MOSFET Protection

Discussion in 'General Electronics Chat' started by DJ57, Mar 31, 2014.

  1. DJ57

    Thread Starter New Member

    Oct 12, 2011
    9
    0
    Hi,
    I am working on Flyback application and using CoolMos 600V FET.While increasing the Input Voltage(Vin), the input current after exceeding the value of 300mA rushes to 1A that breaks the region and destroys the MOSFETs(Reults in short circuiting of drain to source). However drain to source voltage(on Oscilloscope /Vpp) in this case remains below the breakdown voltage(600V). In my application I need to switch about the breakdown voltage but the MOSFET is being destroyed several times before breakdown voltage. This is the main problem in my applicaton.
    Thanks in advance for the suggestions and guidance..
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,127
    3,048
    Take a look at how the manufacturer defines breakdown voltage and compare that to what you are doing. Probably not the same.

    If the MOSFET isn't meeting it's spec, ask the manufacturer.
     
  3. crutschow

    Expert

    Mar 14, 2008
    13,016
    3,235
    Sounds like you need to add a zener or transient voltage suppressor rated at about 500V across the MOSFET drain-source junction. Chances are you are getting a short transient across the transistor of >600V that is blowing the device. Note that 600V at 1A is a 600W pulse.

    What is the output load for the flyback transformer?
     
  4. DJ57

    Thread Starter New Member

    Oct 12, 2011
    9
    0
    The load values are variable i-e in range of 15 to 45 ohms.However I am currently working on 45ohm.The input current draw get higher while reducing the load values that is also a challenge.
     
  5. DJ57

    Thread Starter New Member

    Oct 12, 2011
    9
    0
    The increase in load values result in low current draw but Vout is higher than expected.
     
  6. ian field

    Distinguished Member

    Oct 27, 2012
    4,415
    784
    From "exceeding the value of 300mA rushes to 1A" - that sounds like core saturation causing punch-through that destroys the MOSFET.

    When the core in your flyback inductor is full and can't take any more lines of flux - Xl vanishes just like that and all you have left is the DC resistance of the copper winding wire.
     
  7. crutschow

    Expert

    Mar 14, 2008
    13,016
    3,235
    I believe Ian has the correct diagnosis. The inductor/transformer is saturating, causing the current to suddenly increase and zapping the transistor. You need to shorten the on-time of the pulse so that the inductor doesn't saturate, even at the highest input voltage. You can monitor the inductor current and note when it starts to saturate by the rapid increase in current. Always keep the peak current below that.
     
  8. ian field

    Distinguished Member

    Oct 27, 2012
    4,415
    784
    In most commercial PSUs, the MOSFET has a current sensing resistor in the source lead - these days the current sensing volt drop disappears into the control chip, but in ye olde days the volt drop was fed to the B/E junction of a transistor who's collector clamped the drive to the chopper device.

    This was pretty much a standard circuit on the self oscillating switchers based on the blocking oscillator.
     
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