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Putting many diodes in series for a HV diode.

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Patrick

Tue Jun 21 2011, 07:31PM

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

Ive seen a few mentions here on the forum about putting many diodes in series to make a single Hv diode, my question is...

[Given a square wave of 100kHz at Hv with a steep V rise and fall at 10-20kV,at a Duty cycle of about 50%.]

(1) Do I have to use avalanche rated diodes?

(2) Would the HER108 be a good chioce? I have 700+ Pcs.
]her108.pdf[/file] (says nothing about avalanche, so I presume its not rated.)

(3) What about the "balancing" rule, ive been told to multiply the PIV by 500 to find the parallel resistence needed? (so in my case PIV =1Kv, so 1000V x 500 = 500k Ohms)

I have 1000+ Pcs of 10M Ohms, 1/4 watt resistors. So Id rather use 10M than 500k.

Problem: If I follow this rule, 1000V / 500k = 0.002 amps, 0.002 amp x 1000V = 2 watts for each sereis resistor! Thats a pretty hefty load.

Dr. ISOTOP

Tue Jun 21 2011, 07:39PM

Registered Member #2919 Joined: Fri Jun 11 2010, 06:30PM
Location: Cambridge, MA
Posts: 652

You can:
(1) Use avalanche rated diodes
(2) Balance with resistors and capacitors
or
(3) [my favorite, since it is easier than (2) and cheaper than (1)] build a string of at least twice the desired standoff voltage.

Patrick

Tue Jun 21 2011, 07:43PM

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

bwang wrote ...

You can:
(1) Use avalanche rated diodes
(2) Balance with resistors and capacitors
or
(3) [my favorite, since it is easier than (2) and cheaper than (1)] build a string of at least twice the desired standoff voltage.

Bwang and others, what do you think of the 100Khz and Trr in ns... i need oppinons on this please, will that cuase problems with balancing on/off times and voltage?

Proud Mary

Tue Jun 21 2011, 07:49PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

The tolerances and characteristics of diode rectifiers have become much more homogenous over the last forty years as manufacturing techniques have evolved, and one tends not to see the strings of equalizing resistors and capacitors once thought necessary in days gone by.

Rather than adding a whole lot of dissipative supplementary components - which themselves may fail - I'd suggest very generous de-rating. With HER108, the RMS max is 700 V,
so I'd treat it as though this figure was 350 V. This gives you a lot of slack for voltage drop differences between the individual diodes

It's usually much more expensive to dismantle a malfunctioning circuit than it is to avoid malfunction by generously de-rating inexpensive parts.

Patrick

Tue Jun 21 2011, 07:54PM

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

Proud Mary wrote ...

It's usually much more expensive to dismantle a malfunctioning circuit than it is to avoid malfunction by generously de-rating inexpensive parts.

I tend to agree with your point Proud Mary. And with fewer types of parts (even with more D's) , lower cost and less to go wrong (no more R's and C's).

Proud Mary

Tue Jun 21 2011, 08:15PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Patrick wrote ...

Proud Mary wrote ...

It's usually much more expensive to dismantle a malfunctioning circuit than it is to avoid malfunction by generously de-rating inexpensive parts.

I tend to agree with your point Proud Mary. And with fewer types of parts (even with more D's) , lower cost and less to go wrong (no more R's and C's).

I think all EE courses should have an industrial economics module, because 99% of real world EE is concerned with manufacturing products or providing services which must compete in the marketplace.

Patrick

Tue Jun 21 2011, 08:18PM

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

Proud Mary wrote ...

I think all EE courses should have an industrial economics module, because 99% of real world EE is concerned with manufacturing products or providing services which must compete in the marketplace.

Yes! You are right, I had professor who went over all of the common modes and costs of failure for EE people, but he just got laid off here at Chico.

Adam Munich

Tue Jun 21 2011, 10:12PM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

Proud Mary wrote ...

The tolerances and characteristics of diode rectifiers have become much more homogenous over the last forty years as manufacturing techniques have evolved, and one tends not to see the strings of equalizing resistors and capacitors once thought necessary in days gone by.

It's gotten to the point where even some x-ray manufacturers just string up diodes, even kodak.

Patrick

Tue Jun 21 2011, 10:25PM

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

OK, good enough then ... HER108's in series it is.

Ash Small

Wed Jun 22 2011, 12:13AM

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

It's called 'Production Engineering' (part of most engineering courses), something which most 'academics' have little time for...

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