Using avalanche rectifiers in series for higher voltages - equalizing resistors? - Page 1
Forum section: electronics / projects

author: cvanc
author_rank: Frequent Contributor
timestamp: January 03, 2020, 05:32:10 pm
content: Hi all-Is it a valid general statement that avalanche rectifiers used in series for higher voltage do NOT need equalizing resistors? I haven't used them before in this way and I want to verify.I'm thinking of using some version of BY228 for my project, but really it's a question about avalanche rectifiers in general. Thanks for any clarity you can provide.Example datasheet attached.

author: MagicSmoker
author_rank: Super Contributor
timestamp: January 03, 2020, 05:58:42 pm
content: Technically correct, but not the best practice, really. The main advantage of avalanche rectifiers is that they don't need protection from transients (up to their max energy rating, anyway). The same mechanism of controlled avalanche enables them to self-equalize when wired in series, but that requires one or more diodes operating near their breakdown knee resulting in greater power dissipation. So, it's still a good idea to use static equalization (ie - a parallel resistor across each diode), but there is no need for dynamic equalization (ie - a parallel RC across each diode).

author: HighVoltage
author_rank: Super Contributor
timestamp: January 03, 2020, 06:08:52 pm
content: Depending on your project but in general it will work without any problems.I have used theVishay BYT62diodes in such a way and it worked well for 3 of them in series.

author: David Hess
author_rank: Super Contributor
timestamp: January 03, 2020, 08:11:03 pm
content: Quote from: cvanc on January 03, 2020, 05:32:10 pmWhat makes avalanche rectifiers different is that they are fabricated to have a very uniform junction so the entire junction breaks down at the same time instead of just selected spots which could lead to localized overheating and damage. So they can be reliably used in series without balancing but as MagicSmoker points out, thermal constraints still need to be considered. There is probably some derating rule for operating them this way but offhand I do not know it.

author: cvanc
author_rank: Frequent Contributor
timestamp: January 03, 2020, 08:34:31 pm
content: Thank you all, it's much appreciated.OK so it should be possible to do this without resistors. But I'd like this to be robust under fault conditions so let's talk about the correct resistor value, I will probably add them.Is 1 Meg a reasonable guess? Is there a way to calculate the optimum value? I took a quick look and didn't find much.(Of course the voltage rating of the resistors needs to be adequate; I'm thinking Vishay VR25 or VR37. Datasheet attached.)

author: MagicSmoker
author_rank: Super Contributor
timestamp: January 03, 2020, 10:19:28 pm
content: Quote from: cvanc on January 03, 2020, 08:34:31 pmOh, there's all sorts of rules and recommendations for this, but I've found a good balance is to use a resistor value that passes 100uA at the diode's max PIV rating or 10M, whichever is less. This covers most situations for most diodes (as long as they aren't run at >100C, anyway).No need to think too much about this because reverse current in most diodes is the very definition of a moving target.

author: David Hess
author_rank: Super Contributor
timestamp: January 03, 2020, 10:44:13 pm
content: Quote from: MagicSmoker on January 03, 2020, 10:19:28 pmThat is how I do it but do not forget about capacitive balancing at higher frequencies where the capacitance and mismatch of the capacitance of the diode junction becomes significant.

author: MagicSmoker
author_rank: Super Contributor
timestamp: January 03, 2020, 10:53:38 pm
content: Quote from: David Hess on January 03, 2020, 10:44:13 pmYep, dynamic balancing is definitely necessary for a series string of non-avalanche diodes, but for avalanche diodes it's okay to just use a static balancing resistor across each one.

author: cvanc
author_rank: Frequent Contributor
timestamp: January 04, 2020, 12:05:13 am
content: Quote from: MagicSmoker on January 03, 2020, 10:19:28 pmThat's what I found when I went looking, conflicting opinions and not a lot of hard math.As for HF balancing, my specific task is at powerline frequency so I don't think it's needed.Thanks all!

author: vk6zgo
author_rank: Super Contributor
timestamp: January 04, 2020, 02:36:20 am
content: Quote from: David Hess on January 03, 2020, 08:11:03 pmThe best rule is "use big ones"Running rectifiers well below their max ratings looks like overkill, but it usually ensures long life.A case in point is the really large ones NEC used for the HT supplies of their transmitters.Over decades associated with that equipment, I have never encountered a rectifier failure, or even heard of one.Pye, back when they still made transmitters (mid to late 1980s) used much smaller rectifiers, supposedly within their ratings.They would "blow up" if you looked sideways at them.We went through so many of them, & the supply time from the UK was so long & slow, we joked that it would almost have been easier to send someone to the UK to pick up replacements from the factory.

author: cvanc
author_rank: Frequent Contributor
timestamp: January 04, 2020, 09:34:17 am
content: Quote from: vk6zgo on January 04, 2020, 02:36:20 amThat's kind of my mindset here. This project has many risk factors, and if there's any part of it I can just overkill and not worry about again I would like to do it. (Within economic reason, of course...)Thanks again, everyone. Very helpful.