On Mon, 20 Sep 2004 14:08:51 +0100 peter.chadwick@Zarlink.Com writes:
Good morning Rich & Peter,
>
> 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.
Thus, creating a known operating condition. Without the R's, one is
relying upon the random reverse resistances & the unspecified avalanche
characteristics of common diodes to set the voltage distribution.
Without the R's, a string of unmatched and/or non-avalanche rated
diodes might, or might not, exhibit a higher reverse breakdown capability
but, this figure would wander around with temperature as it effects
leakage resistance. One diode in the string (likely the best, lowest
leakage, unit!) would be the most stressed and prone to early failure.
> 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.
C's are used for transient protection in a low frequency rectifier
application. They should be selected to be large enough to swamp the
diode's intrinsic C to but, not so large as to drive excessive avalanche
energy into the individual diodes during an "event".
I believe that modern commercial high power rectifier assemblies use
some form of varistor protection across the individual diodes to avoid
the secondary effects of C's.
> Another is the reverse recovery time.
Not really an important factor in a 50/60 Hz sine wave operation.
Though, I'll note here that the first rectifier to recover in a string
will slow the recovery of the remaining diodes since their recovery
mechanism will then be the relatively slow recombination of the minority
carriers, rather than sweeping them out by reverse current flow.
>
> Experience also suggests that for best reliability, buy diodes from
> reputable manufacturers - some of the far Eastern ones are distinctly
dodgy
> from a reliability viewpoint.
>
> 73
>
> Peter G3RZP
>
>
73,
Marv WC6W
P.S. -- Here are two more good references on the topic:
Rca Silicon Power Circuits Manual, SB-50, 1967, pp. 23-26
International Rectifier Superpower High Voltage Rectifier Columns,
SR-370-A, 1960, p. 3
*
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