Discussion in 'The Projects Forum' started by allrudd, Jun 20, 2009.

  1. allrudd

    Thread Starter New Member

    Jun 19, 2009
    I got my H-Bridge running using the IR2110. With the High Rail at +12VDC I'M getting about 15VDC amplitude pulse going to the High Side Gate through a 1k resistor. I'm driving a small, brushed model airplane motor with a prop for testing, so I am getting loading as I crank up the RPM.
    I am seeing the High Side Mosfets(IRF540) getting hot while the corresponding Low Side FETs stays cool. I would expect both FETs to run about the same Temp if, in fact the High Side FET is fully turned on, is that true?
    I tried doubling the Bootstrap Cap(.33uf) to .66uf and cutting it in half to .16uf with no effect on the pulse amplitude going to the gate.
    I read somewhere that the Gate Voltage must be 8vdc above the High Side Rail, is that true. I want to eventually drive a 24vdc motor, how in the world can I get the Gate pulse up to 32vdc.
    Also, I'm using a universal 1N4004 for the Bootstrap Diode, could that be keeping my voltage from pulsing up to 20vdc which is what I was expecting.
    Thanks for your help,
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    1N4004's are not really suited to working much above 100 Hz. A Schottkey rectifier might do much better.

    Your high side FET is not turning on with only 15 volts on the gate. The gate needs to run about that high above the source terminal (10 volts is the minimum turn on voltage).
  3. SgtWookie


    Jul 17, 2007
    Do you mean that you have a 1k resistor between the gate and the IR2110? If so, you would be causing your MOSFETs' gate to charge and discharge slowly, which would cause operation in the linear region and resulting power dissipation in the MOSFET.

    How did you measure this 15v pulse? The gate voltage should be measured relative to the source terminal. If your circuit uses a ground that's common with your O-scopes' ground, that could present a problem; as the O-scopes' ground probe is generally connected to earth ground. You'll need to isolate one or the other from earth ground.

    The high side and low side MOSFETs should be running about the same temp.

    For standard power MOSFETs, Vgs should be 10v in order to ensure that the MOSFET is fully turned on.

    The 1N4004 has a rather high Vf (around 1.1v) when carrying maximum current, and it is very slow to turn off, which are the reasons why Beenthere suggested that it won't work very well for your application.

    Consider using something like a 1N5822 3A Schottky diode instead. Digikey carries these for $0.45 each, less if you buy 10 - and they'll ship SMALL orders via USPS 1st Class.
  4. Tahmid

    Active Member

    Jul 2, 2008
    A UF4007/BYV26 will do as well. I think schottkys aren't completely necessary, ultrafast diodes are also ok.