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High Speed Diode Strings for High Voltage Rectification.

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Patrick

Mon Oct 20 2014, 07:06AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

I have some questions about diodes. (HER108)

First, i use to string 1N4007 diodes together, back my high school days in 1998. back then i used resistors and caps to equalize the switching conditions.

Second, Is this still needed for long strings? and what percentage should i de-rate?

Third, when the data sheet quotes 75 nS Trr, do series'ing these up in large numbers start lengthening the Trr time?

Fourth, i plan to epoxy pot these as a bridge rectifier of 50 or so kV, so i dont want it to die easily.

Fifth, is each diode having 20pF going to be a problem?

Finnaly, theres not much i can do about the capabilities of these diodes, (all these makers are pretty close) but i see some common diode strings in special applications, and they seem to work OK.

Im planning on making a zig-zag tray on my 3D printer. laying the diodes in the PLA and filling it with epoxy. That'll be a nice little module.

PLA and other plastics for insulation purposes.

Sulaiman

Mon Oct 20 2014, 01:13PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Why not use UF4007 ? ... just as fast, similar ratings and MUCH cheaper.

(you are probably looking at purchasing 100's of them)

Series connecting diodes does not change Trr

40 or 50 kV at high frequency will need a lot of diodes / space and GOOD insulation.
I would rather build a few 50 kV single diode 'sticks' (I sometimes terminate the 'sticks' with a ring crimp for easy mounting)
and arrange four as a bridge;
. if one string dies it will not make the whole lot scrap
. more flexibility in mounting arrangements / configurations

Ash Small

Mon Oct 20 2014, 03:51PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Patrick wrote ...

and what percentage should i de-rate?

Most people seem to say 50%

Patrick

Mon Oct 20 2014, 06:37PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Sulaiman wrote ...

and arrange four as a bridge;
. if one string dies it will not make the whole lot scrap
. more flexibility in mounting arrangements / configurations

But i need a small module to sit near the planar transformers. other wise i'd keep them seperate.

Antonio

Mon Oct 20 2014, 06:58PM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

The 1N4007 diodes are not avalanche-rated. If you exceed their blocking voltage they are easily destroyed. I like to use microwave oven diodes for high-voltage rectification. They are much more robust, and have lower leakage than a string of 4007s too. I don't know about their speed.

Patrick

Mon Oct 20 2014, 07:33PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Antonio wrote ...

The 1N4007 diodes are not avalanche-rated. If you exceed their blocking voltage they are easily destroyed. I like to use microwave oven diodes for high-voltage rectification. They are much more robust, and have lower leakage than a string of 4007s too. I don't know about their speed.

it seems like HV microwave makers like Rectron only list the 60Hz ratings in data sheets. i think thats becuase of the microwave AC transformers, that we all know weigh 900lbs. Maybe with the newer SMPS microwave power supplies gaining ground, we'll see better diodes.

johnf

Tue Oct 21 2014, 06:58PM

Registered Member #230 Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 284

Use the high speed diodes from inverter type microwaves

radiotech

Tue Oct 21 2014, 07:30PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

If one diode burns open you may have a minor sun burning in your epoxy.
All the diodes strings I saw had capacitors across them and lived under
transformer oil. Rating 100 kV 1 amp. 60Hz. What is your f ?

If I recall,they were 1500volt , 1nF ceramic.

Patrick

Tue Oct 21 2014, 09:11PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

radiotech wrote ...
Rating 100 kV 1 amp. 60Hz. What is your f ?

Im planning on 30-50kV-ish (100kv string down rated), (10 mA or less), at 70-150 kHz. so that rules out standard mains rectifiers.

And "nuclear incandescence" is a risk we all take here on the forum, from trying to stretch a component. Besides its a trophy and point of pride too

radiotech

Wed Oct 22 2014, 05:00PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

Nuclear incandescence has been known to lift a problem (literally) off the
plate off the plate of the electrical and on to the chemist's.

The rate of rise of temperature of organic insulators can release more energy
than might have been expected from the power source. This is documented
in early IEEE papers on circuit breaker failures.

If your sticks form a bridge your output wave will be twice the frequency
of the input, and lifted off the ground reference. This will impact ripple
current rating of capacitors.

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