[Amps] HV Supplies and Diodes

Thu Jan 3 18:50:32 EST 2008

The 1980 ARRL handbook section on power supplies does recommend using
resistors across each diode in a string.

The 2000 ARRL handbook recommends NOT using resistors across each diode in a
string. 
Times have changed!

Diodes these days have an avalanche area in them that makes them operate
like a zener when maximum reverse voltage is seen. So if one diode in a
string has a lower reverse breakdown voltage its zener will conduct a small
amount of current while protecting its junction. Since current in a series
circuit is the same everywhere, the amount of current conducted will be
limited by the total leakage current of the whole diode string. (very small)

By placing resistors in parallel with these diodes you may force excess
reverse current through a diode that has reached avalanche because the
resistors provide a low impedance source of current. 
Without the resistors there is only the very high leakage resistance of the
whole string to supply current.

73
Gary  K4FMX

> -----Original Message-----
> From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com] On
> Behalf Of Larry
> Sent: Thursday, January 03, 2008 2:36 PM
> Cc: AMPS
> Subject: Re: [Amps] HV Supplies and Diodes
> 
> FWIW department:
> 
> Some observations noted over the last 40+ years designing, building, and
> testing power supplies:
> 
> 
> Determining the required ratings for power supply rectifiers, no matter
> if they are vacuum tubes, mercury vapor, selenium, or silicon, can be
> easily done using the information presented in this paper:  O.H. Schade,
> "Analysis of Rectifier Operation", Proc. IRE, Vol 31, No. 7, July 1943.
> 
> This information is also reprinted in various forms in ARRL publications
> and manufacturers app notes. The most comprehensive source of design
> data I have come across is the "Motorola Silicon Rectifier Data Manual"
> for 1980, Series A. Everything a guy needs to know about silicon
> rectifiers is contained in this one book. Other manufacturers may have
> similar guides but I have not run across them. Also, Motorola
> discontinued this series of design data books in the 90's sometime.
> 
> Much has been said over the last few days about diode PIV and equalizing
> components but the information has been, in my opinion, misleading and
> sometimes incorrect.
> 
> As important or probably even more important than the PIV rating of the
> diode is the "Ir" or maximum reverse current rating. In a series string
> of diodes, the reverse current characteristics of the individual diodes
> will be the dominant factor in determining the PIV seen across each diode.
> 
> "Ir" is not as consistent as "PIV" and is highly temperature dependent.
> The reverse current is typically measured at the rated reverse bias of
> the part and is indicative of the effective resistance of the part when
> reversed biased at that particular voltage. If you run the part on a
> curve tracer, the the reverse current (and therefore the resistance)
> will change in a more or less linear fashion until the avalanche voltage
> is reached. Unfortunately there is no guarantee that the reverse current
> (and the resistance) at any given reverse bias will be the same for
> every diode.
> 
> In fact, the spec for a 1N4007 runs from 0.05 microamp (typical) to 10
> microamp (max) at 25° C junction temperature and from 1.0 microamp
> (typical) to 50 microamp (max) at 100° C junction temperature.
> 
> Lets look at a string of two diodes with 1000 volts peak reverse voltage
> across them. If one diode has say 1.0 microamp reverse current at 500
> volts reverse bias and the other has 3.0 microamp reverse current at the
> same reverse voltage the effective series resistances are 500 megohms
> and 166 megohms. Assuming a linear reverse current curve, one diode will
> have a voltage about 750 volts across it and the other will have about
> 250 volts across it. No problem if both diodes have a PIV rating of 1000
> volts or more but you can see what happens if the PIV across the diode
> string is actually 1500 volts instead of 1000 or if there is a wider
> variation in "Ir" characteristics..
> 
> Recommendations for eliminating the compensation components are based on
> the presumption that the individual diodes have virtually identical
> reverse current characteristics *AND* are operated with identical
> junction temperatures *AND* there are far more diodes in the string than
> you might actually need in order to cover up any inconsistencies.
> 
> If you can buy "lots" of diodes and they are all the same date code and
> you insure they are all operating at the same junction temperature (NOT
> ambient but junction) and put 3x what you really need, you can be pretty
> sure that it will be reliable.
> 
> Now hams being as cheap as they are will never go to a major
> semiconductor manufacturer and buy a batch of diodes with identical date
> codes. They will buy them at a fraction of that cost and wind up with
> many date codes from a distributer or even many manufacturers from a
> "surplus" source. In this case the reverse resistance characteristics
> will be all over the map and the only way to control the PIV each
> individual diode sees is to swamp out the "Ir" by using a parallel
> resistor across each diode with a resistance much lower than the reverse
> resistance of the diode.
> 
> Historically, over the last 40 years I have never seen a diode that came
> off a major manufacturers production line that didn't conform to the
> above characteristics.
> 
> In the distant past, silicon diodes were expensive and manufacturers did
> not want to use more than the bare minimum required. Resistors (and
> capacitors) were 100 times cheaper than diodes. Made sense to use them
> to reduce the number of diodes required.
> 
> The junction capacitance of a typical rectifier diode is small enough
> that the impedance differences between individual diodes at 50/60 Hz is
> insignificant. However, the old Motorola, GE, RCA, etc, tube type
> commercial mobile radios used multivibrator DC-DC converters running at
> around 2 kHz. The risetimes and transients involved in those systems
> were fast enough to warrant compensating individual diode junction
> capacitance with external parallel capacitors. I suspect this is where
> the idea of putting capacitors across every diode in the world came
> from. I have never seen a commercial/military power supply running from
> 50/60 Hz mains have compensating capacitors.
> 
> In those instances where high frequency transients are expected, usually
> a simple capacitor across the transformers winding is enough to fix the
> problem. I will frequently incorporate this cap no matter if I use
> compensating resistors or not.
> 
> As far as deciding how much PIV you need, once you have "enough" to
> match the design maximum PIV expected, anything more is at the
> discretion of the designer as to how much he wants to trade off
> reliability vs. cost. Personally, if I have the space available, I will
> use about 3x the expected PIV (diodes are cheap these days!).
> 
> What is the expected PIV? It depends......
> 
> Assume a typical single phase full wave bridge rectifier running into a
> capacitor filter:
> 
> Each leg of the bridge will see a reverse voltage equal to the maximum
> DC output voltage. This does not include any possible transients or
> surges nor any safety margin. I usually make each leg 3X this number.
> 
> Each leg of the bridge will see an average current equal to 1/2 of the
> DC output current. This means a bridge made from 1 amp diodes will
> handle up to 2 amps of output current assuming the peak repetitive
> current rating is not exceeded and the temperature is controlled.
> 
> The transformer secondary voltage will be 0.707 times the output DC
> voltage (roughly depending on many component characteristics) so
> therefore the transformer secondary current rating will need to be 1.414
> times the output DC current. Nobody ever gets this right!
> 
> The peak current through each diode leg will be *EIGHT* times the output
> DC current! Pay attention to the peak repetitive current rating of the
> diodes. They derate very fast with temperature and this IS a critical
> parameter.
> 
> Junction temperature is a big factor in reliability. The heat generated
> in a diode junction is due primarily to the current through the junction
> and the forward voltage drop. The heat is mostly dissipated through the
> LEADS and not through the epoxy body. Every major manufacturer gives
> derating curves for various mounting schemes. I have never once seen
> encapsulation recommended. Potting the diodes in epoxy will decrease the
> allowable forward current and the maximum PIV by a significant amount
> due to the inability of the diode leads to transfer heat efficiently
> when embedded in an insulating material. Commercial manufacturers of
> encapsulated diodes take this into account when designing the stack. If
> you don't do likewise when encapsulating your own, be advised that you
> will probably be disappointed in your efforts.
> 
> Although I sometime stack diodes without compensating resistors due to
> space considerations, I normally use the resistors. I'm careful to use
> resistors that are appropriate and that the diode/resistor strings are
> mounted in a fashion which allows proper heat dissipation. Using this
> approach I have had many many years of proven reliability using "bargain
> basement" diodes in HV applications both in Choke input and Capacitor
> input supplies.
> 
> YMMV
> 
> 73, Larry
> 
> Larry - W7IUV
> DN07dg
> http://w7iuv.com
> 
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