3 DC supplies from 2 transformers ?

Discussion in 'General Electronics Chat' started by ARMDeveloper, Oct 3, 2007.

  1. ARMDeveloper

    Thread Starter New Member

    Aug 16, 2007
    8
    0
    I'm building a circuit that needs 3 voltages.

    15ishVDC to run a small motor
    90ishVDC to run a large motor
    and 105ishVDC to fire the high side MOSFETs that control the 90VDC motor.

    My question is this... can I generate these three voltages from 2 transformers ?

    I've got a small 10VDC transformer and a large 60VDC transformer.

    Can I connect them in series and tap 10VAC for the small motor and 60VAC for the large motor and 70VAC for the MOSFET supply ?

    What if I built a voltage doubler from caps and diodes that got me 180VDC and then I used a voltage divider to drive the high side MOSFET ?

    Thanks.
     
  2. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    If the small motor does not share common with the rest of it, you can make this work. You'll have one side of the small motor at +60 and the other side at +70.

    Have you looked at boost converters for driving high-side MOSFETs?
     
  3. ARMDeveloper

    Thread Starter New Member

    Aug 16, 2007
    8
    0
    The small motor does share a common side with the rest of the circuit. There is more I am not telling you about.

    The problem with the charge pump MOSFET drivers is that they don't work at 100% duty cycles.

    How else can I generate 105ish VDC from a 90ish VDC and 15ishVDC supply ?

    Is there some sort of charge pump IC that would let me generate 15VDC on top of a 90VDC reference ?

    Thanks
     
  4. ARMDeveloper

    Thread Starter New Member

    Aug 16, 2007
    8
    0
    The easiest way seems to be to add a tiny 10VAC transformer and a tiny bridge rectifier and cap, so that I can generate 70VAC.

    I need 100% duty cycle on the 90VDC motor, so I have to be able drive the high side MOSFETs continuously.

    Anyone see anything simpler ?
     
  5. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    Second xfmr and bridge will work for high-side. Add a regulator with chip's ground tied to your motor supply positive. I've done this to run H-bridges with four N-channel MOSFETs.
     
  6. John Luciani

    Active Member

    Apr 3, 2007
    477
    0
    If you do not need fast switching speed on the FET International Rectifier (IR)
    makes a photovoltaic gate driver --- PVI1050 and PVI5050. You can get 5-10V
    output. The output current is in the 5-10uA range. The input to output isolation
    is around 2500V. You can probably series and parallel the PVI's.

    Be careful of the low gate drive current. Depending on your circuit you may be
    dissipating a fair amount of power in the FET while you are charging the gate
    capacitance.

    There may be other high-side switching ICs out there. Search at IR, On-Semi, and
    TI.

    (* jcl *)
     
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