Voltage inverter help

Discussion in 'The Projects Forum' started by cjeung, Jun 3, 2011.

  1. cjeung

    Thread Starter New Member

    Mar 31, 2011
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    I've been trying to hack out a solution for a circuit that requires +20v and -20v, 3-5A w/ balanced ground (obvious?) from a DC source. I've had no luck sussing out a solution so now I'm asking.

    I've tried working with a 555timer voltage inverter circuit where I've got the 555 output controlling an n-ch Mosfet. I was able to create an inverted voltage, w/ 1.5v drop, but when I put into rest of circuit (+20/-20, gnd) the inverted voltage drops. If a circuit diagram will help, I can get one.

    I could use any suggestions and help would be appreciated.
    Any takers?
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Well, a schematic is pretty much mandatory if you want any useful input on your existing circuit. Without a schematic, we can burn up a lot of time and space on the board making guesses about what you have created, and not really get anywhere.

    Just saying that you have a 555 timer connected to an N-channel MOSFET really doesn't do much to explain your circuit.

    But to give you something to think about, I've grown rather fond of Linear Technology's LT1170/71/72 family of switching regulators:
    http://www.linear.com/product/LT1170
    For 3A output, you'd need an LT1170.
    The datasheet:
    http://cds.linear.com/docs/Datasheet/117012fg.pdf

    Take a look on page 14, the top schematic - "Positive-to-Negative Buck-Boost Converter" - which is just about what you need.

    I simulated a version of that schematic using an LT1171 as a variable negative power source; it worked quite well in simulation and was really very efficient. I've attached an image of the schematic.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    Here, I took the above schematic, changed out the regulator to an LT1170, replaced the pot R1 with a fixed resistor, and ran the simulation for you; attached.

    It's showing about 88.5% efficiency; that'll drop down a bit in the real world because not all of the components have all of the parasitics modeled - but even 85% efficiency wouldn't be bad for this simple of a circuit.
     
  4. cjeung

    Thread Starter New Member

    Mar 31, 2011
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    SgtWookie,
    Thanks very much for your assistance. It looks like it will certainly help on my project, and is definitely worth my effort in testing. I will try to post my schematic soon so that I can gain some insight into my theoretical ideas and process. I'm fairly new at working with complex circuits and am equally new at posing questions. This is my first post/thread on allaboutcircuits, and again, appreciate your help.
     
  5. cjeung

    Thread Starter New Member

    Mar 31, 2011
    8
    0
    Here is my schematic of the voltage inverter that I attempted to use to create +20v, -20v ,3-5A power supply. Aside from the voltage drop to the negative side, and uncertainty of the current output, is there any information I could learn from that doesn't allow it to work for my purposes?

    Thanks
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Sorry it took me awhile to get back to you; I'm not on the forums much lately.

    Your circuit is using capacitors and diodes to invert the supply voltage. That works OK for up to around 100mA load current or so, but not for higher currents; it's just not practical. You really need a switched inductor solution, like the one I provided previously.
     
    cjeung likes this.
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