Unable to turn off High side mosfet in Full bridge configuration

Discussion in 'General Electronics Chat' started by ken_tcs, Jun 17, 2008.

  1. ken_tcs

    Thread Starter New Member

    Jun 10, 2008
    I have design and simulated a full bridge DC to DC converter. During the implementation, I have problem turning off my high side mosfet.I am driving my mosfet using mosfet driver IR2101. For the bootstrap capacitor, I used 4.7uF and the diode is an ultra fast recovery diode BYV27-100. Intially, I suspect the problem arise from the gate driver, I did some off line testing with the gate driver. I use the High side output to drive a single MOSFET with a resistor connected at the source and feedback the source voltage to Vs pin of the IR2101 driver. The MOSFET is able to turn on and off perfectly.I move on the test with two mosfet and the high side mosfet is unable to turn off again. May I know is there something that I have left out that has caused this problem.Any help is greatly appreciated.All testing schematic are attached below in the pdf file schematic.
  2. ams0178

    New Member

    Aug 5, 2008

    It appears to me like you might have a problem with your driving scheme. If the high output driver works in a test situation and then when you put the device in its half bridge configuration it doesn't work. Since there is a pin on the half bridge driver for the high input and the low input check your driving signals into the chip. Do you have an oscilloscope? Make sure there is ample delay between turning a high and low side on/off to account for MOSFET turn on/off times. Also test one half of the bridge at a time to make it easier. You may want to use the IR2104 because it only takes one input. I just ordered some. The high side is in phase with the input while the low side is out of phase with the input. A pin on the chip allows you to put a resistor on to ground to create more dead time between the switching. This allows easier software because you don't need to worry about turning both the high and low on at the same time.

    Good Luck, Alan

    (Out of curiosity, are you using a micro to drive the chips? Which one?)
  3. nanovate

    Distinguished Member

    May 7, 2007
    Do you have a schematic of your actual circuit including the gate driver?
  4. Nando37

    New Member

    Aug 15, 2008

    Examining the schematic, I see that you are having a brigde without diode protection of the intrinsic Fet diodes that are turned ON when the FET is turned OFF, then the opposite side FET is turned ON causing a heay cross current that over heat and burn the FETS.

    Also the drivers have slow turning ON/OFF times, if this time is not carefully taken care when turning OFF/ON the FETS and in addition the driver current of these units may be too low to fully discharge the Gate capacitance FAST before the other side is turned ON.

    Cross-current will occur if the over all system does to NOT have precise switching times consideration -- common problem in power supply design.

    RC network is not proper for these types of bridges..

    Let us know if this is your solution to the problem.