On Mon, 2004-09-20 at 16:02, R.Measures wrote:
> On Sep 20, 2004, at 7:38 AM, Steve Thompson wrote:
>
> > On Monday 20 September 2004 14:08, peter.chadwick@Zarlink.Com wrote:
> >> Rich said:
> >>> Please explain why equalizing resistors have merit.
> >>
> >> Because if the reverse leakage differs between diodes, the reverse
> >> voltage
> >> distribution also differs. The one with the least leakage ends up
> >> with the
> >> highest voltage across it, which could exceed the breakdown volts.
> >> With
> >> resistors, the voltage distribution across the diodes is fixed by the
> >> resistor ratios.
> >>
> >> If the capacitances aren't matched, that will also affect the voltage
> >> distribution. That's one reason why it is not wise to mix diodes of
> >> the
> >> same nominal type from different manufacturers without some
> >> equalisation.
> >> Another is the reverse recovery time.
>
> > If I understand it correctly, equalising resistors might hurt you only
> > if you
> > get a reverse voltage spike that exceeds the specified breakdown of
> > the diode
> > chain. In many, if not most, power supplies will a modest mains filter
> > all
> > but eliminate that likelyhood?
>
> Steve - In the example I gave, adding equalizer resistors would cause
> the lesser piv diode to fail and take out the other diodes in the
> series string without any spike having come along.
Sorry, but I don't have that example to refer to right now. If the
diodes are modern avalanche types and the current in the resistors is
much greater than the leakage in the diode then I can't see how the
chain can break down at significantly less than the specified reverse
voltage. The data I have for 1N540* gives 10uA at 25C and 100uA at 150C
max. at the specified voltage. A 470k resistor will pass around 2mA,
which will dominate the control of voltage sharing.
Steve
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|