Way of controlling MOSFET.

Discussion in 'General Electronics Chat' started by moorejohn90, Sep 21, 2012.

  1. moorejohn90

    Thread Starter Member

    Dec 9, 2011
    57
    0
    Hi guys, I want to know the way of controlling out put (ic in BJT) current from MOSFET. I know definitely and have experience with BJT transistor that use bi with HFE to get desire ic. But I don't know about MOSFET. If anyone can, show me the way with formula if is also used as BJT.
    Thanks. :)
     
  2. Dyslexicbloke

    Active Member

    Sep 4, 2010
    420
    19
    I am no expert, but have learned here that MOSFET's in general make poor followers and don't work well when only partially turned on.

    However there are several types of FET and to get a propper answer you are going to have to specify a bit more, for example is it N or P channel, is it enhancement or depletion mode.

    Having said all that I also know that you can get audio amps that are FET based so at least one type of FET must work in a partially on state buy as I said I am no expert so couldn't tell you what that might be.

    see http://en.wikipedia.org/wiki/Field-effect_transistor

    'Types of field-effect transistors' about half way down.
    I will watch this thread with some interest as I need to learn more myself.
    Hope you enjoy the reading
    Al
     
  3. cork_ie

    Member

    Oct 8, 2011
    348
    58
    Me too.

    All I know is they are ideal for PWM and switching circuits, where they are switched hard on and hard off.
     
  4. Potato Pudding

    Well-Known Member

    Jun 11, 2010
    684
    92
    Output current sensing is pretty common for a variety of FET switched circuits.

    Two things I said in that sentence that are pretty much key.

    Current Sensing - meaning use of feedback. You seem to want a formula and a reliable set response - the "That will work!" system build approach. That has an inherent acceptance of error that becomes a problem as you head towards the higher power levels where FETs make a huge difference. You do better both in performance and reliability when you control the output by sense and regulation, by several orders of magnitudes in both those criteria which means that the product is going to be about 1000 times better. It is hard to get to better than 5% accuracy without feedback, easy to get 99.5% with it. Risky to run above 60% of device rating without feedback without problems. Easy to run at 90% of device rating when using feedback control and have no problems.

    Switched circuits. FETs are great for switching so to get for example 2 amps out of a continuous/max 10 amp circuit you might switch it on and off at 20% duty cycle and average the output. In a crude system without sense feedback you would still be vulnerable to loading and unloading. For example a motor on a vehicle rolling downhill could turn into a generator and going uphill it might stall.
    But by switching you save energy. Two states for the FET switch. Off with near zero current means a near zero power product, and on with a near zero voltage across the FET means another near zero power product. Switching losses are low. No heat sink needed in many cases, which means cheaper and lighter weight, and less wasted energy.

    But that is enough of explaining why you likely want something else. Here is what you asked for, transconductance.
     
    Last edited: Sep 21, 2012
  5. Potato Pudding

    Well-Known Member

    Jun 11, 2010
    684
    92
    There are whole OP AMPS that are designed with linear FETs that work very well.

    Most Often --
    Class D which are almost like a type of switching power supply - well filtered and fast enough response to use the Audio signal as their voltage setting.

    You are not wrong about the FET working in linear mode as I made mention of their use in OP AMPS.

    JFETs are often used as a current source. Try some sims of a JFET with a ramp signal input.
     
  6. Dyslexicbloke

    Active Member

    Sep 4, 2010
    420
    19
    Interesting, thanks.
    I don't want to hijack a thread so will not comment further.
     
  7. moorejohn90

    Thread Starter Member

    Dec 9, 2011
    57
    0
    Hi guys, Thanks you for all of yours reply.
    Today, I made a test with IRFZ44N(N channel MOSFET). I hope that this will help to understand someone like me.
    There are two side have to consider.
    1- Supply the voltage that enough to open Drain to Source (~1 to 15V, I used 5v and 3.3v)
    2- Use Ohm's Law for getting desired current (Drain to Source or Drain current or ic in BJT).

    That all. Thanks
     
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