mosfet h-bridge circuit

Discussion in 'General Electronics Chat' started by adrenalina, May 10, 2011.

  1. adrenalina

    Thread Starter Active Member

    Jan 4, 2011
    Hello everyone. I "designed" a circuit for a n-channel mosfet circuit and just wanted your opinion on what you think of it. I am not using any chips for the mosfet drivers its all discrete components. I simulated the circuit in multisim and measured the rise time on the load and it's about 4us and the fall time about 5us. is this too slow? If I don't plan on using PWM are the times good or should they be better? how can I improve the times? I put a darlington pair in the circuit and the time improved. it was about 11us rise time and it dropped to the 4us.
    Feel free to criticize all you want. I attached the circuit.

    NOTE: The function generator at the top would actually be a 555. the two sources and switches would be the inputs.
  2. ifixit

    AAC Fanatic!

    Nov 20, 2008
    Hi adrenalina,

    For manual switching, the rise and fall times are more than enough. Having the BC108 in there will limit the maximum gate voltage, which limits Rds. Keep the gate voltage =>5V to ensure minimal Vds at 7 amps Id.

  3. shortbus

    AAC Fanatic!

    Sep 30, 2009
    The mosfets need at least 10V on the gates to completely turn on. The 'high side mosfets need to rise to 10V above the source voltage when on.

    Why not just use a dedicated gate driver chip to do what you want? Probably less cost and definitely smaller PCB for the project.
  4. adrenalina

    Thread Starter Active Member

    Jan 4, 2011
    In the simulation the charge pump is giving about 18v and I also measured the Vgs on the high side which is about 11v with the load. so I think there is no problem with the Vgs voltage on the high side.

    The problem with using a mosfet driver IC is that I can't find any supplier in my country, so I would have to make an international order from the US. If I do order, which IC would you recommend?
  5. Barnaby Walters


    Mar 2, 2011
    I made a similar circuit lately with P and N channel mosfets using a Maxim driver IC, the 4427. I'm in the UK, and Maxim sent me 4 free samples in a nice little box ;) Worked perfectly, the result can be seen in this thread: Robotics Platform

    The FETs I used were fairly standard, cheap power MOSFETs off ebay. I can't remember the numbers, I can find them if you need them.