Bootstraped totempole .. will this work?

Discussion in 'General Electronics Chat' started by Dyslexicbloke, Jun 6, 2011.

  1. Dyslexicbloke

    Thread Starter Active Member

    Sep 4, 2010
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    Hi folks ..
    I think I am probably going to need a high side driver for a couple of N-Channel FET’s
    I have searched here and the web and now think that the attached should work ….
    I am attempting to bootstrap a totempole driver made from a couple of common BJT’s
    The PWM chip has two outputs each with the ability to work at 100% duty but on alternate clock pulses.
    SUB1 will have an identical partner in parallel, apart from S1, which will be fed from the complimentary output of the PWM chip (SG3525)
    [​IMG]
    Firstly ….
    Will the attached work?
    Secondly ….
    Would it be better with a single base resistor and a complimentary switch in the lower leg of the floating driver, base to source?

    I am not expecting anyone to suggest values just comment on the strategy ...

    As always… any and all comments welcome.
    Thanks
    Al
     
    Last edited: Jun 6, 2011
  2. magnet18

    Senior Member

    Dec 22, 2010
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    Your image be brokened.
     
  3. tom66

    Senior Member

    May 9, 2009
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    It depends on the Vdrop of the 1 ohm resistor, but from a 12V supply and assuming negligible Rds(on), that would put Vdrop at 12V and Vgs at -2V, so it wouldn't work if the duty cycle was 100%. HOWEVER, if less than 100%, it could possibly work, but the maximum duty cycle will be determined by the gate current required and the two resistors. I've seen SMPS buck converters use this configuration before. But they had a maximum 95% duty cycle before the boost current wasn't enough to sustain the output.

    You can use a gate driver IC like TC4427A to accomplish this task. It will be much easier.
     
  4. Dyslexicbloke

    Thread Starter Active Member

    Sep 4, 2010
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    Sorry I dont follow that .... but I am not contradicting you, I just want to understand.
    If S1 is open and Vs is essentally 0 wouldnt the cap in Sub1 charge to 10v less the diode threshold?
    and if it dose then when S1 is colsed VGS would be at Cap voltage untill it discharged wouldnt it?

    What am I missing?
     
  5. tom66

    Senior Member

    May 9, 2009
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    That would be fine. But you say your chip goes to 100% duty cycle. If that is the case the boost voltage is never generated.
     
  6. Dyslexicbloke

    Thread Starter Active Member

    Sep 4, 2010
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    Ah 100% per output ... each output is activated on alternate clock pulses so the max duty cycle per driver is 50% ... I think
    you have me wondering now.....

    The chip specs require outA and outB to be commoned with diodes to achieve a single ended 100% duty

    Al
     
  7. tom66

    Senior Member

    May 9, 2009
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    Oh okay. But 50% would significantly reduce the amount of power you can deliver to your load.

    Bottom line is, the switch must never be on 100% of the time.
     
  8. Dyslexicbloke

    Thread Starter Active Member

    Sep 4, 2010
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    50% yes I know but two drivers essentally 180 Deg apart so 100% drive but with each stage only doing 50% of the work ...

    Ooooo saying that just realised ther is a significant issue with my strategy.
    VS will be dependant on BOTH FET's so everything you said RE 10%v duty is absolutly vcorrect, the cap acnt charge ....
    So now what?

    Al
     
  9. tom66

    Senior Member

    May 9, 2009
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    One other way is to use a boost converter to generate say 24V from 12V. It only needs to provide ~10mA continuously so you may even be able to get away with a switched capacitor converter.

    OR, if you can get a cheap one with low Rds(ON), use a P-ch fet. To turn it on you just connect the gate to ground (far easier!) Have a pull-up to the drain pin to keep it off during normal operation. Then use a small NPN to ground the pull-up when a logic signal turns on, turning the P-ch on. Make the pull up around 1k for slow switching (1kHz or less.) For faster switching you need a more advanced driving system.
     
  10. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    May be missing something in this discussion but.... The schematic in the first post shows a HIGH side configuration of the mosfet. Post #3 links to a LOW side mosfet driver. How are the two item compatible?

    A HIGH side Nmosfet gate must be at least 10V higher than the D/S voltage when turned on. That would mean 22-42V in this schematic.

    On almost every forum when the use of a high side driver is needed everyone steers clear of the dedicated driver chips. Why? They will try every thing under the sun but a high side drive IC. Can someone tell me the reason?
     
  11. tom66

    Senior Member

    May 9, 2009
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    My error. I wasn't looking at the right chip. Microchip also make high side drivers: here is one example.
     
  12. Dyslexicbloke

    Thread Starter Active Member

    Sep 4, 2010
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    shortbus ...
    In my case I have everything but a high side driver to hand and wanted to see if I could learn something in the process.
    That said, I need to get this working and will be looking for a driver chip or monolythic isolated supply just about now.

    EG. http://cpc.farnell.com/xp-power/il0512s/converter-dc-dc-2w-12v/dp/PW02212

    The output of this, or something simmilar, could be stacked on top of my upper rail providing suficiant supply to run the PWM chip and drive the FET's I expect.

    THe other option I have would be to simply buy a solid state relay ... But where is the fun in that?

    Al
     
  13. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    OK, I understand wanting to learn. That's what I'm doing too:)

    I'll ask another question then, how will using the component you linked to drive the high side mosfet? You still need to get the gate to source voltage plus 10V when the mosfet is turned on? Or am I missing something right in front of me?

    In this application note from IRF; http://www.irf.com/technical-info/appnotes/an-978.pdf on pages 17-18 it tells how to ad a 'charge pump' to a high side driver to keep the driver working without using PWM if needed.

    The on going project I'm working on/at has caused me to research this stuff and after a few years it's even starting to make sense. Didn't mean to hijack your thread, just thought we could help each other out.
     
  14. shortbus

    AAC Fanatic!

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  15. Dyslexicbloke

    Thread Starter Active Member

    Sep 4, 2010
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    Great info thanks ...

    Below is what I was thinking of doing with the DC/DC converter which will essentally level shift the whole control circuit .. I think
    I think it will work like this ...

    Its supply, 5V, is derived from the variable upper rail and referenced to ground.
    Its output, which is isolated 12V, is has its lower leg connected to S on the FET with its upper leg floating and supplying the driver ...

    No I have three rails - Gnd, 10-30V and 12V above S ...

    The PWM chip and totempole driver subcircuit reside between S and S+12V and therfore have 12V at all times which will allow me to supply VGS for 100% duty.

    It also occurs to me that I could use 2 FET's, paralelled apart from G, and drive them directly from the PWM chip.
    Its outputs, which are active on alternate clock pulses, are designed for the purpose.
    [​IMG]
    I'd apriciate your thoughts
    Al
     
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