Author Topic: Using avalanche rectifiers in series for higher voltages - equalizing resistors?  (Read 1492 times)

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Offline cvancTopic starter

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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.
 

Offline MagicSmoker

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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).



 
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Offline HighVoltage

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Depending on your project but in general it will work without any problems.

I have used the Vishay BYT62 diodes in such a way and it worked well for 3 of them in series.


There are 3 kinds of people in this world, those who can count and those who can not.
 
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Offline David Hess

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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.

What 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.
 
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Offline cvancTopic starter

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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.)
 

Offline MagicSmoker

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Is 1 Meg a reasonable guess?  Is there a way to calculate the optimum value?

Oh, 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.
 
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Offline David Hess

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Oh, 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.

That 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.
 
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Offline MagicSmoker

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...do not forget about capacitive balancing at higher frequencies where the capacitance and mismatch of the capacitance of the diode junction becomes significant.

Yep, 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.
 
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Offline cvancTopic starter

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Oh, there's all sorts of rules and recommendations for this

That'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!
« Last Edit: January 04, 2020, 02:53:35 am by cvanc »
 

Offline vk6zgo

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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.

What 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.

The best rule is "use big ones" ;D
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.
 
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Offline cvancTopic starter

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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.

That'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.
 


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