[Amps] HV Diodes

Carl km1h at jeremy.qozzy.com
Sat Oct 18 12:27:57 EDT 2014


Ive been told by several diode manufacturers that adding all that parallel 
resistor and capacitor crap, as they usually called it, could actually be 
bad for the circuit.

It took decades before the ARRL finally got with it and removed all their 
antiquated drivel once they acquired a team of real engineers to volunteer 
and review the HB for serious updating across the board. I dont know if 
thats finished yet as I dont buy a HB very often and only then one a few 
years old at a hamfest for $5 or so.

FWB flatpacks, at least in the LV category, do generate hash that can be 
heard in audio circuits and seen on a scope of course. A .01 across each leg 
takes care of that. I suspect they havent changed the technology in them 
since the 60-70's as the same part # are still around.

Carl
KM1H



>
> ##  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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