High Voltage Low Current - Reversing Polarity

Discussion in 'The Projects Forum' started by airplane100000, Aug 28, 2016.

  1. airplane100000

    Thread Starter Member

    Aug 2, 2016
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    I am attempting to design a circuit which reverses the direction of a high voltage 5ma dc through a 100k load.
    The circuit currently achieves a high voltage by means of a flyback converter powered by a 12V(cells in a series). The output is fed through a full bridge rectifier to produce aprox. 500V DC, through a 100k load giving 5ma.

    My goal is to cause the current on the load to be reversed at a given interval, let's say 100hz. In theory all I would need to do is switch the output leads from the rectifier to the opposite side of the load. This is proving problematic for me, being that this involves high voltages and I am having trouble determining which components I would need to accomplish this task. I would imagine it would involve transistors acting as an h-bridge or something of the sort, fed by a high voltage low current driver chip (does that sound right?).

    It would be of great help if anybody could give some relevant advice.
    -Thanks
     
  2. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Yes, you have the idea.
    Does it run at the 100 Hz rate all the time or are there times it needs to stay on for long periods? I ask because it will be pretty easy with some high voltage FETs and a bootstrap driver. Or maybe just the driver.
     
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  3. airplane100000

    Thread Starter Member

    Aug 2, 2016
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    While the device is on (by the momentary pushing of a button), I would want the polarity on the load to be reversed every 5ms (given 100hz, 50% duty cycle signal). The signal does not necessarily have to be 100hz, it can range between 100hz-2khz, depending on what is most appropriate. The frequency will not change, that is if I choose 100 hz the frequency will always be 100hz.
    The entire device is not to be activated for more than several seconds, and would be deactivated by release of the push button.
    I hope that answers your question.
     
  4. ronv

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    Nov 12, 2008
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  5. airplane100000

    Thread Starter Member

    Aug 2, 2016
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    Thank you for the offer, at the moment it appears that I only need assistance on the secondary side of the transformer.
    My response is delayed because I have for two days been attempting to figure out how exactly to work that chip.
    If I understand correctly, I would need either two of these "Half-Bridge Dirvers" or one "Full-Bridge Driver" chip for this circuit.
    It is not how this cheap feeds a high enough voltage to the mosfets. Am I supposed to connect a high voltage to it in some way? I attempted to replicate the set-up in the datasheet in a simulator and its not working so far - not sure what the cap/resistor values should be.
    Also, what timer chip is ideally used to provide the square wave signal?
    To your question, I don't think I can specify what this circuit is driving because it may be proprietary, and it might not be mine to disclose.
    But I think I know why you're asking, its not a puppy if that's what you meant...
     
  6. ronv

    AAC Fanatic!

    Nov 12, 2008
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    oow, I love secrets.
    No, I was asking because if the load is inductive you might need some clamp diodes in the attached circuit. So I just left the wires.
    A 555 Ic would probably work for the signal.
    http://www.interfacebus.com/555-IC-Square-Wave-Oscillator-circuit.html
    The only high voltage needed is your 500 volts. The IC adds 12 volts to that if you run it from 12 volts.
     
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  7. airplane100000

    Thread Starter Member

    Aug 2, 2016
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    Well that was extremely helpful, I got it to work.
    I'm wondering, would this Full-Bridge chip be a better alternative? ( IRS2453(1)D(S) )
    It seems to be the same, and has a self contained oscillator.
    I don't know if the Vcc should concern me, as it says min 10V; my battery is 12V with pretty high IR. If it's around 8-9 would it not work?

    If the above chip isn't good, could I use similar chips from inifneon's line but that supplies a lower current (they're cheaper), would it make a difference in this case?

    Also, could the mosfets be switched to cheaper lower current ones? And can the low side be of a lower voltage rating than the ones on the high side, or not?
    -Again thanks for the help, it looks like I'm finally nearing the end of this project
    -
    Edit: While I'm at it, wondering if there is a benefit to using a full bridge rectifier at the secondary of the transformer instead of just one diode?
     
    Last edited: Sep 1, 2016
  8. dannyf

    Well-Known Member

    Sep 13, 2015
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    a H-bridge will do. 500v is nothing.
     
  9. dannyf

    Well-Known Member

    Sep 13, 2015
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    BTW, as the current is low, you likely can live with using just two half bridge drivers.
     
  10. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Perfect!
    I don't know what you are running off your 12 volts, but usually a battery (lead acid) is dead at about 11.5 volts and unless you look for a logic level FET you should have around 8 volts to make sure the FET is turned on good.
    5 ma is pretty low current so the filter cap on the power supply can still be pretty small with just the 1/2 wave rectifier. Do you have 50 or 60 Hz. power?
    Lower current from the driver is probably not a problem.
    I found this FET.
    http://www.infineon.com/dgdl/Infine...N.pdf?fileId=5546d46249be182c0149c7acf11e1e92
    The low side sees the full 500 volts so it needs to be the same.
     
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  11. airplane100000

    Thread Starter Member

    Aug 2, 2016
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    Well the frequency on the flyback converter at the moment is 500khz, because I don't think the transformer I'm using functions at lower frequencies. The whole purpose of this is to lower the frequency of the current on the 100k load. If there are better ways to go about this I'll be glad to hear. I might take you up on your earlier offer if it's okay with you...
    I posted this thread with the information pertaining to the the flyback converter on the power section of the forum (it's not seeming to get any replies): http://forum.allaboutcircuits.com/threads/tiny-1-50-transformer-help.127031/
    If you would like to take a peak it would really help.
    In other news, I'm going to order the full-bridge driver now and see how it works.
     
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