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    Stacking diodes for high voltage rectification

    Would a stack of 100 series uf4007 diodes actually be able to rectify ~95Kv if properly insulated so as to prevent any arcing?

    Seems the plastic encapsulation would give way to arcing to the other side of the silicon junction which is under 2mm^2 large.

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    Re: stacking diodes for high voltage rectification

    see this thread
    Bridge Rectifier won't Rectify

    you would need some way to guarantee that all of the diodes carried their share of the voltage

    since the uf4007 is rated at 1000 V, 100 of them is 100,000 V, > 95 kV
    it should work, but all the secondary issues will bite you see the thread above.

    i prefer a larger margin. maybe 70% or so instead of 95%

    you might look at the K100F
    http://www.voltagemultipliers.com/pr...urrent-diodes/


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    Re: stacking diodes for high voltage rectification

    Quote Originally Posted by wwfeldman View Post
    see this thread
    Bridge Rectifier won't Rectify

    you would need some way to guarantee that all of the diodes carried their share of the voltage

    since the uf4007 is rated at 1000 V, 100 of them is 100,000 V, > 95 kV
    it should work, but all the secondary issues will bite you see the thread above.

    i prefer a larger margin. maybe 70% or so instead of 95%

    you might look at the K100F
    http://www.voltagemultipliers.com/pr...urrent-diodes/

    From the mentioned site K50UF looks ideal but uf4007 is faster. Those specialty diodes are likely much more costly than standardized uf4007 parts.

    Whats done to make sure every diode in the stack has same amount of voltage across it?
    Last edited by Zak28; 19th August 2019 at 22:27.



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    Re: Stacking diodes for high voltage rectification

    you would need an RC snubber across each diode as well as voltage sharing resistors ( 1Meg-ohm - 4.7 Meg Ohm ) across each diode so that the voltage is shared when off.


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    Re: Stacking diodes for high voltage rectification

    100 uf4007 plus 100 R & C for snubber plus 100 voltage sharing resistor
    vs
    10 K50UF plus 10 R & C for snubber plus 10 voltage sharing resistors

    now compare board space, parts cost, inventory cost

    what speed do you really need?


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    Re: Stacking diodes for high voltage rectification

    I would suggest 220pF ( 2kV ) and 4k7 ( at least 600V rated ) for the RC snubber for 50/60Hz rectification


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    Re: Stacking diodes for high voltage rectification

    This is for 200khz-1Mhz - diodes would be 1n4007 for mains voltage rectification. Infact there are many inexpensive high voltage diodes but they are slow such as those found in micowave ovens.



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    Re: Stacking diodes for high voltage rectification

    This is the first time you have mentioned 200kHz or higher, unless you have significant experience in this field - I would suggest abandoning this endeavour.

    There are no commercial products that generate > 50kV at greater than 50kHz - for a good set of reasons

    you will discover these reasons if you try and build.


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    Re: Stacking diodes for high voltage rectification

    But there is hardly any current, its well under 5mA which is a small fraction of the diodes current rating.

    Whats the cause for this to not work?



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    Re: Stacking diodes for high voltage rectification

    You'll find out pretty quick if you try to rectify any thing greater than 1kV at 200kHz... do you have any wiring inductance, Tx leakage inductance, diode capacitance ....


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    Re: Stacking diodes for high voltage rectification

    But there isn't any capacitance since each junction is already ~18pF (1kv SMA schottky) and series stacked should only reduce them further. The inductance isn't enough to cause ringing since its smt diodes with no leads and wide traces.

    Is the primary issue parasitics causing the stack to ring between diodes?
    Last edited by Zak28; 20th August 2019 at 03:18.



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    Re: Stacking diodes for high voltage rectification

    do you know the leakage inductance of your transformer?

    - - - Updated - - -

    Let us assume the low figure of 18pF per diode, at 200kHz this is 44k-ohm, lets say we have 900 volt reverse and 1V forward across the diode at the 200kHz, this is approx 315Vrms ( AC ) giving a capacitive current of 7mA (much higher peak current for a square wave applied voltage), if the true capacitance is a bit higher say 36pF the current is 14mA rms + peaks, this would be 900 x 0.14 = 12.6 VA out of the transformer - just for this one 1kV stage ( x100 for 100kV = 1260VA), before any real power is being delivered. This also ignores the losses of the snubbers you will find necessary to use to stop the diodes going pop. Say we use 33pF(3kV) and 1k-ohm, losses due to snubber will be 2.916W at 200kHz ( x 100 = 291.6 watts), so each 1k-ohm resistor needs to be 5W. Also let us assume the reverse recovery of the diodes is a bit worse than the data sheet suggests - if we drive the diodes with a square wave rising from zero to 900V in 250nS say the dv/dt in the diode capacitance alone will be: [i/C = dv/dt] = 64.8mA for 18pF - it is likely the reverse rec current will be a few times higher than this for square wave drive - all this current builds up in the Tx leakage inductance - then the diodes try to turn off with the current still trying to flow backwards thru them - if they go overvolts then they go bang, without snubbers there will be a LOT of HF ringing - radiating RFI every where - including into your 200kHz control ckt - with snubbers the ringing and overvolt spike may be suppressed to low enough levels to stop the diodes going pop - but they will absorb a lot of heat at 200kHz if they are big enough...

    So you see - a few issues...

    One way to compensate for the above is to have a sine wave voltage drive out of the Tx as this limits the dv/dt at the zero xing - peak dv/dt lower at 20kHz than at 200kHz - but perhaps you know better than me ...


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    Re: Stacking diodes for high voltage rectification

    This was to rectify high frequency sines in am envelopes modulated by low frequency sines.

    But it seems that you pointed out unless the output is capable of high power outputs diode stack will distort the output.



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    Re: Stacking diodes for high voltage rectification

    The dv/dt of a 900Vpk sine wave at 200kHz, at the zero xing ( where it is maximum) is, I think , 1.13V/ns, or 282V in 250nS, so a little slower than my estimated 900V in 250nS for an applied square-ware - although the leakage of the Tx and the capacitance of diodes + snubbers would naturally slow the Volt rise time for a real circuit ...


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    Re: Stacking diodes for high voltage rectification

    Omitting the RC snubbers would still make distortion on the output?



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    Re: Stacking diodes for high voltage rectification

    without the snubbers and sharing resistors the diode with the lowest leakage current will break down in reverse first, then the next lowest leakage current, etc ...


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    Re: Stacking diodes for high voltage rectification

    Is it fine to omit RC snubber and only put a resistor across each diode instead?



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    Re: Stacking diodes for high voltage rectification

    The usual solution is to use diodes with avalanche characteristic that can be series connected without voltage sharing means.


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    Re: Stacking diodes for high voltage rectification




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    Re: Stacking diodes for high voltage rectification

    Such as these
    They should work if you can keep the reverse power dissipation within limits. What is the load you are going to use?


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