[sdiy] how to double Si diode voltage rating

[ASSI]

On Dienstag 26 Februar 2008, Roy J. Tellason wrote:
> I've read fairly recently that almost *no* rectifiers are being made
> any more at under 1000PIV,  and that the ones you buy that are
> labeled as such are the higher-rated parts simply labeled
> differently.

Funny you should say that, as (somewhat related to the date-yourself 
contest going on in another thread) almost exactly two decades ago I 
was operating a sorter for 25A diodes.  We did have a decidedly bimodal 
distribution on the breakdown voltage (the two modes centered around 
500V and 1000V) until some clever chap had the idea to freeze off the 
diodes from the dicing tape instead of peeling them.  That change 
resulted in a very narrow distribution around 1200V and we actually 
tossed everything that was binned below 900V.  The diodes were stamped 
with whatever voltage class and manufacturer label was the order of the 
day.

OTOH, if the particular series in question does not have voltage ratings 
going up that high, proper engineering would result in a lesser median 
breakdown voltage.  I do not, however, expect this practise of extreme 
downbinning to be common except for power diodes as discussed above, 
where it simply doesn't make sense to tune the diffusion recipes for 
each breakdown voltage.

On the original question of putting diodes in series or parallel: it 
used to be fairly common to series connect diodes without any further 
precautions, but todays diodes have not enough leakage for that IMHO.  
You'd need symmetrizing high-ohmic resistors in parallel to the diodes 
to keep them at about the same voltage in reverse bias.  The same goes 
for connecting them in parallel: you'd need resistors in front of each 
diode to keep one from running away.  If the diodes had a really lousy 
dynamic resistance and carefully matched forward drop, you might get 
away without the resistors, but that sort of defeats the purpose of 
getting more current - just use a better diode and have less waste 
heat.  If you want to become creative with parts you already have at 
hand, replacing a diode with a diode connected (collector and base tied 
together) transistor can be done.


Achim.
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