mosfet/ npn

Discussion in 'The Projects Forum' started by ryajhi, Mar 4, 2013.

  1. ryajhi

    Thread Starter New Member

    Feb 28, 2013
    The project that I'm building requires 2 amps of 12 volt switching at 50khz. However the 2 transistors I have laying can only handle 1.75 amps each, so is it possible to connect them in parallel for total of 3 amps? and also, if you stack 2 mosfets/bipolar in series will you get double the max voltage?
  2. takao21203

    Distinguished Member

    Apr 28, 2012
    1. You'd typically observe one of them will hog 70% of the current.

    2. No. I don't think.
  3. JDT

    Well-Known Member

    Feb 12, 2009
    Firstly, if you want to control 2A, your transistors need a safety margin of at least 100%. So use transistors rated at 4A or above. The same applies to the maximum voltage rating.

    MOSFETS are better at switching at high frequencies than bipolar transistors. Either way, you need to drive them carefully to get them to switch amps at 50KHz.

    Transistors can be paralleled to increase the current rating but steps have to be taken to get them to share the current equally.

    Bipolar transistors are worse in this respect because they will conduct more when they are hotter. So they can thermally "run-away": the one that is taking most of the current will get hotter and take still more current.

    MOSFET transistors have the opposite effect so in many cases they are easier to parallel.

    I would not try to use series connected devices to increase the voltage rating. Can be done but not easy.
  4. JMac3108

    Active Member

    Aug 16, 2010
    Yes, always provide plenty of margin on the voltage rating to account for spikes and other transients. 2X might not always be possible though. measure your switching edges with a scope and if you have large spikes consider adding a snubber to reduce them.

    Do not use the current rating alone to select a mosfet or Bipolar in a power application. Always perform a thermal calculation to see how hot the device will get at your load current.