Rich says:
>Not a good wager. Without equalization, neither diode's piv limit is
>exceeded and reverse current is virtually zero. With equalization the
>101v diode is toast.
Sorry Rich - NO. Let me repeat:
> > If the PIVs are that different, the capacities are likely to be so
>
Now, when two capacitors are placed in series ACROSS DC, let alone AC, the
voltage divides inversely proportional to the capacitance. So the smaller
capacitance gets the most voltage. As the smaller capacitance tends to go
with the higher diode reverse voltage for a given leakage, that means that
the higher voltage diode gets more of the volts, and the lower voltage diode
gets less.
Next factor is the cause of the leakage. It could be straight electric field
breakdown across the barrier, or it could be surface leakage, or it could be
a crystalline defect in the silicon - defects need not cause a catastrophic
problem in large geometry devices, depending on exactly what they are. All
these leakage causes have different V/I characteristics, so once the
voltage exceeds an amount that is the 'reverse voltage measured at some
specified current' we can get even more rapid differential voltage
distribution.
It might be such as to improve matters. However, 'Murphy rules'. So it
probably won't improve matters, but will screw up the voltage distribution
between diodes even more.
The bottom line there is that if the surge is to be 250 volts, you need 250
volts worth of diodes under all conditions, and assuming equal voltage
distribution - preferably 300 volts or even more. Keeping surges within
bounds is also useful, but that's why we have varistors and AC rated
capacitors.
73
Peter G3RZP
--
FAQ on WWW: http://www.contesting.com/ampsfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|