Exploding mosfets/shorting DS

Discussion in 'General Electronics Chat' started by doug08, Dec 2, 2013.

  1. doug08

    Thread Starter Member

    Jan 30, 2011
    A large majority of the times when I connect the inverter circuit to a 12V Lead Acid battery. The circuit worked perfectly using (2) stp60nf06's, but I needed to limit the inrush with a .1 ohm resistor at start up, otherwise they explode. The fixed inrush resistor is bypassed 1/2 to 1 second after the connection to the battery is made, to allow full current. The inverter operates at 60hz. The circuit used a 555 IC. I used the inverter several times with no issues at all. The other day I fired it up, and the power came on, but the mosfet shorted DS once the inrush resistor was bypassed. The circuit supplies around 350 Watts of power using my rewound MOT transformer, the stp60 mosfet. Once the mosfet went short, my 30A fuse blew. Why am I having this problem? Driving me nuts. I checked the frequency at both gates, and 60 HZ is there. I tried 2 new irf3205, and they IMMEDIATELY short out or explode. I then tried irfz44n's, and they worked as long as full power was not applied to the circuit(leaving the .1 ohm inrush resistor in series with the CT of the transformer). As soon as full power is applied, it explodes. If I connect the mosfets to the circuit using smaller jumper wires, the inverter works 90% of the time. As soon as I solder them back in, they fail. Clearly the smaller wires are further reducing the inrush current. Something is not right here. This calls for an expert with the solution.

    Thank you!

    Schematic here:

  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    With that simple switching arrangement there is probably a brief period when both FETs are conducting at the same time and passing excessive current. Another possibility is spurious high frequency oscillation around the switching points.
    Last edited: Dec 2, 2013
  3. doug08

    Thread Starter Member

    Jan 30, 2011
    I thought of that possibility. What can be done? Reduce the resistor values to the gates? BTW, I added a 470 ohm resistor on the other mosfet which did not have one. I noticed when the mosfet goes short, it destroys the 555. By adding the 470 ohm, I eliminated that problem. This inverter works fantastic, if I can only eliminate the mosfet problem. Having too large a transformer a problem?

  4. David Pate

    New Member

    Oct 29, 2013
    You may have to debounce some of the switching. There must be no possibility of two FET's on at the same time; and consider the time it takes for a FET to turn off. Also you need to consider any possibility of a FET turning on by itself; I know from experience that a FET that is not turned off hard can turn itself on with the slightest charge on the gate. I have a bridge circuit with small FET's handling the 5 volt logic to 12 volt gate switching. There is an external circuit wired to the small FET's, and if for any reason there is a poor connection half the bridge will turn itself on and cause a short circuit. Essentially an unconnected gate will turn on.
  5. tracecom

    AAC Fanatic!

    Apr 16, 2010
    I would add pull-down resistors to the gates of both MOSFETs.
  6. ronv

    AAC Fanatic!

    Nov 12, 2008
    You might try something like the resistor diode shown here:

    It will slow down the turn on, but keep the turn off about the same, reducing the cross over.

    I also lowered the base drive resistor a little to make it turn off a little faster.