Power MOSFET choices?

Discussion in 'General Electronics Chat' started by samuel.whiskers, Aug 6, 2014.

  1. samuel.whiskers

    Thread Starter Member

    Mar 17, 2014
    I'm currently using a BUZ71 MOSFET in the arrangement below - it is a rocket launcher igniter - the load is a rocket igniter connected to long cables.

    I've measured the current flow through the igniter with a shunt - scope trace of voltage drop across shunt shown. Shows current flow of nearly 8A for 140ms.

    The figure from the datasheet suggests I might be pushing the limit of this MOSFETs safe operation?? I've done a LOT of test pulses of 1.5s through 1 Ohm resistors as dummy loads, with nothing catastrophic happening so far.... I do prefer to have a good safety margin with these things when possible.

    Can anyone suggest a higher power MOSFET I could drop in as a replacement??

  2. Jony130

    AAC Fanatic!

    Feb 17, 2009
    You don't understand the datasheet correctly.
    Rds(on) limitation tell as that Rds(on) is so large the Vds_full_on will also be large.
    Because the maximum current for the relevant Vds is limited by the component’s Rds(on).
    For your MOSFET has Rds(on) = 0.1 Ohm.
    So for Id = 8A Vds cannot be smaller than
    Vds = 8V *0.1 ohm = 0.8V.
    This voltage drop across the MOS will produce the heat and power dissipation.
    P = 0.8V * 8V = 8W.

    Also you don't need to worry about the MOS because you don't launch the rocket very often. So the MOS will have the time to cool down.

    As for the MOS with the lower Rds(on) check this IRFZ44n or BUZ11.
  3. crutschow


    Mar 14, 2008
    You have some errors/typos in your math. It should be:
    Vds = 8A * 0.1Ω = 0.8V
    P = 0.8V * 8A = 6.4W

    That doesn't affect your conclusions. :)
  4. samuel.whiskers

    Thread Starter Member

    Mar 17, 2014
    Thanks gents - happy the MOSFET should be fine....
    I still don't really understand that chart it seems... the RDS(on) limits still has me confused.