Problem with Mosfet Drive

Discussion in 'The Projects Forum' started by mvandrunen, May 30, 2008.

  1. mvandrunen

    Thread Starter New Member

    May 21, 2008
    I'm looking for help on this mosfet drive circuit. The IC is a dual low side driver from IR. I have the Vs pulled up to 12 v. I am switching two IR Mosfets at 40khz. Each drain is connected to one side of a bifilar wound transformer which has the common winding at 115 vdc. This is a 300 watt load as the current measured at the drain is 2.6 amps. The problem is I can get only one mosfet to switch with the load attached. If I remove the load, both FETS switch. The buck boost transformer is connected to a transducer load.

    Any ideas why the FETS would not switch with the load attached? I was wondering if I need to increase the Vs supply?

    Thanks for your help.:confused:
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    Looking at the drawing, I see you do not have the current limiting resistors between the outputs and the gates of the FET's. Adding resistors of 100 - 470 ohms is a good idea. You can refer to the data sheet on your FET's to see if there is a recommendation.

    Placing a snubber diode in anti-parallel with the FET's is pretty necessary. There is an internal diode in the FET structure, but it cannot handle the voltages and currents induced by a collapsing field in a transformer. An IR HexFred might be best - something like a HFA08TB60PBF (600 v, 4 amp, $2.63 from Digi-Key).

    You might wish to check your FET's before going further. You should see the diode between the drain and the source, but the terminals should show no conductance otherwise.
  3. nanovate

    Distinguished Member

    May 7, 2007
    You might want to introduce a dead band also to prevent shoot-through since FETs can have different turn off and turn on times.
  4. mvandrunen

    Thread Starter New Member

    May 21, 2008
    Question for Beenthere,

    With the circuit as shown, as soon as power is applied, I blow a fuse and also a mosfet. Before applying power, I read 0/4 ohms from drain to source/ DO you think this snubber diode is the cure? Also, what do you mean in anti-parallel? I assume this snubber diode goes across each side of the transformer coil. I would assume I would need two of these snubber diodes.

    Thanks for your help
  5. MorskiBG

    New Member

    Jan 20, 2009
    I have the similar problem. I'm trying to control DC motor with PIC through IR4427 and IRL1404Z. With R=200oms between IR4427 and gate, motor starts and immediately stops, Vs=9V. Measured voltage of the PIC's output pin, which drives IR4427 dropping fats to 0V. Without load (motor) everything is just fine. I started to increase R and motor started to spin longer. Finaly with R = 70 kOms (70k) I obtained steady motor's work. This is huuuuge value of seris R (between driver IC and MOSFET gate) is very strange for me .I achieved similar results, after MOSFET's replacement with "non logical" IRF3205. Any explanations ?
    Last edited: Apr 30, 2009
  6. tkng211

    Well-Known Member

    Jan 4, 2008
    It may be caused by the high frequency response of the MOSFET. Suggest using a resistive load (2 light bulbs) to replace the inductive load and test if the circuit works fine. Then put the inductive load back and see what the result will be. If the fault exists with inductive load only, a low pass filter should be added in serious with the Driver IC power supply to avoid unwanted high frequency feedback through the supply rail.
    Better put a 100nF ceramic capacitor in parallel with the elctrolytic 1uF capactor.
  7. Tahmid

    Active Member

    Jul 2, 2008
    I can see a few rooms for improvement in your circuit design.

    1. Add a resistor between output pins of the driver and the gate. I would say 100ohm is too high. I normally use 10 ohm - 22 ohm. Also use an anti-parallel signal diode across the gate resistor.

    2. Add an anti-parallel diode across each MOSFET. The diode must be ultrafast type.

    3. Add snubbers (RC) from the MOSFET drain to the centre tapping of the transformer.

    4. Use MOSFETs with higher VDS Rating as the minimum VDS rating here for each MOSFET should be (2 X 115V = 230V). So, I would say, use a MOSFET with a VDS rating with more than 300V.

    5. Add an inductor between the 115VDC and the centre tapping of the transfomer to prevent high current surges.

    6. I see that the output is not DC but High Frequency AC. To convert that to DC, use 4 Ultrafast Rectifiers (MUR480) to rectify high frequency AC to DC.

    I hope I could be of help.
  8. MorskiBG

    New Member

    Jan 20, 2009
    Thanks for the answers. Update:This is a battery driven application (8 NiMH cells) I put 10 ohms gate resistors - just the same - random duration "on" time of motor (DC brushed). So, I replaced motor with "weaker" one (lowest magnetic flux ) and everything started to work. I'm already put a 100n and 4,7u tantalum cap in parallel between Vss and Vdd of driver IC. So, how to urge on the "strongest" motor?