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High voltage smoothing

  1. Oct 8, 2011 #1
    Hi. I've built a (3 in fact) negative HV power supply continuouly adjustable from 0 to - 8 kV. It uses a CCFL inverter followed by a voltage multiplier and a regulation loop. In order to eliminate regulation peaks I would like to smooth final voltage by using a 10 nF / 15 kV cap (connected to ground) , but ... I not completely sure but it seems that I already did that in the past and the result was the destuction of several diodes ad caps of the voltage multiplier. Is there a condition to fill in order to properly realize this smooting ?
     
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  3. Oct 8, 2011 #2
    Put a current limiting HV resistor in series with the output, then take the voltage feedback from the output side of the resistor. The resistor serve as the current limiting and feedback from the output side of the resistor will compensate the voltage drop across the resistor. You just have one more pole to deal with but it is easy. The filter works even better with the resistor. We did a lot of HV supply design, never ran into any problem. Just pay attention on the closed loop feedback.
     
  4. Oct 8, 2011 #3

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    A conventional voltage multiplier already has such capacitance from output to ground, so the addition of further capacitance should not be the cause of blowing up components.

    You say '..and a regulation loop..'. Care to expand on this? Maybe this is where the problem lies.
     
  5. Oct 8, 2011 #4
    I don't think extra cap at the output will cause it to burn. But if the charge of the internal caps is being discharged through the diode without current limiting, that spell trouble. Those high voltage diodes are not as rugged as the regular diode. HV discharge can be very high current even though the capacitance are very low because of the amount of energy store due to the voltage. You are talking about hundreds of amp discharge in nano seconds. Even Transorbs need to have current limiting resistor. Notice I specified HV resistors that have long body to avoid creepage. If you use normal resistors, an arc will cause the charge to jump over the resistor body and you loss the current limiting effect of the resistor. This is very important in HV design.

    Also one very important thing, you need to check to make sure there is no conductive materials( metal) left floating. Every bit of metal has to be tie to a known potential. Loose metal will be charged up in HV environment and start arcing to the next door. This is very hard to trouble shoot. Some PCB design actually ached rev number in copper and that is deadly in HV environment. You have to scrape them off or tie it to ground. Once it arc, all bets are off.
     
    Last edited: Oct 8, 2011
  6. Oct 8, 2011 #5

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    Incidentally, what diodes are you using? 'Regular' diodes (1N40XX) aren't quite quick enough for 40kHz off CCFLs. They just about do it, but may begin to overheat, so they'll look like they are working with no load, but flunk out with any further loading on the stack.

    I've used them before in this application, rectifying CCFL, but for a teeny current (uA - feeding Geiger tubes), but it's not a clever idea if you need it to be robust.
     
  7. Oct 8, 2011 #6
    High voltage diodes are very long, nothing of the 1N40XX!!!

    When a high voltage arc, it take out big diodes. 1N4001 or 02 etc. is only 1A diodes, I am talking about burning 5W diodes from HV arc!!!! We burned transorbs like P6KEXX or bigger!!! You don't let current avalanche.....period. Everything is about current limiting in HV design, component do not survive arcing. Discharging a 5' RG59 ( 30pF per foot) is enough to burn a lot of things if inrush current is not limited. Take my word for this. Do a calculation to discharge a 500pF cap charged to 10KV in 1nS and you'll appreciate the amplitude of the instantaneous current.
     
  8. Oct 8, 2011 #7

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    Not sure I agree with the wording - to 'burn' implies heating to some 'hot' degree, but 10kV on a 500pF is 25mJ. For sure, voltage breakdown can be very damaging - and even more so if there are inductive effects that push the voltage higher still, but 'burn' implies more than just a few mJ!
     
  9. Oct 8, 2011 #8
    In 1nS!!! When You put all the power in such a short period of time, power rating does not reflect the current handling capability as heat has no time to transfer out and ALL localized in one spot. Burn is burn, I don't really care about whether it smoke or not, they usually shorted out......still cold!!!! You never see burn in form of smoke like the normal sense that it get hot, smoke and die.

    I am telling you from years of experience designing 15KV power supply and high power HV supply and I learned all these the hard way. You'll be surprised that 5' RG59 can burn some serious stuff when arc.
     
  10. Oct 8, 2011 #9

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    I respectfully disagree/don't understand your point. Whether nS or ms, X mJ of energy will raise whatever it is heating to the the same temperature. The specific heat capacity of a thing is its specific heat capacity.

    Thermal conductance away from some wire bond/junction/whatever won't happen much faster or slower on that timescale, if we are talking mJ. That's why if you look at any component sheet it says that derating/thermal issues are related to pulse energy rather than current, once below a ms or so.
     
  11. Oct 8, 2011 #10
    Actually not. Discharging 5' of RG-59 (Z = 75 ohms, v/c = 66%) will produce a 15 ns long pulse. If it is charged to 10 kV, the pulse current is 67 amps. RG-59 has a delay length of about 7.5 ns, so the pulse length will be 15 ns long at 5 kV when discharged, and the output impedance is 75 ohms, so the instantaneous current is 5 kV/75 ohms = 67 amps.

    See for example pages 7 and 8 in
    http://web.missouri.edu/~kovaleskis/ApplEMandEP/Lectures/Lecture-5.pdf [Broken]

    Bob S
     
    Last edited by a moderator: May 5, 2017
  12. Oct 8, 2011 #11
    Instantaneous curent will raise the temperature at one local spot. Heat takes time to transfer out.

    If you work in IC design and testing, you'll see. In pcb, we cut trace with exato knife. But in IC, you can't do that. What we do is use needle probe and stick on both end of the trace that needed to be cut on the die and then just discharge a small cap. You can see the trace under microscope just pop. The trace supposed to be able to conduct a few amps but it will pop by the cap. The reason is it does not have enough time to transfer all the heat out at one time even the pulse is very short by high current.

    As I said, whether you agree or not, this is real life. As I said, I have been in this field and anyone in the field know exactly what I mean. Go download schematic from Bertan or other HV supply manufacturers and you'll see they ALWAYS have a resistor to isolate the bridge or other rectifiers from the output and take the voltage to feedback to regulate the voltage.

    Particular if you have voltage multiplier that have caps in between diodes directly, you discharge the cap through the diode, you burn the diode almost every time.
     
  13. Oct 8, 2011 #12
    You might be right, it might be only 7.5nS as it is not a round trip. but as you can see even the 5' coax give 67amp for 15nS and that is enough to burn a lot of things. Discharging a cap is in hundreds of amps.
     
    Last edited by a moderator: May 5, 2017
  14. Oct 9, 2011 #13
    Many thanks for all your replies. To give you more information the 3 voltage multipliers used 10 stages composed by 4.7 nF / 2 kV and double 1N4007 ( 2 serialized diodes ). One of the multiplier has to produce 15 µA and the 2 others something in the order of 1 µA. Frequency is 32.5 kHz and amplitude at the output of CCFL is around 800 V.
     
  15. Oct 9, 2011 #14

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    Sounds like you may be suffering the same 'issue' I ended up addressing. I replaced 1N4's simply because they weren't fast enough. First-pass inspection looks like they work, but actually they are struggling.

    Is that 15uA into the storage caps of the rectifier, or 15uA from the whole circuit - remember losses in the caps will suck up load, which you may be adding to by adding further external capacitance, taking the diodes over what they can cope with, as they will be struggling with the switching rate.

    For such applications, I now tend to use 8kV diodes from a Chinese ebay seller [look up '8kv diodes'] that does 10x Trr=150ns 8kV diodes for $8 (or 80 for $25), shipped. Use those, or similar.

    You have to think about Trr when rectifying above a few kHz....
     
  16. Oct 9, 2011 #15
    I don't think Trr is a factor using in rectification purpose. Trr is in nS, which is much faster than the transformer and operating frequency of 32KHz. The Trr is only important in for arc protection. That is the reason we only use either Transorbs or Schotky diodes for clamping.
     
  17. Oct 9, 2011 #16
    For this low current, the more reason to put a HV resistor in series with each diodes to limit the current.

    I never heard of HV supply burning diodes because of the filter caps, When things burn in HV supply, it usually cause by arcing. I don't know the detail of your design and how it is implemented, so I can't exactly pin point the cause. Read the post #4 where I explain things that can really blow stuff. Without arcing, HV and normal supply work the same, even with multiplier, they all work.

    We designed 10mA 6KV supply from ground scratch and even wind our own transformer. We ran at much higher frequency ( hundreds of KHz) in order to lower the number of turns. We tested the ruggedness by putting as spark gap and let it spark and make sure it survive continuous arcing at the output. That is some power.

    Remember the trickiest part is the floating metal around the circuit, they arc every time. The way to find those are to arc the output in the dark, you will see those floaters arc also. I even gave them the name "sympathetic arc"!!! Get rid of all the arcing, the HV supply is every bit as rugged as any other switchers.

    Most people don't appreciate the amount of energy radiated out in such a short time of an arc, just look at all the computer freeze from the large EM radiated out.
     
  18. Oct 9, 2011 #17

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    Show me a datasheet with the Trr for 1N4X, then. They are so slow it's not even quoted! These things are intended for 50Hz mains rectification, not multi kHz. You need 'fast' diodes for this. Not 'ultrafast', but certainly quicker than mains rectifiers.
     
  19. Oct 9, 2011 #18
    PN junction by nature is at least in MHz region. No diode are too slow for 50KHz!!!! I think you better show me the physics to back up why you think it would be too slow. I am not an expert in transit time, but it is much shorter than 20uS which you are implying about less than 50KHz. Even if you consider it as square wave that has 1,3 5.....harmonics. The 3rd harmonics is only 5X32KHz=160KHz which is over 5uS period!!!!

    This is really the first time I even read question about diodes not fast enough in power supply application.

    FYI, the limitation of a switcher is the core. You are limited to around 100KHz to 200KHz. If I have my way, I would have them run in 2MHz to cut the turns of the coil. . We can get over 10 volts per turn with higher frequency. The kind of diode has never been an issue. You ever design a switching or HV supply as a real product? I don't count experiment of voltage multiplier in school or read a few books as real experience!!!!
     
    Last edited: Oct 9, 2011
  20. Oct 10, 2011 #19

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  21. Oct 10, 2011 #20

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    I've made a patent application for a prototype of a stackable supply module design for cheap manufacture that cost about $20 in parts and can drive up to 5mA at 20kV (variable, according to input power voltage), which I have been using for my HV work for several months now, showing no signs of any failure or fatigue, and can also withstand short-circuit, and also has a floating output so either side can be grounded, or lifted high to stack them.

    I've also built a more complex HF PSU that can drive 4 channels simultaneously in quadrature at up to 6MHz and 500V, with a nominal pulse rating of 50A between them. It's not easy to do that, components just don't work like the data sheets say they should over 2MHz in power applications. Looks easy on paper, but the learning exercise is steep. You should try it out rather than leaving it as a hopeful wish, because the challenge is wholly different to running a few 10's kHz. I run 200kHz switcher circuits with BJTs! But MHz requires MOSFETs, and they need to be coaxed into being driven that fast by tuning the gate. Switching at zero load is straightforward enough with signals, but try it with a power/reactive load.

    If we are comparing notes, what are your HV PSU achievements and what makes them unique?
     
    Last edited: Oct 10, 2011
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