+ Post New Thread
Page 1 of 5 1 2 3 ... LastLast
Results 1 to 20 of 91
  1. #1
    Full Member level 4
    Points: 981, Level: 7

    Join Date
    May 2019
    Location
    Roskilde, Denmark
    Posts
    220
    Helped
    9 / 9
    Points
    981
    Level
    7

    Pulse charging capacitor through diode - problems

    Hi friends.

    I am trying to pulse charge a few capacitors through diodes, here is the schematic below. Q3 and Q4 are both OFF in this test. C6 is the problem area.

    Click image for larger version. 

Name:	load.png 
Views:	45 
Size:	16.1 KB 
ID:	153768

    Input pulse

    Click image for larger version. 

Name:	SDS00191.PNG 
Views:	15 
Size:	40.7 KB 
ID:	153769

    Voltage across C4 with ref. to 0 (zero) everything is as expected.

    Click image for larger version. 

Name:	SDS00192.PNG 
Views:	11 
Size:	33.8 KB 
ID:	153770

    Voltage across C6 with ref. to 0 (zero), now why can't this be like C4? the only difference between C4 and C6, is that when charging C6 there is an extra diode D3 in the charging path. If I short out D3, then C6 trace becomes exactly like C4. And I tried several other diodes, same problem.

    Click image for larger version. 

Name:	SDS00190.PNG 
Views:	12 
Size:	36.1 KB 
ID:	153771

    I measured the voltage drop over D3, according to the datasheet the Vfm is 1.7V and I measured it to 410mV - so it never turns on, why?

    Click image for larger version. 

Name:	SDS00197.PNG 
Views:	10 
Size:	41.2 KB 
ID:	153776

  2. #2
    Full Member level 4
    Points: 981, Level: 7

    Join Date
    May 2019
    Location
    Roskilde, Denmark
    Posts
    220
    Helped
    9 / 9
    Points
    981
    Level
    7

    Re: Pulse charging capacitor through diode - problems

    I did some further investigation, it seems to me it's a design error with that D3, see sim below. I had the feeling when I saw this design proposal in a IEEE paper, but then I doubted myself and went with the original proposal.

    Click image for larger version. 

Name:	ltsim29.png 
Views:	14 
Size:	82.8 KB 
ID:	153782

    So I guess I have to use a MOSFET instead of a IGBT for Q3, unless someone here have experience with "pre-forward-biasing" a diode, and D3 can be forward-biased permanently for that matter doesn't have to be switched in any way.



  3. #3
    Advanced Member level 5
    Points: 19,100, Level: 33
    Achievements:
    7 years registered
    crutschow's Avatar
    Join Date
    Feb 2012
    Location
    L.A. USA Zulu -8
    Posts
    3,677
    Helped
    874 / 874
    Points
    19,100
    Level
    33

    Re: Pulse charging capacitor through diode - problems

    You can't have only a diode in series with a capacitor.
    Think about it.
    Once the capacitor charges through the diode, there's no path for the discharge current.
    Zapper
    Curmudgeon Elektroniker


    1 members found this post helpful.

  4. #4
    Full Member level 4
    Points: 981, Level: 7

    Join Date
    May 2019
    Location
    Roskilde, Denmark
    Posts
    220
    Helped
    9 / 9
    Points
    981
    Level
    7

    Re: Pulse charging capacitor through diode - problems

    Quote Originally Posted by crutschow View Post
    You can't have only a diode in series with a capacitor.
    Think about it.
    Once the capacitor charges through the diode, there's no path for the discharge current.
    Yes I know, but it's a solid state Marx Generator, you turn Q4 on then all caps are connected in series and a spark discharges all the caps.

    https://www.researchgate.net/publica..._Configuration



  5. #5
    Advanced Member level 5
    Points: 19,100, Level: 33
    Achievements:
    7 years registered
    crutschow's Avatar
    Join Date
    Feb 2012
    Location
    L.A. USA Zulu -8
    Posts
    3,677
    Helped
    874 / 874
    Points
    19,100
    Level
    33

    Re: Pulse charging capacitor through diode - problems

    Well, your first circuit had diodes in series with some of the capacitors with no discharge path shown, and that won't work.
    Zapper
    Curmudgeon Elektroniker


    1 members found this post helpful.

    •   AltAdvertisement

        
       

  6. #6
    Full Member level 4
    Points: 981, Level: 7

    Join Date
    May 2019
    Location
    Roskilde, Denmark
    Posts
    220
    Helped
    9 / 9
    Points
    981
    Level
    7

    Re: Pulse charging capacitor through diode - problems

    Quote Originally Posted by crutschow View Post
    Well, your first circuit had diodes in series with some of the capacitors with no discharge path shown, and that won't work.
    If you are ref to the first circuit in the LTspice sim (post #2), yes then you are right it won't work (at least not reliably) that's what the sim shows. But that was how the IEEE paper suggested to do it and since they claimed it worked in their lab circuit, I adopted their design proposal.

    And there is no discharge path shown, becauseat the moment the discharge is uninteresting if I can't charge the caps to start with. I think the discharge part will present a completely new set of problems if I manage to solve the charging problems.



  7. #7
    Full Member level 4
    Points: 981, Level: 7

    Join Date
    May 2019
    Location
    Roskilde, Denmark
    Posts
    220
    Helped
    9 / 9
    Points
    981
    Level
    7

    Does Solid State Marx Generators actually work?

    Hi Friends

    I'm rephrasing my previous post as I maybe have not expressed myself clearly enough.

    This is a standard topology for a SSMG take from this paper Solid stateBipolar Marx GeneratorTopologies: A Comparative study

    Click image for larger version. 

Name:	Screenshot from 2019-06-18 12-38-44.png 
Views:	11 
Size:	34.5 KB 
ID:	153793

    The question is: How does C2 get charged? As you can see C2's charging path is from plus to minus (+) Ddc C2 Df1 (-).

    I have made a simulation of it because I could not get it to work in my real life circuit, the diode Df1 (D2 in the sim) will not become forward biased, and neither will it in the sim. But people all over the world claim that it works, so why can't it work for me?

    Click image for larger version. 

Name:	ltsim29.png 
Views:	17 
Size:	95.4 KB 
ID:	153794



  8. #8
    Full Member level 6
    Points: 2,035, Level: 10

    Join Date
    Jan 2019
    Posts
    378
    Helped
    94 / 94
    Points
    2,035
    Level
    10

    Re: Does Solid State Marx Generators actually work?

    you're circuit does not look like the Marx Generators I found on the internet

    1) https://en.wikipedia.org/wiki/Marx_generator
    2) https://www.instructables.com/id/Bui...arx-Generator/
    3) https://www.penguinslab.com/marx.htm


    your circuit appears to be the charging model, Figure 3, from this site:
    https://www.ijert.org/research/solid...NV3IS01033.pdf

    i expect it will work if you build it per items 2 and 3 above.



    •   AltAdvertisement

        
       

  9. #9
    Full Member level 4
    Points: 981, Level: 7

    Join Date
    May 2019
    Location
    Roskilde, Denmark
    Posts
    220
    Helped
    9 / 9
    Points
    981
    Level
    7

    Re: Does Solid State Marx Generators actually work?

    Quote Originally Posted by wwfeldman View Post
    you're circuit does not look like the Marx Generators I found on the internet

    1) https://en.wikipedia.org/wiki/Marx_generator
    2) https://www.instructables.com/id/Bui...arx-Generator/
    3) https://www.penguinslab.com/marx.htm


    your circuit appears to be the charging model, Figure 3, from this site:
    https://www.ijert.org/research/solid...NV3IS01033.pdf

    i expect it will work if you build it per items 2 and 3 above.
    Hi wwfeldman

    Thank you for your input, but none of the proposed are solid state.



  10. #10
    Full Member level 6
    Points: 2,035, Level: 10

    Join Date
    Jan 2019
    Posts
    378
    Helped
    94 / 94
    Points
    2,035
    Level
    10

    Re: Does Solid State Marx Generators actually work?

    Quote Originally Posted by Swend View Post
    Hi Friends
    The question is: How does C2 get charged? As you can see C2's charging path is from plus to minus (+) Ddc C2 Df1 (-).
    Click image for larger version. 

Name:	ltsim29.png 
Views:	17 
Size:	95.4 KB 
ID:	153794
    C2 gets charged when switches Tdc, Ta1 and Te1 (in the schematic) are closed

    When the power supply goes to 0 V at the end of the pulse, it looks like a short, NOT an open
    by Kirchoff's law, the voltage across the capacitor get distributed across the other components.
    the diodes are reverse biased, so there is no current flow
    with one diode, all of the Vcap is Vdiode
    with two diodes, Vcap is evenly split

    so position C1 has voltage = Vcap - V diode, or what amounts to 1/2 V expected

    use a differential probe across the capacitor, not a single ended probe relative to ground

    I used LTspice to simulate, except I did not use an inductor.
    I saw no inductors in the SSMG
    i also did not find any differential probes, but you can change the probe to a reference

    use the simulation software with an eye toward the basic stupidity of a computer program
    you as the user need to be smart



    •   AltAdvertisement

        
       

  11. #11
    Full Member level 4
    Points: 981, Level: 7

    Join Date
    May 2019
    Location
    Roskilde, Denmark
    Posts
    220
    Helped
    9 / 9
    Points
    981
    Level
    7

    Re: Does Solid State Marx Generators actually work?

    Hi Friend

    Quote Originally Posted by wwfeldman View Post
    C2 gets charged when switches Tdc, Ta1 and Te1 (in the schematic) are closed
    I'm afraid it doesn't, neither in the sim nor in a real life circuit.

    Quote Originally Posted by wwfeldman View Post
    I saw no inductors in the SSMG
    Here is one with inductors for your kind ref https://apps.dtic.mil/dtic/tr/fulltext/u2/a635206.pdf

    Anyway, in the meantime I think I solved my problem by "pre-forward-biasing" the diode, at the cost of a additional galvanic isolated power supply, but it gets the job done the cap gets fully charged now.

    Click image for larger version. 

Name:	Screenshot from 2019-06-18 15-31-29.png 
Views:	8 
Size:	49.9 KB 
ID:	153800



  12. #12
    Full Member level 6
    Points: 2,035, Level: 10

    Join Date
    Jan 2019
    Posts
    378
    Helped
    94 / 94
    Points
    2,035
    Level
    10

    Re: Does Solid State Marx Generators actually work?

    @swend

    its likely you know something i don't

    but i think you're not understanding what LTSpice is telling you

    When the power supply goes to 0 V at the end of the pulse, it looks like a short, NOT an open
    by Kirchoff's law, the voltage across the capacitor get distributed across the other components.
    the diodes are reverse biased, so there is no current flow
    with one diode, all of the Vcap is Vdiode
    with two diodes, Vcap is evenly split

    so position C1 has voltage = Vcap - V diode, or what amounts to 1/2 V expected, since the diodes are identical.

    use a differential probe across the capacitor, not a single ended probe relative to ground

    the simulation software does what you tell it to do.
    you need to understand how it works and what it does
    and you need to interpret the out put appropriately

    specifically, in post 7, the traces are Vc1
    as you know, voltage is measured relative to some point
    in all spice programs, unless otherwise specified, it is always the ground
    that's why every spice program insists that you specify ground

    with one diode, all of the voltage across the capacitor is across the reverse biased diode
    with two (identical) diodes, half of the voltage across the capacitor is across each reverse biased diode

    when you plot Vc1, it is relative to ground
    with the second diode, you get half the voltage.


    1 members found this post helpful.

  13. #13
    Full Member level 4
    Points: 981, Level: 7

    Join Date
    May 2019
    Location
    Roskilde, Denmark
    Posts
    220
    Helped
    9 / 9
    Points
    981
    Level
    7

    Re: Does Solid State Marx Generators actually work?

    Quote Originally Posted by wwfeldman View Post
    but i think you're not understanding what LTSpice is telling you
    Yes you could easily be right about that I'm a LTspice noob. However the problem is that I do not fully understand what you are saying, first of all I'm not sure what circuit/measurement you are referencing in each paragraph, so please start with that it would help me a lot.

    I do understand the part where you talk about C1 (in the sim) and differential probe, so here is the sim with a differential probe, and yes indeed the sim looks like it charging the cap.

    Click image for larger version. 

Name:	Screenshot from 2019-06-18 20-01-27.png 
Views:	5 
Size:	44.2 KB 
ID:	153807

    But I can assure you it doesn't in the physical circuit, not without the "pre-forward-bias" (post #11). Please look at my post #1 and the D3 (D2 in the sim) trace with differential probe in the physical circuit - D3 never ever turns on, and that was the original problem.



  14. #14
    Super Moderator
    Points: 260,253, Level: 100
    Awards:
    1st Helpful Member

    Join Date
    Jan 2008
    Location
    Bochum, Germany
    Posts
    45,447
    Helped
    13829 / 13829
    Points
    260,253
    Level
    100

    Re: Does Solid State Marx Generators actually work?

    I try to understand what your simulation has to do with the post #1 circuit. I believe that the latter doesn't work as expected, but what do you exactly expect?

    The imagined design problem in the post #2 simulation circuit has turned out as misunderstanding of Ltspice usage.

    Furthermore I recognize only few similarities between the circuit quoted in #7 and your original circuit.

    - - - Updated - - -

    The full bipolar generator circuit is shown in Fig.2 respectively simplified Fig.9 of the quoted paper. It has little in common with circuit from the other paper, which needs a bipolar pulse to charge all four capacitors. The post #1 circuit looks like a correct implementation of the generator suggested in the paper. Unfortunately you don't clearly mark the output terminals, neither the paper does. I admit, it took me two minutes to understand where the output is.

    Voltage across C6 with ref. to 0 (zero), now why can't this be like C4?
    Now I see the similarity with your erroneous Ltspice simulation. Voltage across C6 simply can't be measured ground referenced, it must be measured differentially.


    1 members found this post helpful.

  15. #15
    Super Moderator
    Points: 260,253, Level: 100
    Awards:
    1st Helpful Member

    Join Date
    Jan 2008
    Location
    Bochum, Germany
    Posts
    45,447
    Helped
    13829 / 13829
    Points
    260,253
    Level
    100

    Re: Does Solid State Marx Generators actually work?

    To clarify the output point

    Click image for larger version. 

Name:	mg.PNG 
Views:	6 
Size:	70.8 KB 
ID:	153814



  16. #16
    Full Member level 6
    Points: 2,035, Level: 10

    Join Date
    Jan 2019
    Posts
    378
    Helped
    94 / 94
    Points
    2,035
    Level
    10

    Re: Does Solid State Marx Generators actually work?

    FvM's circuit requires a control that follows a pattern like:

    s1 on s2 off
    s1 and s2 on
    s1 off S2 on
    s1 and s2 on
    repeat

    the current in the inductor needs a path at all times

    the only exception is if you know the current in the inductor goes to 0
    before you turn off s1 or s2



  17. #17
    Super Moderator
    Points: 260,253, Level: 100
    Awards:
    1st Helpful Member

    Join Date
    Jan 2008
    Location
    Bochum, Germany
    Posts
    45,447
    Helped
    13829 / 13829
    Points
    260,253
    Level
    100

    Re: Does Solid State Marx Generators actually work?

    Yes. Fig. 3 in the paper linked in post #4 is a simplified schematic. You should review the original paper for related questions and more elaborated circuits. I was posting it solely for better understanding of post #1 and the problems raised about it.



  18. #18
    Full Member level 4
    Points: 981, Level: 7

    Join Date
    May 2019
    Location
    Roskilde, Denmark
    Posts
    220
    Helped
    9 / 9
    Points
    981
    Level
    7

    Re: Does Solid State Marx Generators actually work?

    Quote Originally Posted by FvM View Post
    Now I see the similarity with your erroneous Ltspice simulation. Voltage across C6 simply can't be measured ground referenced, it must be measured differentially.
    Yes that is also what wwfeldman noted previously. But does it matter? I mean I measured (post #1) voltage across D3 (DS3 in the simplified schematic in post #15) differentially, and it read 410mV and the datasheet says 1.7V - so D3 never turns on?



  19. #19
    Super Moderator
    Points: 260,253, Level: 100
    Awards:
    1st Helpful Member

    Join Date
    Jan 2008
    Location
    Bochum, Germany
    Posts
    45,447
    Helped
    13829 / 13829
    Points
    260,253
    Level
    100

    Re: Does Solid State Marx Generators actually work?

    I measured (post #1) voltage across D3 (DS3 in the simplified schematic in post #15) differentially, and it read 410mV and the datasheet says 1.7V - so D3 never turns on?
    If D3 never turns on, how is C6 other terminal jumping to 600V? There's no other current path provided.

    1.7V specification is for 1A diode current, you also have junction capacitance dynamically reducing the forward voltage. To see if C6 is charged, you'll measure its voltage with a differential probe.



  20. #20
    Full Member level 4
    Points: 981, Level: 7

    Join Date
    May 2019
    Location
    Roskilde, Denmark
    Posts
    220
    Helped
    9 / 9
    Points
    981
    Level
    7

    Re: Does Solid State Marx Generators actually work?

    Quote Originally Posted by FvM View Post
    If D3 never turns on, how is C6 other terminal jumping to 600V? There's no other current path provided.
    Well I thought anything is possible and I don't know everything, so perhaps it could be because the voltage presented itself on the other side of D2 (schematic in post #1) due to e.g. PCB creepage or perhaps evil spirits. In any case I was confused and frustrated.

    Quote Originally Posted by FvM View Post
    1.7V specification is for 1A diode current, you also have junction capacitance dynamically reducing the forward voltage. To see if C6 is charged, you'll measure its voltage with a differential probe.
    In the meantime I decided to dump the inductor and current return path circuitry, it reduces component count considerably and I have no need for the voltage boost as my power supply can easily reach +/-600V. And perhaps I will later charge both negative and positive simultaneously if it doesn't add too much complexity. Here is the differential C6 measurement (x100)
    Click image for larger version. 

Name:	SDS00200.PNG 
Views:	2 
Size:	18.1 KB 
ID:	153832

    And here is the final output (x100), I'm missing 400V but I think it's because the caps have time to self-discharge a bit before Q4 (schematic in post #1) is switched on. Load is 100Kohm on the output btw.
    Click image for larger version. 

Name:	SDS00199.PNG 
Views:	5 
Size:	19.1 KB 
ID:	153833
    Last edited by Swend; 19th June 2019 at 17:53.



    •   AltAdvertisement

        
       

--[[ ]]--