MOSFET H Bridge for a beginner. I'm missing something but I don't know what.

Discussion in 'The Projects Forum' started by pencil364, Feb 15, 2015.

  1. pencil364

    Thread Starter New Member

    Feb 15, 2015
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    0
    Hello for the first time!

    I'm a physics major who is getting into electronics seriously for the first time ever. I have a model boat that I'm designing a speed controller for. A very smart coworker of mine suggested an H bridge and gave me some MOSFETs. The intent at the end is to use the bridge to control 9.6V out of an RC battery with PWM from my Arduino. For simplicity's sake right now I'm using a 12V motorcycle battery and a 5v control source with one switch for each direction. The motor pulls 10A at high load and 12V (tested). Here's what I've built:

    [​IMG]

    When I use it I get some very funky output and some very hot MOSFETs. It hardly puts out half a volt to my motor... Thankfully the polarity reversal does work though.

    The MOSFETs are N-channel NXP PSMN8R0-40PS, datasheet here: http://www.nxp.com/documents/data_sheet/PSMN8R0-40PS.pdf

    I feel like I'm missing something fundamental about my MOSFETs or just MOSFET design in general.

    Please help and thank you!
     
  2. crutschow

    Expert

    Mar 14, 2008
    12,993
    3,227
    Yes, you are missing several fundamental things.

    If you look at the data sheet you will see that MOSFETs require +10v gate-source voltage to fully turn on (look at the RDSon value).
    Thus the bottom transistors are not fully turning on with only a 5V signal.

    And the upper transistors are operating in the source-follower mode so the source voltage (to the motor) can be no higher than the gate voltage and will actually be several volts below that due to the transistor's threshold voltage.
    This means there is about a 10V drop across the upper transistors when they are on. No wonder they get hot.

    The bridge needs to have P-MOSFETS for the upper transistors so they can be fully on (unless you go to a bootstrap driver for them to generate the necessary gate voltage).

    How fast is the PWM signal?
    You may also need a driver stage to rapidly switch the large gate capacitance of the MOSFETs.
     
  3. pencil364

    Thread Starter New Member

    Feb 15, 2015
    3
    0
    Thank you so much for your reply, it's exactly what I was hoping for.

    Based on my understanding, to solve I need to drive my PWM to at least 10 volts. Would just using a 12 volt bootstrap driver and keeping my N-MOSFETs work? I have 2n2222s at work that I can use.

    Also I was wondering if using the battery as a source for both the motor and the driver was a problem? This is all going in a boat so less batteries = less problems :).

    The Arduino's PWM operates at 500Hz.

    I'm not sure I understand the gate capacitance problem. Is the issue that they have a charge time and may not respond quickly enough with only 5V? Would driving with a higher voltage cause a faster response in the FETs?
     
  4. crutschow

    Expert

    Mar 14, 2008
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    3,227
    Can you purchase other devices?
    They make bootstrap driver ICs that would significantly simplify the circuit, such as one of these.
     
  5. pencil364

    Thread Starter New Member

    Feb 15, 2015
    3
    0
    I could but frankly I really enjoy learning the circuits and understanding everything. My job involves a lot of electronics that I don't necessarily have to understand but it certainly helps if I do. On top of that I have certain resources already available to me through work and I would rather use those.
     
  6. crutschow

    Expert

    Mar 14, 2008
    12,993
    3,227
    Okay, here's a discrete half-bridge circuit with a bootstrap driver.
    Below is the LTspice simulation of it
    You need to duplicate the circuit for the other half of the bridge.
    The top MOSFET uses a bootstrap capacitor which generates a gate voltage that stays at least 10V above the source to fully turn on the MOSFET.

    Edit: Just noticed that the 2N2369 transistors are only good to 15V so they need to be higher voltage devices such as the 2N3904 (at least the bottom one does).

    Bootstrap.gif
     
    Last edited: Feb 16, 2015
    cmartinez likes this.
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