Can I operate the recommended MD90 F18 and MD90U25 replacements without over potting them?

The recommended replacement for the MD90F18 is the SXF20FF3. The recommended replacement for the MD90U25 is the SXF6523. If the diodes are operated at full rated voltage, the voltage stressed between leads will exceed the general design guideline of 10V/mil in air. It is recommended that the isolation voltage between leads be increased through the use of a conformal coating, the use of a dielectric liquid or vapor. 

Can I operate an axial-leaded 10KV diode without it arcing from lead-to-lead?

A rule-of-thumb for determining whether additional encapsulation is required is the 10KV/inch isolation in air rule, which assumes relatively low humidity and clean air (no particulates on the diode). For example, using the 10kV/inch rule for the 1N6517 diode (5kV, 1A, 70ns), based on body size and distance between leads, it could operate up to 2900V in clean, dry air. Above 2900V, or in less clean conditions, the diode should be encapsulated or operated in oil or some other dielectric fluid.

Many factors can effect whether the 10KV/inch rule is effective in applications. Additional factors include humidity levels, thickness of the glass passivation, body length between leads, ambient temperatures, vacuum levels, and power levels.

Generally speaking, additional isolation should be provided for diodes operating a levels of 5KV or higher.

Can diodes be matched for Trr?

Occasionally in high frequency applications where voltage rise times are fast, matching Trr can be advantageous. This only works for single junction diodes (1KV or less). Individual junctions in high voltage multi-junction diodes (greater than 1KV) cannot be accessed.

Multi-junction diodes work very well in high frequency applications.

Do VMI diodes have avalanche capability?

Yes and no. 100% of our diodes are tested under avalanche conditions at 100uA. Yes, all of our diodes have some avalanche capability.

The real question becomes, “How much?” That varies from diode to diode and depends on several factors including number of junctions, junction area, resistivity of the wafers, the energy or power dissipated, and more.

In avalanche mode, a diode is reverse biased. Leakage current increases exponentially as the reverse voltage increases. Dissipated power increases as leakage current increases, which heats up the diode junction. Heating up the diode junction increases the leakage current, which increases the dissipated power, and so on until the diode fails catastrophically. If the diode is kept cool, it will continue to operate indefinitely.

VMI diodes are tested in such a way as to ensure reverse leakage current is low enough at the rated PIV to help prevent thermal runaway commonly associated with avalanche mode operation.

If the customer knows what the avalanche requirement is, VMI can determine if the diode in mind will meet it.