MOSFET as Switch

Discussion in 'General Electronics Chat' started by Chacabucogod, Oct 29, 2014.

  1. Chacabucogod

    Thread Starter New Member

    Nov 16, 2013
    I've been trying to build an H-Bridge. At first I tried to use BJTs, but couldn't because the voltage wouldn't saturate the pnps on the top. Now I'm trying to use MOSFETS, but it seems I'll gte the same problem since the voltage of the base must be similar to the voltage in the drain for it to work. Is there a way to use MOSFETS with a low voltage gate signal to let large voltages around the darin and source. Thankyou
  2. pwdixon


    Oct 11, 2012
    Look at N and P type devices
  3. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
    hi Chac,
    What is the supply voltage to the H Bridge, motor current and how are you driving it.?
  4. ScottWang


    Aug 23, 2012
    Where is the schematic?
  5. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    Sounds like you're using NFETs at the top of the bridge? If so, their gates will need to be driven several volts above the drain voltage if you want to have them turn on fully. A FET driver with a bootstrap function may be what you need.
  6. MaxHeadRoom


    Jul 18, 2013
  7. cmartinez

    AAC Fanatic!

    Jan 17, 2007
    I've been working around the same problem for a while... and I can tell you that, at least for me, it's been a pain in the you-know-what... the net is full of resources regarding mosfet drivers, and I've tried many of the best circuits out there, but not all work well in different circumstances...
    And yes, Max is right, Tahmid's site is one of the best to be found regarding this subject, I have the greatest respect for this guy.

    So far the only circuit that I've found fully convincing is this one, posted by JDT, it's more expensive than most, but it will do the job.

    As far as I understand the issue, the key lies in using an isolated (or floating) power supply that will reliably generate a voltage between the gate and the source, regardless of voltage between the drain and the source... For you mosfet gurus out there, please kindly correct me if I'm wrong...

    Update!: check out page 11 of this document, it provides some excellent alternatives regarding mosfet drivers.
    Last edited: Oct 29, 2014
  8. Chacabucogod

    Thread Starter New Member

    Nov 16, 2013
    Thank you all. I'll take into consideration what you said Alec_t. Since my source voltage is 24 volts, I'll use a pic(to generate the square wave) and and opamp to raise it all the way to 30 volts. It should work right?

    I'll also take a look at the link. I'll inform you all about that happens
  9. ian field

    AAC Fanatic!

    Oct 27, 2012
    The problem is even worse with MOSFETs as you have to give them at least 8V above Vcc instead of enough voltage to drive current into a 0.7V junction.

    There are high side drivers for MOSFETs in bridge circuits - but you can roll your own; you need a "flying capacitor" charge pump from the mid point (output), feeding a rectifier that charges a reservoir capacitor stacked on top of Vcc. I think its called "bootstrapping" if you want to google for more info.
  10. BobTPH

    Active Member

    Jun 5, 2013
    You do not need an opamp, and it is a poor choice for this application. Get a MOSFET gate driver chip.

  11. Centera

    New Member

    Jul 18, 2011
    Perhaps you can try a different approach. An Audio amplifier is basically a modulated power supply, why not look at a proper (and simpel!) amplifier circuit? I don't see any schematics posted in this topic yet so I am not completely sure if this will work for you or is any way near what you want.

    First a circuit. The circuit you see here is the F5 power amplifier designed by Nelson Pass (

    This circuit is a simplified version. For a complete circuit of this amplifier look for the schematics of the F5 Turbo V3 amplifier, it includes current limiting, overheating protection and other parts needed to get the circuit working.

    The F5 is a very simple DC coupled two stage amplifier design that utilises N&P jfet's in the first gain stage and N&P mosfet's in the output stage. It requires not much drive voltage and the bandwidth extents from DC up to 1 Mhz. Power output can be increased by beefing up the power supply and paralleling output devices.

    Because the circuit does not require that many parts it's obviously very cheap to build. I am about to build one myself in a few weeks, total costs for the parts will be around 75 euro's (excluding the enclosure) for a stereo 25WPC amplifier :)

    Why not use two of these amplifiers in class B (Nelson drives them in class A with up to 3A of idle current) and drive them 180 degrees out of phase of each other?

    If you want I can fire up ye olde spice and work out a circuit for you. What kind of output voltage, current and frequency are you looking for?