## You wont see avalanche diodes used in commercial HV
rectifier assys...used by broadcast. Check out the diode tech notes
by the diode makers. Even they don’t recommend using super fast diodes
for 60 hz use....so the super fast types are not required. You wont see eq
resistors
across diodes either. The diodes are in series, so they are already
equalized. The current through em is the same.
Adding eq resistors will make it worse. The peak dc v is
equal to the B+. Its 1.41 x the xfmr sec AC voltage.
A 1 kv ac sec is 1414 vdc. Its not 1414 x 1.41
Putting a .01 uf cap across each diode is a waste of time.
All those caps are in series, so total C is minimal. 10 x .01uf
caps in series = .001 uf..or 1000 pf. You are better off to put
a .047uf cap... (4700 pf @ 10-15 kv) across each leg of the
fwb. I wouldnt even do that. Put 1-2 x 4700 pf @ 10-15kv
disc ceramic bypass caps from output of FWB.....to chassis.
## On a side note, I measured 506 pf across a 6A10 diode on my lcr
meter. If you are going to put anything across each diode, a MOV
would work better. I tested the movs..and I believe they were 910 pf
each. The movs will conduct well b4 the piv is ever reached. You will
see movs across diodes on commercial broadcast rect assys.
You can also put fused movs across the xfmr primary. You can
also wire several 1 kv rated movs in series, across the sec of the xfmr.
You can series movs for more V...and parallel em for more joules.
## You need to do a re-write on your note below. A lot of water has
passed
below the bridge. Nobody ever blew out a string of 1N5408s..and that’s
with nothing
across em. Just use enough of them... like double or triple the B+
value..per leg.
Most commercial ham amps will only use 50% more piv vs B+..and even
they don’t blow up.
Take the $$ wasted on eq resistors + .01uf caps..and spend it on more
1N5408s...
or better yet... 6A10s.
Jim VE7RF
This really makes one wonder how many times a subject can be covered, in
detail, over and over.
Let's do this. Here is a little ditty I wrote a long time ago concerning
rectifiers. At this point it probably is outdated. Still, it might offer
some insight and some real engineers to come forth and correct it.
I have read many statements, in this thread, that are nothing but
guessing
and speculation. The bottom line is we are talking a small difference in
price to do it right and/or over do it. I always vote for over do myself.
The question of transients is solved using more modern rectifiers or
avalanche diodes.
Paul
WD8OSU
*What About Those Series Rectifiers?*
The use of several rectifier diodes in a series string is often required
to achieve the Peak Reverse Voltage (PRV) or Peak Inverse Voltage (PIV)
required for building a High Voltage Power Supply (HVPS). It makes no
matter if you are talking about a half wave up to and including a full
wave
bridge, one needs to be aware of peak voltages involved and required
component specifications.
Think about that B+ supply for your dream linear. What are we talking
about here, 2 to 5 or even 6,000 volts DC for that 4-1000A? Woof! That?s a
hugh amount of PIV that can burn up an improperly designed string in a
nanosecond. Having personally experienced a string failure I can tell you
it is not a pretty sight. There is a lot of noise and many projectiles
flying about. I can give you a couple options in your design and you can
decide which will achieve the level of operational reliability and
financial burden you can endure.
Remember, we are talking about ?PEAK? voltage. If you are shooting for
3000 volts DC to your plate the ?PEAK? would be 1.41 times this amount or
4230 volts! And, this doesn?t count what they call ?mains over voltage?
typically 5%. That brings us to a grand total of 4441.5 volts. That is
what
your poor little rectifier string has to be capable of handling.
Traditionally, normal, everyday, rectifier, diodes were and are used at
the output of the HV transformer to whip out more or less DC to the filter
network. The first requirement is the current that is going to be drawn by
your favorite tube. Don?t forget we are talking about ?peak? current too.
So, your tube specifications say 1000 milliamps or 1 amp is the maximum
current your tube likes. Guess what? That?s average not peak current.
Instantaneous current can be quite a bit higher. No worries, diodes are
cheap now days and they have relatively high peak current capabilities. I
like to use a factor of 3 and this is really safe. If you think your tube
is going to draw 1000 milliamps during normal operation then a 3-amp
rectifier will be more than enough to cover the load. They can handle peak
currents much higher than your 4-1000A could stand. Hey, we?re only
talking
about maybe 7 cents a piece for the old, reliable, 1N5408.
Now we run into an area that takes up considerable bandwidth on the
Internet arguing about compensation and/or equalization resistors and
suppression capacitors in parallel with the diodes. There is a bit of a
problem when you start stacking a bunch of diodes in series. Especially
when you use really fast switching diodes, which even the cheapies are
pretty quick now. Each diode does have it?s own personality and really
doesn?t conduct or shut off at the same time as all the others. What does
this mean? Well, basically, it means that there may be voltage differences
across each diode and noise induced in the output of the HVPS that you
don?t want. It can even lead to distortion on your transmitted signal.
Like I said, folks in the industry still argue about this. But, I always
figure better safe than sorry. You can take care of the unequal voltage
issue by placing equalization resistors in parallel with each diode. This
insures that each diode sees the same, or close to the same, voltage as
all
the others in series.
Now issue number 2, transient voltages across each diode and all the
little bits of noise they make. When the diodes turn on and off, at
different times due to their personalities again, they make a bit of noise
that you don?t necessarily want. There is an issue of unwanted surges too.
Transients are the little creatures that can cause all kinds of problems
and come from any of a number of sources. The bad thing about them is they
don?t always go with the normal flow of current. They are the things, for
lack of a better explanation, that can make your life miserable and cause
your standard rectifiers to give up the ghost. This is what all the
amplifier-building guys are arguing about all the time. How do you stop
transients?
One old school answer is placing a capacitor in parallel with the diode
and
a resistor to make a happy family. OK, OK, I know, the Handbook says not
to
do this anymore. Well, ah, what can I say? If in doubt, look over
STMicroelectronics application note ?AN443? May 2004 and make up your own
mind. To me a few cents of parts that is not really going to hurt anything
might be worth it. Is it really going to make a difference? There?s not a
whole lot of data out there either way. Some say that their semiconductors
are so good now that you need not bother.
Now, let me give you the ultimate, modern, answer to all the normal
diode?s problems. Introducing the grandson or granddaughter of the 1N
whatever diode family, the avalanche diode. These little guys do what a
normal diode would never do on purpose and survive. Conduct relatively
large amounts of reverse currents on command. What a concept! These dudes
switch on and off on command really quick, don?t introduce noise, and are
all around our power supply friends. Take a look at Dynex Semiconductor?s
Application Note ?AN5370? to read all about the little marvels.
Yea, they do cost a bit more but not that much. As a matter of fact, after
you put the bucks into an equalization network for your favorite 1N5408 it
works out to about the same. Let me give you an example, Fagor makes a
little number that will do 3 amps at 1000 PRV for about a quarter a piece
at Allied Electronics (BYM36E). Not bad!
So, sooner or later we always get to the math of it all. Let me give you
the basics to get done what you need to do. Remember, it is always PEAK!
Take the plate voltage you are shooting for and multiply that by 1.41 to
get peak voltage. This is the minimum PRV your diode string should have in
a full wave bridge I.E. 3000 VDC X 1.41 = 4230 volts peak. This will work
for normal rectifier diodes.
Avalanche diodes are a bit different, look out now, a safety factor of 2.4
to 3 times has to be added to the mix. With that in mind, 3000 VDC X 1.41
X
(a minimum of 2.4) = 10152 PIV. That jumped right up there didn?t it? Yes,
you need more avalanche diodes than normal rectifiers. Now, don?t forget
about this little thing called ? mains over voltage?. That pumps in
another
1.05% to the equation. Let?s see, we were up to 10152 PIV (10152) X 1.05 =
10659.6. So, you get 11 avalanche numbers is each leg and your set to go.
You really didn?t think you were going to get all this great avalanche
stuff for free did you? There is always a trade off isn?t there?
The bottom line is using the avalanche diode is the best way to go for a
very dependable HVPS and at the price they are affordable, even if it
takes
more to get the job done.
*Col. Paul E. Cater, WD8OSU, is a Cambridge, Ohio native that returned to
the area in 1994 after retiring from government service. Mr. Cater is a
veteran and has been a technician, electronics maintenance officer, field
service engineer, instructor, signals analyst, and the director of
training
for a major government contractor. He currently resides at historic
Prospect Place Mansion in Trinway, Ohio. *
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