Using multiple diodes in series to increase voltage rating

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How legit is this? I had always assumed that it wouldn't work, as you couldn't guarantee that the diodes had the same voltage drop across them (due to manufacturing variations) - but I'm seeing this 240V dual diode used in series to act as a 320V diode here (figure 6, page 10). Maybe they can get away with it as the diodes are on the same die?

Thanks!

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nleahcim wrote:
as you couldn't guarantee that the diodes had the same voltage drop across them (due to manufacturing variations)
They don't have to have the same voltage. Each diode voltage just has to be within specification.

Each diode in the example can take 240 V. If they are absolutely identical each would take 180 V, so there is 60 V headroom, 33 %. Up to a 240 V / 120 V split everything would be fine (ignoring any derating one shold better do, too).

To get a 240 V / 120 V split the diodes need to be rather different. That is unlikely for two diodes in one package.

Stealing Proteus doesn't make you an engineer.

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ArnoldB wrote:
nleahcim wrote:
as you couldn't guarantee that the diodes had the same voltage drop across them (due to manufacturing variations)
They don't have to have the same voltage. Each diode voltage just has to be within specification.

Each diode in the example can take 240 V. If they are absolutely identical each would take 180 V, so there is 60 V headroom, 33 %. Up to a 240 V / 120 V split everything would be fine (ignoring any derating one shold better do, too).

To get a 240 V / 120 V split the diodes need to be rather different. That is unlikely for two diodes in one package.


Hi - I should have been more clear in my first post. My question is if this is considered to be an acceptable design practice and, if so, what sort of derating factor you need to use.

I agree that just adding the maximum voltage ratings of two diodes in series does not make sense. But what sort of safety factor would one use? And would you only want to do this when the parts are on a single die?

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Its a common technique for high voltage diodes. As for the pitfalls, I'm not sure.

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High value resistors (hundreds of Kohms)in parallel with ea. diode will equalize the voltage drops in the off state. You might research the optimum value. Junction capacitance will come into play at higher freqs.

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I am to lazy to run the numbers. Take the diode equation two times, resolve to a relation of the two voltages V1 / V2 (or something similar) and evaluate the susceptibility with respect to diode parameters.

Stealing Proteus doesn't make you an engineer.

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I once worked around a particle accelerator that irradiated molecules making up PVC piping that was used in irrigation in the desert.

The voltage rectifier had to handle about 100,000 volts DC. It was comprised of a massive number of diodes and resistors as RickB described above.

The goal in this case wasn't so much the Forward Voltage (Vf) rating of the diode. Rather, it was the Reverse Voltage (Vr) rating as, the goal was to be able to block about 100,000 volts without putting the diodes into reverse breakdown.

The Vf rating of say, a 1N4007 general purpose diode is about 0.65 volts. The Peak Inverse Voltage (PIV) of a 1N4007 is rated at 1,000 volts. So, if you needed a diode that was able to block 100,000 volts using 1N4007 diodes, a series string of at least 100 diodes would be required. But the drawback would then be that a Vf of about 65 volts would be created.

While back in the mid-1970's, Silicone diodes were really just becoming available/popular, that was the best choice at the time. But today, it's Better I think, to use a diode that is specifically engineered for the purpose.

You can avoid reality, for a while.  But you can't avoid the consequences of reality! - C.W. Livingston

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I've looked into this in the past, and my understanding is that modern diodes (even modern 1N400x) have a controlled breakdown characteristic, so they act like zeners when presented with overvoltage (a voltage greater than their reverse breakdown voltage). This is as opposed to suffering catastrophic breakdown under overvoltage.

With this controlled breakdown characteristic, all the diodes can safely find a spot on their V-I curves where they are happy, and the reverse current is very low. Of course you'd still use conservative numbers of diodes, figuring maybe 50%-60% of the rated voltage per diode. Diodes are SO cheap these days.

In the past, equalizing resistors were used to prevent the catastrophic breakdown that earlier diodes suffered. But with modern diodes, such resistors make things worse, not better. Resistors themselves will drift and fail (moreso under high voltage), blowing out perfectly good diodes.

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To be honest, I don't remember is they were Si or Ge diodes, though, I don't think 1N4007 diodes were even in existence back then. I only used 1N4007 as an example as, they are a diode that many would be able to relate to.

The fact is, those diodes could have even been Selenium diodes, but that would have taken up much more then the 30 feet or so as Selenium diodes are so much larger.

Speaking of Selenium diodes, I haven't seen any of them for a very long time...

You can avoid reality, for a while.  But you can't avoid the consequences of reality! - C.W. Livingston

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Back in "the day", as they say, strings of diodes designed for high voltage had a shunt resistor and a capacitor across each diode.

The low frequency issue is reverse leakage. Reverse leakage is much more variable from diode to diode. It is not something that is part of process control as long is it is "low enough". At higher frequencies, it depends a lot on junction capacitance. Again, for regular diodes, that is NOT something that is tightly controlled.

You saw strings of diodes in oscilloscope high voltage supplies, for example.

My hunch is that there are actual diodes designed for high voltage (and rated for modest currents).

Jim

 

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