Thursday, March 22, 2012

Multi-junction High Voltage Diodes

VMI Multi-junction High Voltage Diode
 Once in a while we get a customer inquiry about connecting diodes in series to get a higher reverse blocking voltage in their application.

Sometimes they express concern about one diode having a faster Trr than the others. The thought is that when the applied voltage starts to switch negative, the faster diode would hold off the entire PIV until the rest of the diodes in series could catch up. The question is whether it would cause a catastrophic failure by shorting out first one diode, then the next, and the next, and so one, until the whole string failed.

Fortunately, that is rarely the case. For one thing, reverse voltage does not happen instantaneously. There is a slew rate, and if the reverse recoverytime, Trr, of the diode is sufficiently fast for the application, it will block before the full reverse voltage is experienced, thus preventing a cascade of failing diodes.

Secondly, anytime you work with a diode greater than roughly 1200 Vrwm, there is more than one diode junction in the package. A 10kV diode used multiple internal junctions. It is the rough equivalent of connecting ten single junction diodes in series, in a much, much smaller package.

Multi-junction diodes are made from the same wafer and diffusion lot, so variations in Trr are kept to a minimum. With continuous improvements made to the diffusion and manufacturing processes, process variations have been further minimized. All of which goes towards improved product reliability and quality.  And you know that's good for you!

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