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How to step down high-current negative DC voltage

Discussion in 'General Electronics Discussion' started by hopperms, Feb 21, 2015.

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  1. hopperms

    hopperms

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    Jan 22, 2013
    I'm trying to step-down a -7V source into -1V supply that can handle 10A. I tried using a few linear regulators in parallel (to handle the current), but as expected, they overheated fairly quickly even with a heat sink on them. I've looked into getting a DC-DC converter instead, but can't seem to find any that can handle anywhere near the current that I need.

    Does anyone have any ideas as to how I can achieve this without excess heat?
     
  2. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    You'll definitely need to use a switching converter of some kind. Low-voltage output with high efficiency normally means a synchronous rectifier, but I don't know of any synchronous controllers that are able to work as negative regulators.

    Is it really essential that the positive side of your output and the positive side of your input supply are at the same potential?

    Can you describe your requirements and your project in more general terms? There may be an alternative approach that will meet your needs.
     
  3. hopperms

    hopperms

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    Jan 22, 2013
    Thanks for your response!

    I'm trying to generate a 1kHz alternating current to power some electromagnets that will, in total, have amplitudes of +/-10A. I am using a 5V center-tapped transformer to get approximately +/-7VDC after filtering. I've found a DC-DC converter for the positive supply without an issue (haven't ordered it yet), but can't find one for the negative.
     
  4. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    How are you planning on switching the current?

    How about generating a single 1V supply and reversing the polarity to the electromagnets using an H-bridge?
     
  5. hopperms

    hopperms

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    Jan 22, 2013
    I've actually already built that part of the circuit using an MCU with a DAC to switch the current using mosfets. After I finished building it, I realized that an H-bridge would have been much more efficient (I'm still learning). This is just a rough prototype that doesn't have to be perfect, just functional.

    If you don't think there are any other ways to regulate the voltage, I can just work with what I have and get a better heat sink.
     
  6. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    As I said, I would use a switching supply with synchronous rectification. Have a look on Digi-Key - they have a huge selection, all parameterised and ready to choose from, with links to data sheets.

    You haven't gone into much detail, but one method that comes to mind would be to combine the linear MOSFETs with the H-bridge. On one side of the H-bridge, turn ON the bottom MOSFET, so that side of the electromagnets is at 0V, and on the other side, bias the top MOSFET to produce the desired voltage or current. When the polarity needs to reverse, do it the other way round.

    You shouldn't need a great deal of heatsinking. Worst case dissipation would be in the middle of the linear region, where you would have 0.5V across the active MOSFET and 5A flowing, I guess, which is only 2.5W dissipation.

    I'd be happy to try to help further if you post some schematics and more details of the project.
     
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  7. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Have a look at the LT8709. I am not sure it's lowest output voltage though, might not be 1V
    Adam
     
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  8. hopperms

    hopperms

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    Jan 22, 2013
    @KrisBlueNZ - I appreciate the help, but as I was saying, I can't really redesign the entire circuit at this point. I will be rebuilding the circuit in the future and it will have to be as efficient as possible, so I will definitely be using an H-bridge for that.

    @Arouse1973 - Thanks for the suggestion! I think this might work!
     
  9. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    No probs
    Good luck
    Adam
     
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