varies with temperature, resistivity, and dopant concentrations. The concentric use of matched wafers ensures closely matched dice that track with temperature much better than discrete diodes. Also, each die can survive while operating in the reverse avalanche mode, in the event that one chip recovers slightly faster than another.

The worst-case temperature of the center junction is often perceived as an uncontrollable, frightening malady, when it is in fact, merely a matter of thermal management. Silicon, as it happens, is a very good conductor which compares favorably with some aluminum alloys. As a result, it provides an efficient thermal path through the multi-junction stack to the leads.

An important factor in any high-voltage application is corona, a partial discharge that results from ionization of air in a gap that is under voltage stress. Over time, the presence of corona destroys insulation and is particularly destructive in trapped-air voices near high-voltage sources. The geometry of the multi-junction deep-diffused diode is cylindrical and void-free.

Moreover, it is not subjected to corona if it is encapsulated in a dielectric. Also, the replacement of several diodes with a single multi-junction diode often leads to substantially fewer corona problems in potted assemblies because unnecessary air traps are eliminated.

Last revised: 21 May 2014