Charge pump

Discussion in 'General Electronics Chat' started by shortbus, Mar 11, 2013.

  1. shortbus

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    Sep 30, 2009
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    Why does a charge pump for a boot strap circuit on a high side mosfet circuit and a steady voltage doesn't? If they both are putting the same voltage into the cap, why does the pulsing voltage work and steady voltage not?
     
  2. thatoneguy

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    They switch the capacitor between in parallel with the source to in series with the source.

    Charge in parallel, discharge in series.

    Or am I missing your question?
     
  3. shortbus

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    You may be answering my question, but it's going over my head. :) I'll post an attachment from another thread.

    I can't see where it is going from parallel to series. Looks all series to me. The mosfet I'm trying to switch is strictly high side,with no low side to charge boot cap, like in a half-bridge.
     
  4. thatoneguy

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    See Pages 2-3 of IR App Note 1052

    --ETA: IR AN978 App Note has more details and math for better understanding.

    This type of bootstrap only works with a pulsed input signal, such as PWM. If the level will be steady, then a "bootstrap cap" + diode is "pulsed DC-Voltage Adder", as opposed to charge pump device (that we were both thinking of), needs to be used, since non-pulsed DC doesn't ever allow the cap to charge through the diode. Think of the PWM signal, in combination with the diode, doing the switching in place of traditional MOSFETs in Charge Pumps.

    ETA:

    From Wikipedia simpler description
     
    Last edited: Mar 11, 2013
  5. shortbus

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    Thanks,thatoneguy. I was calling it a charge pump because that's what most all of the literature calls them. I haven't seen the AN 1052, but will be reading it.

    Wish I had started trying to learn this "magic" of electronics earlier in my life. But I guess you got to let out some smoke to make a circuit work. Thanks again.
     
  6. thatoneguy

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    It seems all methods using capacitors for gate voltage are called Charge Pumps these days, which does a bit to muddle the issue.

    I'm not sure how the distinction should be made between "charge in parallel, switch to series" and "charge cap, add different voltage on negative plate" should be, other than "charge pump" and "bootstrap". Especially since a lot of designs mix and match the terms like they are the exact same method.
     
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