[AMRadio] High Voltage Power Supplies


Jim Candela jcandela at prodigy.net
Sun Oct 30 11:00:18 EST 2005




Typo, Those diodes I mentioned are both 1000 volts
PIV. I incorrectly stated 100 volts.
1N5408 Data sheets:
http://www.datasheetcatalog.com/datasheets_pdf/1/N/5/4/1N5408.shtml
1N4007 data Sheets:
http://www.datasheetcatalog.com/datasheets_pdf/1/N/4/0/1N4007.shtml

Article about reverse breakdown voltage in a diode:
http://www.voltagemultipliers.com/html/bvr.html

excerpt from above:

"
In the past, the term controlled avalanche diodes
referred to those diodes that had "sharp" breakdown
characteristics and which would survive over-voltage
with controlled reverse current. The term
non-controlled avalanche diodes typically referred to
those diodes that had very high reverse currents at
considerably lower voltages than the breakdown
voltages. This high reverse current leads to
overheating and very round breakdown curves.

Rectifiers are generally subjected to a peak inverse
voltage (PIV) test to identify their breakdown
characteristics. This test is performed by applying
60-hertz half-wave reverse voltage of sufficient
amplitude to initiate breakdown. During the test, the
reverse current is usually limited to 50µA. The
resulting waveform is observed on an oscilloscope to
determine the sharpness of the "knee" at the point of
breakdown. Both planar and deep-diffusion processes
yield controlled avalanches under PIV test
conditions."


--- Jim Candela <jcandela at prodigy.net> wrote:

> Eddy,
> 
>     It might be helpful if you describe that power
> supply topology. Is it full wave with transformer
> center tap, full wave bridge, and is the filter a pi
> type, choke input filter, etc. If this is an option
> for you, make a sketch, and scan it into a JPEG
> file,
> and upload it to the net soemwhere. I have net space
> if you email it to me. This way we can all look at
> your schematic, and comment better without guessing.
> 
>    In general diodes fail from two transient
> factors.
> The first is current surge, and the second is
> avalanche reverse voltage breakdown. Since you added
> a
> step/start circuit already, the current surge issue
> should be contained. That leaves reverse breakdown.
> As
> Brian mentioned, a transformer primary varistor
> (like
> V130LA10A, for 115 volts, or V250LA10A, for 220
> volts)
> might help from power line transients.
> 
>     If you look at the Bill Orr handbooks around
> 1970,
> Bill goes into detail describing diode failure
> modes,
> and ways to protect them. This includes a custom
> series R-C across the transformer secondary, and
> across the filter choke (if choke input). Today's
> diodes are tougher, and this precaution is often
> unnecessary so long as good diodes are used, and the
> diode PIV rating is at least 2X what the formulas
> state you need. However diodes like the 1N4007 (1A
> 100
> PIV) need protection, whereas diodes like the 1N5408
> (3A 100 PIV) are a lot more rugged. In researching
> diodes, look for the term "controlled avalanche".
> These are the best because they can take repetitive
> PIV spikes beyond rating without turning into a
> piece
> of wire. The 1N4001 to 1N4007 series are not in this
> catagory. Unfortunately the data sheets sometimes
> omit
> the "controlled avalanche" term.
> 
> Regards,
> Jim Candela
> WD5JKO
> 
> --- Ed Swynar <gswynar at durham.net> wrote:
> 
> > 'Morning All,
> > 
> > I'm curious as to why I have to replace the
> silicon
> > diode strings in my 2500 VDC power supply about
> > every 8-10 years, or so...I just went through the
> > exercise again early this morning --- turned on
> the
> > B+ to my 2 x 813 linear, & got nothing back for my
> > trouble but smoke from inside the enclosed p.s.
> > unit...
> > 
> > Opened it up, & the resistor across the relay in
> my
> > time delay circuit --- transformer primary side
> ---
> > was fried. I checked the conductivity of both legs
> > of my diode string (it's a full-wave set-up), &
> sure
> > enough, one leg was A-OK, but the other displayed
> > conductivity on BOTH sides of each diode...not
> good.
> > 
> > Is this the reult of transients / voltage spikes
> > somehow "overwhelming" the diodes...? 
> > 
> > It used to be more frequent prior to the
> > incorporation of the delay circuitry --- but,
> > despite its presence, this HAS to be at least the
> > second time that a re-build was in order.
> > 
> > Any thoughts re. a dose(s) of possible
> "preventative
> > maintenance" for the future...? I'm at a loss as
> to
> > anything else here that I might try...
> > 
> > Thanks in advance...
> > 
> > ~73!~ Eddy VE3CUI - VE3XZ
> > 
> >
>
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