Re: Series or Parallel???

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Dave Lewis by way of Terry Fritz <twftesla-at-qwest-dot-net>" <hvdave-at-earthlink-dot-net>




> In my experience, diodes of any type in series give a higher voltage rating
> (Sum). VT Diodes in parallel give more current. Solid state diodes in
parallel,
> unless almost perfectly matched to the usec in response time, soon give more
> smoke.
> 
> Matt D.
> G3-1085

The problem with parallel diodes is really due to the negative
temperature coefficient of the forward voltage drop rather than reverse
recovery time.   As one of the diodes gets a bit hotter than the others,
it hogs more of the current thereby making it hotter still.  This
continues until just one of the diodes is conducting a large majority of
the current, which will soon fail if the current is more than the diode
alone can handle.   This is usually the case since the reason multiple
diodes were paralleled in the first place was because the current was
too high for just a single diode.

Reverse recovery plays a key part in how diodes are stressed when placed
in series. Miss matches in trr can cause extra power dissipation on the
faster diodes as the string transitions from forward bias to reverse
bias.  This is because the recovery current for the slower diodes has to
avalanche through the faster ones which are already fully turned off. 
The avalanche voltage is higher than the rated voltage for the diode.  
Usually this is not much of a problem for most applications and indeed
many diodes are rugged enough to handle getting avalanched at low duty. 
If the diodes are already getting hot due to high forward currents
and/or bad heat sinking then miss matches in trr could be the straw that
breaks the camels back.

Regards
Dave Lewis