High-Power MOS H-bridge, complete with fires.

Discussion in 'The Projects Forum' started by Severe, Mar 25, 2011.

  1. Severe

    Thread Starter New Member

    Mar 25, 2011
    Hey guys,
    Workin on a project requiring a bi-polar current flow of rediculously high pulse (~80A). A high-force solenoid actuator if you must know. Everything from the uC down to the solenoid works perfectly, but only below 14V. The coil/driver circuit resistance requires at least 19V and we're running 2x 11.1V lithium poliymer batteries for the pulse source.

    Basically! our problem lies on the PMOS in the H-brigde. Set up correctly it operates exactly as expected with 1 of the 11V batteries but goes on fire with 2. Using a programmable source we've established that it heats up violently fast at ~13.9V, even with low current (1k load resistance for testing).
    The MOSFET's used are the STP80PF55 series, rated at Ids max = 80A (320A pulse) and Vgs max = 16V, Vds max = 55V @Vgs=0, thus Vgate can handle up to 55V??

    Our original fault was forgetting about the Vgs max rating so then implimented a simple NMOS, through a potential divider, regulation of the gate voltage on the PMOS. This works as expected giving a ratio and keeps Vgs controllable, but when the ratio is set so the voltage needs to be above 14V to trigger the PMOS threshold, it heats up insanely fast when it begins to turn ON, even though the Vgs drop still remains only 4V.

    I've thrown together a basic schematic on free software I have at home so no home made libraries/simulations but I really need help, sick of burning my fingers tbh lol.

    Basic schematic

    The gate inputs are driven by the uC through an optically isolated source of 5V, thus the voltage at the MOS gate don't have a common ground with the Drain/Source (doesnt seem to be a problem below 14V). The variable resistor merely represents the solenoid load.

    Links to MOS data sheets.

    Pretty much ran out of ideas, hope you can help.
    Last edited: Mar 25, 2011
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    Consult your data sheets. 5 volts is just past the point where the FET's are beginning to conduct. You really need 10 volts gate to source to get full conduction.

    If your IRF620 turns full on, the voltage on the ST80Pf gate will be 20 volts. The data sheet indicates +/- 16 is the maximum allowable.
  3. John P

    AAC Fanatic!

    Oct 14, 2008
    It looks to me as if your STP80PF55's are backward. Did you really install them this way?

    Also, I worry about the timing of the turn-on of the upper transistors versus the turn-off of the lower ones, and vice versa. Are you sure they'll never be on together during switching?