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  1. #21
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    Adding 2nd stage to Solid State Marx Generator fails

    Hi Friends

    After my one stage Solid State Marx Generator started working, I have added a 2nd stage, but it does not work as advertised in this paper it should be a simple matter of adding stages

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    When adding the 2nd stage I expected that it would add ~1000V to the output, but instead it produces the same output as one stage ~2000V. I notice that the voltage across Q3 and Q5 is only 600V, half of the single stage Q3 voltage of 1200v, which is the reason for the low output. But I have no idea why it does that, 1200 is expected on both Q3 and Q5.

    DUAL STAGE (not working as advertised)
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    SINGLE STAGE (working)
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    Last edited by FvM; 20th June 2019 at 22:28. Reason: Merged with previous similar topic thread



  2. #22
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    Re: Does Solid State Marx Generators actually work?

    UF1007 breakdown voltage is a sufficient reason to get not more than 2000 V output from your circuit. You may evaluate this point by reducing the DC input voltage.

    In addition it's not clear how the measurement loads the circuit. I'm stumbling upon the term "ground clip", there should be a fully differential high impedance probe.



  3. #23
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    Re: Does Solid State Marx Generators actually work?

    Hi FvM

    Quote Originally Posted by FvM View Post
    UF1007 breakdown voltage is a sufficient reason to get not more than 2000 V output from your circuit. You may evaluate this point by reducing the DC input voltage.
    Yes that could be a reason, so first I changed D3 and D7 (post #21) to BY203 2000V and then I lowered input voltage to +/-300V, but the output is still the same ~1100V regardless of one or two stages. And if I remove one or the other stage, then remaining stage works fine.

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    Quote Originally Posted by FvM View Post
    In addition it's not clear how the measurement loads the circuit. I'm stumbling upon the term "ground clip", there should be a fully differential high impedance probe.
    Sorry I forgot to mention that load on output is 100Kohm resistor. I don't have a differential probe yet.



  4. #24
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    Re: Does Solid State Marx Generators actually work?

    you can fake a differential probe by measuring the voltage at each end and subtracting
    if your 'scope has two probes, it may do the math for you
    both probes must be referenced at the same point



  5. #25
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    Re: Does Solid State Marx Generators actually work?

    Quote Originally Posted by wwfeldman View Post
    you can fake a differential probe by measuring the voltage at each end and subtracting
    if your 'scope has two probes, it may do the math for you
    both probes must be referenced at the same point
    Thank you for your suggestion, I thought I did fake a differential probe by floating the scope and measure as I did (see schematic in thread #21 where I indicated how the probe was connected)?

    - - - Updated - - -

    I made this sim perhaps it can give some clue to what is wrong, and yes maybe I'm interpreting it incorrectly.

    I have draw a schematic that shows how the caps and diodes are configured during final output stage where full 3600V (6 caps x 600V) output is expected, I kept the component refs from the schematic thread #21.

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    On the left is how it is in the circuit. I'm wondering why it doesn't charge up to 3.6K. And as you can see the voltage C6 C9 and C10 is the same, it's like these caps doesn't exist.

    On the right, it looks like if I remove either D1 D2 D6 or D8, then there is a voltage difference on C6 C9 and C10, but it still charges up to only 2.3KV. So I'm wondering if D1 D2 D6 D8 in fact are shorting out C6 C9 and C10?

    - - - Updated - - -

    I see the problem in the sim thread #25, the diodes are only 800V, so now i quintupled them and it charges to 3600V. As you can see on the right circuit in the sim below there is an even spread 600V across each cap, but that is only because I cut the connection from C5/6 to the diodes (marked in red). So I think now that a additional switch is required during output stage to cut this connection, would you agree?

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    - - - Updated - - -

    Here is my problem: D8

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    So this design will not work for additional stages. And user Easy Peasy warned me against believing everything on the internet, but I thought I was safe this time because it was a IEEE paper.

    Well back to the drawing board


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  6. #26
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    Re: Does Solid State Marx Generators actually work?

    So this design will not work for additional stages. And user Easy Peasy warned me against believing everything on the internet, but I thought I was safe this time because it was a IEEE paper.
    I agree it can't work. They claimed the circuit could be extended, apparently without ever checking it. Lesson learnt!



  7. #27
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    Re: Does Solid State Marx Generators actually work?

    OK, I have a new approach Development of High Voltage Solid State Marx Generator for Liquid Applications, but I must admit I found it on the internet so I checked the publisher, at least they were not listed in List of Predatory Journals.

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    Then I made this sim with four stages just to be on the safe side

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    It seems to be working right out the box, so if nobody has any objections or warnings, then I will start designing a PCB.



  8. #28
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    Re: Does Solid State Marx Generators actually work?

    Why are there so many switching elements in your circuit ? It should be a straightforward RC network with a single switching power source.



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    Re: Does Solid State Marx Generators actually work?

    Marx generator needs at least one switch per stage, more if active charging is intended. The suggested circuit looks reasonable.


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    Re: Does Solid State Marx Generators actually work?

    Quote Originally Posted by kripacharya View Post
    Why are there so many switching elements in your circuit ? It should be a straightforward RC network with a single switching power source.
    Yes, you would be right if the final output was 800V, I choose 800V because that was the diode with the highest voltage rating in ltspice and that was just to check the topology. In reality I will have 8 stages of 1200V so the output will be ~10KV pulses at 20KHz.



  11. #31
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    Re: Does Solid State Marx Generators actually work?

    OK, I got my PCB's and assembled them and problems start...

    I have two stages, when I test each one independently it delivers 1100V on the output, however when I connect both together (see below) the output remains 1100V when it should have been the double. I checked both caps charge up to 1100V and measuring the Qdischarge IGBT's across Source and Drain they both show ~0V when ON, which is expected. So why only 1100V on the output (measured across R1) with 2 stages?

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    Re: Does Solid State Marx Generators actually work?

    If I understand right, the circuit worked in your simulation, thus there's obviously a problem with the implementation, either actual circuit or control pattern.



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    Re: Does Solid State Marx Generators actually work?

    Quote Originally Posted by FvM View Post
    If I understand right, the circuit worked in your simulation, thus there's obviously a problem with the implementation, either actual circuit or control pattern.
    Yes, it's an implementation problem for sure.

    Sequence of events: I assembled four stages, tested all four individually and they worked. Joined stage 1+2 they worked (output 2200V), felt confident that everything was a-ok and added stage 3+4 so had 1+2+3+4 connected together but output was 1100V. Messed around stage 1+2 for four hours couldn't get them to work together so I wrote thread #31 for help. Subsequently tried with only stage 3+4 (output 2200V) they worked. So now I'm trying to compare whats different between 1+2 vs. 3+4 and so far can't figure out what's wrong with 1+2.



  14. #34
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    Re: Does Solid State Marx Generators actually work?

    I found the culprit, it was Qcharge1 that was defunkt. So now it looks like this and the output with one stage 1100V, two stages is 2000V, three stages 2700V, four stages 1800V - disappointing to say the least.

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    So I measured across the caps (200V/DIV), and for C1 and C4 it looks like it should:

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    But for C2 and C3 the caps are completely discharged instantly, that's not good, but why?

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    Here is the commutation
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    Last edited by Swend; 30th June 2019 at 20:59.



  15. #35
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    Re: Does Solid State Marx Generators actually work?

    I did some more investigation.

    With stage 1+2+3 = 2700V

    With stage 1+2+3+4 = 1800V

    With stage 2+3+4 = 2700V

    So the problem seems to occur only when the fourth stage is added, but it doesn't make any sense to me, here is the analysis.

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    Re: Does Solid State Marx Generators actually work?

    i guess i missed it
    please show how the capacitors get charged, as the only path through the caps is the discharge path



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    Re: Does Solid State Marx Generators actually work?

    Quote Originally Posted by wwfeldman View Post
    i guess i missed it
    please show how the capacitors get charged, as the only path through the caps is the discharge path
    Charging is Qsupply1, Qcharge(1-4) = ON and Qdischarge(1-4) = OFF so follow the red and blue lines to each capacitor, the charging is all caps in parallel.

    Discharge is Qsupply1, Qcharge(1-4) = OFF and Qdischarge(1-4) = ON so follow the green line through eachcapacitor, the discharging is all caps in series.



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    Re: Does Solid State Marx Generators actually work?

    Red + blue is charge path, as indicated in the drawing. Also explained in the post #27 link.



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    Re: Does Solid State Marx Generators actually work?

    My few bits... I could be totally wrong here, but thought to mention anyhow. Maybe it'll trigger some ideas somewhere.

    1) Would it be feasible to share your gate drive circuitry ? It seems to me that in your discharge cycle, in the later stages the IGBT's emitter/ collector will be raised to ever higher voltages. How do you drive the gate in such a scenario ? Some error there vis-a-vis higher voltage operation might explain why each stage works independently but fails when cascaded.

    2) What is the 'delay' time in your commutation ? Maybe that's too small - not enough time for some of the charge IGBT's to switch off fully & hence shorting out one complete stage.

    3) The voltage drop across Discharge IGBT should surely be lesser than the diode drop in the following stage ? If not, then again this would short out an entire stage during discharge. Maybe put in 2x diodes in series to ensure ?

    Do let me know if these are obvious/ irrelevant points, and I'll go back to spectator status.



    - - - Updated - - -

    And oh !! What about that free-wheeling diode in the IGBT's ? Doesn't that come into the picture somewhere ?



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    Re: Does Solid State Marx Generators actually work?

    Quote Originally Posted by kripacharya View Post
    My few bits... I could be totally wrong here, but thought to mention anyhow. Maybe it'll trigger some ideas somewhere.

    1) Would it be feasible to share your gate drive circuitry ? It seems to me that in your discharge cycle, in the later stages the IGBT's emitter/ collector will be raised to ever higher voltages. How do you drive the gate in such a scenario ? Some error there vis-a-vis higher voltage operation might explain why each stage works independently but fails when cascaded.

    2) What is the 'delay' time in your commutation ? Maybe that's too small - not enough time for some of the charge IGBT's to switch off fully & hence shorting out one complete stage.

    3) The voltage drop across Discharge IGBT should surely be lesser than the diode drop in the following stage ? If not, then again this would short out an entire stage during discharge. Maybe put in 2x diodes in series to ensure ?

    Do let me know if these are obvious/ irrelevant points, and I'll go back to spectator status.



    - - - Updated - - -

    And oh !! What about that free-wheeling diode in the IGBT's ? Doesn't that come into the picture somewhere ?
    Hi kripa

    1) Sure, please see my thread #1 the gate drive is identical for all igbt's (but now using 10/100ohm for off/on). But three cascaded stages do work, it's when you add stage 4 then the stages 2 and 3 does not work. (see thread #35). Then when you remove stage 1 or 4, the three remaining stages work.

    2) It's 40uS as can be seen in the scope traces in thread #34. But yes timing could be an issue so I experimented with various delays at various points, even tried to turn the Qd's in sequence forward/reverse with various delays in between, but none of it made any difference unfortunately.

    3) I will check it.

    4) These igbt's don't have any fw diode, but if they did it still works in the sim.



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