1. Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.
    Dismiss Notice

Why use MOSFET instead of BJT?

Discussion in 'General Electronics Chat' started by StupidDum, Dec 2, 2004.

  1. StupidDum

    StupidDum New Member

    Joined:
    Nov 3, 2003
    Messages:
    242
    Likes:
    0
    Location:
    Malaysia
    Any significant advantages?
    and what are the common application where MOSFET is preferred over BJT?
     
  2. Oznog

    Oznog Active Member

    Joined:
    Apr 21, 2004
    Messages:
    2,879
    Likes:
    11
    Location:
    Austin, Tx
    Very significant advantages. MOSFETs don't need current on their control pin, but require more voltage. Some don't turn on fully at 5v, some do. A BJT is limited to something like 0.3v for the lowest voltage drop on the current path, but MOSFETs are only limited by their resistance (rDSon). MOSFETs are usually more efficient switches for power supplies, etc where you want a switch rather than an amplifier.

    So there are lots of other differences in linearity and high speed performance, but they're not really straightforward.
     
  3. Styx

    Styx Active Member

    Joined:
    Dec 19, 2003
    Messages:
    1,743
    Likes:
    9
    Location:
    UK
    FET's are only more efficient because they can be switched alot faster and thus small SMPS can be used.

    BJT can handle alot more current and have a lower on-state conduction losses.

    If you want the best of both worlds check out IGBT's
     
  4. dave

    Dave New Member

    Joined:
    Jan 12, 1997
    Messages:
    -
    Likes:
    0


     
  5. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

    Joined:
    Nov 17, 2003
    Messages:
    39,324
    Likes:
    653
    Location:
    Derbyshire, UK

    Probably the best idea is to look what's used commercially, sometimes MOSFET's, sometimes BJT's - they both have advantages and disadvantages. Generally BJT's are cheaper - so are used more often.

    Reliability wise, there seems very little difference, both MOSFET's and BJT's fail in equal numbers in SMPS's.
     
  6. Optikon

    Optikon New Member

    Joined:
    Sep 23, 2003
    Messages:
    1,729
    Likes:
    2
    Location:
    Cleveland, OH, U.S.A.
    Since you mentioned an advantage of BJT, I'd also like to add that BJT's in general have lower noise characteristics than the similar mosfet in question.
     
  7. TheOne

    TheOne New Member

    Joined:
    Oct 30, 2004
    Messages:
    464
    Likes:
    0
    Location:
    USA
    MOSFET's are less prone to secondary breakdowns
     
  8. Styx

    Styx Active Member

    Joined:
    Dec 19, 2003
    Messages:
    1,743
    Likes:
    9
    Location:
    UK
    and IGBT's are prone to Thyristor lockup if you are not careful

    basically it is horses for courses. What normally definds it for me is what is my driving source and how do I want the output to respond.

    100's of volts then IGBT's
    upto 100V FET

    very rarely do I use BJT except for gate drives since in a push-pull you can switch a FET pretty quickly
     
  9. Oznog

    Oznog Active Member

    Joined:
    Apr 21, 2004
    Messages:
    2,879
    Likes:
    11
    Location:
    Austin, Tx
    Not at all. It's difficult to compare since you there are large and small BJTs and FETs, so it's not fair to pick two out of a catalog and compare them.

    There are many MOSFETs capable of handling quite high currents.

    On the whole, a BJT will consume more power in the on-state. It cannot switch with less than a 0.3v voltage drop, and it takes a lot of base current to do it. Now there certainly are MOSFETs with poor rDS-on that they exceed a 0.3v drop under load, but that's hardly a rule. I commonly use MOSFETs with an rDS-on on 0.02 ohms or less.

    The bipolar's emitter current can be problematic. If you want an on-state voltage drop of 0.3v at 5 amps, you might need to give it 200mA of base current (gain is poor as you get into the overdrive region). Say it's a motor and you wanted to meet that spec so it won't overheat the transistor or stall the motor when heavily loaded. But most of the time your motors only draw 500mA. Well, unless you're doing something funky with the driver, you must end up putting in an emitter resistor that always provides 200mA to the base no matter what the load. You can't really adjust the current to only the magnitude needed to drive the load at the time.

    If you have a 12V supply and a 5V reg, you're sucking 200mA off the 12V supply, or 2.4W, to drive a 500mA load. In practice, you'd choose a more practical base current and live with the added voltage drop & heat in the BJT. Or the far more practical solution is to use a FET. A FET would take no current on its gate under high or low load and there is no prob finding one with a voltage drop <0.05v under a 5 amp load. There are some awesome switching MOSFETs in SO-8 pkgs that handle high currents without generating significant heat due to such low rDS-on.

    MOSFETs can easily be placed in parallel, bipolars cannot unless external emitter resistors are added. The external resistance generates additional efficiency and voltage drop losses.
     
  10. TheOne

    TheOne New Member

    Joined:
    Oct 30, 2004
    Messages:
    464
    Likes:
    0
    Location:
    USA
    Also to operate bipolars at high switching frequencies and high current, you have to prevent the devices from going into hard saturation as this will increase storage times (making it difficult to switch off quickly) but then cause the device to dissipate more heat due to higher Vce-sat.

    One place that MOSFETS were proved to be a poor choice is in low distortion audio power amplifiers as indicated in a research by the author "Douglas Self", a leading figure in audio power amplifier design.
     

    Attached Files:

Share This Page