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  1. #21
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    Re: Stacking diodes for high voltage rectification

    The load appears to be capacitive, certainly nothing inductive - however would ordinary fast recovery diodes work if their package permits lots of power dissipation?

    This is not a capacitor charging application.



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  2. #22
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    Re: Stacking diodes for high voltage rectification

    if you avalanche these diodes at 200kHz, the solder will melt ...


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    Re: Stacking diodes for high voltage rectification

    if you avalanche these diodes at 200kHz, the solder will melt ...
    Avalanche diodes are mostly used in HV diode stacks, e.g. the 8 kV/2.5A stack of Semikron. Avalanche capability means in this regard, that a diode can start to draw breakdown current without wear, in case stack asymmetry causes exceeding of breakdown voltage for individual diodes. Regularly I would expect an uniform voltage distribution however.

    There are devices like Vishay BYG23T-M3, "Ultrafast Avalanche Rectifiers", I wonder what's the feasible current at 200 kHz in terms of turn-off losses.

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


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  4. #24
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    Re: Stacking diodes for high voltage rectification

    Quote Originally Posted by Easy peasy View Post
    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 ...
    What you detailed simply states no RC snubber & resistor combination will permit the diodes to make rectified high voltage since at the minimum of 200kHz each diode has ~15pF of capacity which is ~53K and I = 1kV / 53K = ~18mA which means each diode will be a bidirectional short circuit at those frequencies.

    Considering output of the highvoltage is =<180W @ 100KV its a maximum of 1.8mA which isn't enough to cause heating in the diodes but the energy absorbed by them will not output any high voltage making the stack inoperable to rectify the 100kv high frequency. Avalanche diodes will do the same thing since they all have capacity not to mention the waveform would be greatly distorted if the stack did rectify the AM envelopes.



  5. #25
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    Re: Stacking diodes for high voltage rectification

    The problem is that for the diodes you seek to use - the source must supply a lot of reactive power as well as the real power to provide the DC to the unknown load.

    This is due to the capacitance in the diodes and the arbitrarily high frequency you want to use - if you do a thorough internet search you will find diodes that are nearly up to the job - but they come at a price - common diodes cannot do what you seek to do at 200kHz.


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    Re: Stacking diodes for high voltage rectification

    Quote Originally Posted by Easy peasy View Post
    The problem is that for the diodes you seek to use - the source must supply a lot of reactive power as well as the real power to provide the DC to the unknown load.

    This is due to the capacitance in the diodes and the arbitrarily high frequency you want to use - if you do a thorough internet search you will find diodes that are nearly up to the job - but they come at a price - common diodes cannot do what you seek to do at 200kHz.
    What are those diodes which are nearly capable of doing this? Any diode has some capacity at its junction unless its a very low voltage high current schottky or an RF detector diode.
    Last edited by Zak28; 21st August 2019 at 23:36.



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  7. #27
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    Re: Stacking diodes for high voltage rectification

    http://www.ppmpower.co.uk/products/d...urrent-diodes/

    are what you are seeking - a 1 min google search ...


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  8. #28
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    Re: Stacking diodes for high voltage rectification

    Quote Originally Posted by Easy peasy View Post
    http://www.ppmpower.co.uk/products/d...urrent-diodes/

    are what you are seeking - a 1 min google search ...
    Those have capacitances lower than point contact diodes. They should work - are you aware of any retailers stocking their products?



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