help with IGBT common ground circuit

Discussion in 'General Electronics Chat' started by oneprint, Sep 26, 2011.

  1. oneprint

    Thread Starter New Member

    Aug 24, 2011
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    hi everybody,

    i have a problem that i need some help with. i basically have a switching circuit which uses a 555 to switch an IGBT. this circuit is run using 18vdc. the IGBT switches a circuit that uses about 200vdc.

    the problem i have is that the IGBT has to be ground on both circuits in order for it to work. that is the IGBT emitter is ground on both high and low voltage circuits.

    I have not yet tested the high voltage side of the circuit. i did test with 23vdc as a substitute for the high voltage side of the circuit and everything works fine.

    Can some one tell me if its ok to ground both circuits together i would have thought the low voltage power supply would be exposed to the 200vdc and that sounds dangerous!

    can some tell me how to isolate the low voltage side of the circuit. I know about optocouplers but i dont realy want to use them. also both circuits are battery powered.

    I hope some one can help. thanks in advance.
    :)
     
  2. shortbus

    AAC Fanatic!

    Sep 30, 2009
    4,015
    1,531
    This type of circuit is what gate driver IC's are made for - http://www.irf.com/product-info/hvic/ Don't be alarmed if they say 'mosfet' gate drivers in some of them, both IGBT and mosfets use the same technology in their gates.
     
  3. oldtech33709

    New Member

    Sep 24, 2011
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    You can tie the returns together. The low and high voltage supplies are only connected at one point and the IGBT isolated between the gate and collector.
     
  4. oneprint

    Thread Starter New Member

    Aug 24, 2011
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    thanks for the reply. i can run the IGBT throught the 555 no problem. its just the common ground thats worrying me.
     
  5. oneprint

    Thread Starter New Member

    Aug 24, 2011
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    i assumed that when the high voltage is applied to the collector it passes through the emitter to ground? and if the low voltage timer circuit has the same ground then the low voltage timer circuit is not isolated from the high voltage circuit.

    basically there will be two wires coming from the emitter of the IGBT one to ground in the high voltage circuit and one to the low voltage circuit. So what im asking is that safe? and if not how should it be done correctly.
     
  6. shortbus

    AAC Fanatic!

    Sep 30, 2009
    4,015
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    Are you sure about that? Does the 555 allow enough amperage to switch your IGBT on fast enough?
     
  7. oldtech33709

    New Member

    Sep 24, 2011
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    The gate acts like a capacitor and that is the load your 555 circuit will see. To quickly turn on and off the IGBT you will need driver to source and sink this current.
     
  8. oneprint

    Thread Starter New Member

    Aug 24, 2011
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    im using 15 vdc to drive the 555. so there is enough voltage to power the IGBT. they dont realy need much current and in my test circuit were i used it to switch an L.E.D, the IGBT switched fine.

    basically i used a test 23vdc circuit to power the L.E.D and it switched at high enough frequency.

    I realy need help with the common ground issue and how to make it safe

    :(
     
  9. colinb

    Active Member

    Jun 15, 2011
    351
    35
    Does one have to worry about making switching time fast with IGBTs as with MOSFETs when handling large currents? Is the IGBT gate capacitance an issue? Can the 555 drive it hard enough (supply the large current spike needed to quickly charge/discharge the gate)?
     
  10. oneprint

    Thread Starter New Member

    Aug 24, 2011
    5
    0
    Hi, i have still not been able to find a solution to this. If any one can help me that would be great
     
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