Series switching supplies

Discussion in 'General Electronics Chat' started by BReeves, Dec 2, 2014.

  1. BReeves

    Thread Starter Member

    Nov 24, 2012
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    Can I get away with connecting two switching supplies in series to get a higher voltage. I have an 80 volt 6.5 amp supply and a 36 volt 4.2 amp supply, both are isolated. Wondering if I connect them in series for 116 volts if anything will smoke. I will not load them beyond the lower rated supply.

    Thanks

    <Edit> Looks like I should have checked the search function. Found this with some good info..
    http://forum.allaboutcircuits.com/threads/switching-power-supplies-linking-in-series.64738/
     
    Last edited: Dec 2, 2014
  2. Papabravo

    Expert

    Feb 24, 2006
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    Can you show us a diagram of what you think connecting power supplies "in series" actually looks like? I'm not being flip here, I'm really curious and we can give you a better answer if we know precisely what you are talking about.
     
  3. #12

    Expert

    Nov 30, 2010
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    Well, you said they are isolated and you're not so much of an amateur that you would forget to check that the earth grounded cases are separate from the power terminals on both ends, so I say, "yes". It will work.
     
  4. BReeves

    Thread Starter Member

    Nov 24, 2012
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    Ya spent 50 years working as a technician, mostly on two way radios and microwave hops. My problem is I'm approaching 70 and sometimes I can't remember what I use to know.

    This is a continuation of the LED strobe light experiment I started in the projects section. I have it working with an 80 volt supply and want to see how hard I can hit the LED before it smokes. I can actually go to 160 volts, have the 80 volt supply the 36 volt supply plus a DC to DC supply that will take the 36 volts to 80. Will have to wait till my new Mosfet and cap order comes in. The ones I have now are only rated at 100 volts and the Mosfet is only 40 amps. According to the simulator I should be approaching 60 amps with 116 volts.

    Just something I'm playing with.
     
  5. Sensacell

    Well-Known Member

    Jun 19, 2012
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    It should work fine:
    1) double check with an ohm meter (power off) that the outputs are totally floating.
    2) observe the current limit of the smallest of the two power units, as they share the same current.

    I use two 600 watt 12 Volt supplies to create a single 1200 watt 24 V supply.
    The only trouble with this approach is the awkward voltage steps that happen on start-up, as long as the load is OK with this, then you should have no problem.
     
  6. Denesius

    Member

    Feb 5, 2014
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    It's funny, but in my experience, the cheap switching power supplies and most linear ones have no issues with series hookup (as long as the outputs are truly isolated), but the fancy, extensively protected ones refuse to cooperate. I think the issue has to do with the circuit that turns on the supply: if one supply comes up first, then thru the load the voltage on the output of the slower supply appears reversed. This may trigger the safety circuit of the second (slower) supply, and prevent power up. The only way to handle it is to ensure there's no load until the expected series voltage is present.... oh, wait- no load can prevent some fancy protection circuits from releasing the power supply! Good luck- I don't see you frying anything, but if you only see the voltage of one of the supplies on the output, that's probably what's going on.
     
  7. #12

    Expert

    Nov 30, 2010
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    I have created that problem with a circuit as simple as a board full of 78xx and 79xx regulator chips.
    The cure was to add output capacitance to slow down the start sequence, but that's easier on some 100 ma chips than with 4 to 6 amps.
     
  8. BReeves

    Thread Starter Member

    Nov 24, 2012
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    As long as nothing smokes, that was the main item I was worried about. If it doesn't work because the two supplies are talking to each other I would think a diode or two would solve that issue.
     
  9. #12

    Expert

    Nov 30, 2010
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    The recommended method is a Shottky diode, but I didn't have any that day.:rolleyes:
     
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