Hi Ian,
> W3VVN explained that modern rectifier diodes are designed not to fail by
> the destructive arc mechanism.
Tow or three years back, I called Motorola and talked Dave Hollander.
Dave contributed to the Motorola Rectifier Handbook, and that
handbook has absolutely no information of series HV strings.
Dave couldn't answer the question about "is equalizing necessary",
all he could do was quote amateur publications. He gave me the name
of a semiconductor power rectifier design engineer.
The design engineer laughed when I asked him about "modern diodes".
He said the manufacturing process hasn't changed since the early days
of the 1N400x series of diodes. As a matter of fact, he said there
was LESS control today that years ago. Now the test is one of equal
or better, rather than a curve trace method.
But the conclusion was the same as you have below, with the
exception that the arcing stopped way back in the 60's...... so
"modern diodes" must mean less than 30 years old, hi. As the owner of
several 1960's vintage muscle-cars, I'm not used to "modern"
being that old! ;-) To me that's antique!
> The economics have also changed. PIV is now cheap, but high-voltage Rs
> and Cs probably cost as much as the diodes. Thus it's possible to build-
> in plenty of PIV (at least 2x the nominal) and not use the Rs and
Cs.
Bingo. That's what the engineer said. Use more diodes (at least two
times expected PIV) and the caps and resistors can be removed. Of
course I use the caps, because I like some RF and transient
protection. Now with the darned CE testing, amplifier have to have
bypass caps or diode-generated HARMONICS of the power line
frequency can cause a PA to fail conducted EMI tests.
> There are still problems with mixed-lot diodes having unequal reverse
> leakage at high temperatures. In a 1-2A HV supply it's obviously a high-
> risk strategy to use mixed lots of 1N4007s of unknown origin - they are
> being pushed to the limits of their current rating, and you can bet that
> a cheapskate manufacturer won't use enough of them either. The ideal
> solution is to use strings of diodes that are matched in a high-
> temperature-reverse-bias test. On the other hand, it seems pretty safe
> to use branded 1N5408s (3A rated) from a reputable supplier, all off the
> same reel, and with enough diodes to give a good margin on PIV without
> help from any Rs and Cs.
He also warned me about temperature and the mixed lots. In the same
box, you might get diodes from different countries as well batches,
hi.
> Also, we now have better ways of dealing with mains spikes than
> connecting a C across each diode. Mains filters have never been cheaper
> than they are today. In Europe it's practically impossible to market any
> large power supply without a mains filter to ensure compliance with
> interference regulations - that is, to prevent interference FROM the
> power supply going back into the mains - but the filter also protects
> the PSU components from mains-borne transients.
See, you brought up that nasty CE subject. Caps are cheaper than the
mains filter.
> No individual builds enough power supplies to collect reliable
> statistics on failures. The only reliable sources are commercial PSU
> designers such as W3VVN, which is why I took good care to check the
> background before going into print.
Ameritron has sold ten's of thousands of HF amplifiers using 1N5408's
with matched value .01 uF caps across the diodes. Out of all those
PA's, diode failures are almost unheard of. The early AL-80 (Denny
Had vintage) used 1/4 watt 470k resistors, the resistors would
commonly arc and wipe out the 5408's. Diode failures were common
(along with toasted diodes).
Case history seems to indicate resistors are a problem when under
rated for voltage, and high voltage rated resistors cost more than
the diodes. The capacitors do, however, provide some benefits many
people seem to agree about.
Thank you for the interesting post, I'm pleased the basic information
agrees with what I've learned through other channels. "Classic"
silicon power rectifier diodes apparently do not need equalizing, if
you use enough safety margin and watch the temperature.
73, Tom W8JI
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