N channel Mosfet question DGR

Discussion in 'General Electronics Chat' started by Matter45, May 8, 2013.

  1. Matter45

    Thread Starter Active Member

    Dec 13, 2012
    Hi All!

    Currently experimenting with high voltage mosfets in AC circuitry.

    The problem: my first mosfet blew up. WHY?

    Attached in this post is the schematic that I had setup.

    I understand that during full wave rectification, when the AC wave is in its negative state, produces a negative charge on the netrual side of the circuit and the positive side of the ciruit remains neutral, giving the illusion on the oscilloscope that the positive side of the fully rectified circuit always has a positive potential difference on both negative and positive state of the sine wave.

    Because of this, if the base is not connected to anything and is discharged, when the sine wave is in its positive state, no current flows through the mosfet because both the source and the gate remain neutral. But when the neutral side of the circuit develops a negative potential difference, then all of a sudden the voltage difference between source and gate is now +240V, which turns on the mosfet and blows it up.

    BUT... if the base was negatively charged along with the source, then the potential difference would be 0v.

    Is my theory sound? Am I missing something? Why did the mosfet blow up?

    The mosfet used is STP9NK50Z.

    Looking at the datesheet, under Vdgr in the Absolute Max Rating, I don’t fully understand what Rgs is. It says 20k ohms. Does that mean 125ma of current will flow through the gate to drain if the voltage of drain is 240v? Thats about 3w of heat! so i dont think this is true. what does it mean? or does it mean i need a 20k ohm resistor on the gate (which doesnt make sense to me either)?
    Last edited: May 8, 2013
  2. Sensacell


    Jun 19, 2012
    Theoretically your circuit should do "nothing"- assuming you have everything connected properly and your diodes are rated to sufficient voltage.

    Connecting the Gate and Source together ensures VGS=0, so the device will never turn on.

    Reverse the Source and Drain- the FET's internal diode will conduct- BANG- fried parts.

    This seems like a strange, dangerous and expensive way to learn electronics?
  3. Matter45

    Thread Starter Active Member

    Dec 13, 2012
    LoLz Sensacell, agreed. it is dangerous. this mosfet is part of a bigger circuit which I have already built, and when the mofet blew up, i was like WTF? this shouldnt be happening!

    I just wanted to make it as simple as possible.

    Also, do you know what Rgs is in the Max Rating Vdgr on the datasheet? if you google STP9NK50Z its one fo the first links.
  4. Sensacell


    Jun 19, 2012
    The Rgs is just a test condition I believe.

    The rated breakdown voltage assumes that the gate is tied to source through a 20K resistance.
  5. tubeguy

    Well-Known Member

    Nov 3, 2012
    Could you post the circuit that you built ?
  6. Matter45

    Thread Starter Active Member

    Dec 13, 2012
    The circuit is quite extensive. Do you want the schematic in sections or as a whole?
  7. kubeek


    Sep 20, 2005
    That really depends on the form of presentation. Most preferable would be a pdf with each circuit section on it´s own page. But even a single large schematic can work, but probably not as a huge picture.
  8. Ron H

    AAC Fanatic!

    Apr 14, 2005
    What is the part number of the rectifier(s) in the schematic you posted?