Problem with Vds voltage at MOSFET

Discussion in 'The Projects Forum' started by uhdam, Aug 24, 2015.

  1. uhdam

    Thread Starter New Member

    Jan 24, 2013
    7
    0
    I am working class-E DC-DC converter topology. I have the problem with the drain-source voltage which is not immediately start to rise the Vds voltage after the gate voltage zero. So, what are the values need to tune in order to get the ZVS (zero voltage switching). But, I can get it at another side which is Vds falling to zero. See, the attached Vds file.
     
  2. ian field

    Distinguished Member

    Oct 27, 2012
    4,415
    783
    Can't remember if the circuit has a name - but there is a transistor speed up circuit for driving the gate capacitance faster. A PNP with its emitter to the gate and collector to GND ensures rapid gate discharge when the drive signal goes low, the B/E junction must be bypassed by a diode so that the driver chip can push the gate high. A Shottky-barrier diode minimises drive loss, and its low PIV is protected because the transistor B/E will limit the reverse voltage to 0.7V.
     
  3. #12

    Expert

    Nov 30, 2010
    16,283
    6,795
    Is this what you're thinking of?
     
  4. ian field

    Distinguished Member

    Oct 27, 2012
    4,415
    783
    What I described uses 1 transistor and 1 diode - but your way works too.
     
  5. #12

    Expert

    Nov 30, 2010
    16,283
    6,795
    It's not really, "my way". I have never built this circuit, I just read about it and this is what I remembered when I saw your post. Feel free to modify as required.
     
  6. ian field

    Distinguished Member

    Oct 27, 2012
    4,415
    783
    Some controller chips have Vcc around 30V, which is too high for the 18 - 20V Vgs limit for most power MOSFETs. When Vcc is not higher than that, the totem pole is better, but the transistor/diode speed up may be the only option if there is no suitable supply rail close at hand.
     
  7. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,535
    1,251
    And add a pulldown resistor from gate to source.

    ak
     
Loading...