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MOSFET packaging

Discussion in 'Electronic Basics' started by ErikBaluba, Mar 1, 2006.

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  1. ErikBaluba

    ErikBaluba Guest

    Hi,

    I'am putting together a H-bridge and I have found some interesting SO-8
    MOSFETs with combined P/N channels on the same chip.
    The maximum power-rating for such packages seems to be 2.5W. But many of
    these mosfets have a rating of several amperes continous current.

    If I have a MOSFET like this with e.g. Ids continous = 4A, and Pmax=2.5W,
    does this mean that Vds cannot be larger than 2.5/4 = 0.62V ?

    Am I right to say that the ampare rating for such MOSFETs is not relevant
    for driving motors, except perhaps small lov-voltage motors with low torque
    and high speed?

    Now, for TO-92 or I-Pak power MOSFET the watt-rating is suddenly very high.
    What MOSFET packaging would cover the usage between these extremes? For
    example, I have some very small Lithium-Polymer batteries with several
    amperes discharge current. If I want to drive a 3V motor with 1A current
    using such a battery I would need a mosfets that can handle about 4W to be
    safe. Most of the TO-92 power mosfets I see seem to be completely overkill
    for this, and I also want things to be as small as possible.


    erik
     
  2. DJ Delorie

    DJ Delorie Guest

    Power dissipated by the MOSFET depends on its own Vds, not the drop
    across the load. A 4A mosfet can pass 4A as long as the drop *across
    the mosfet* doesn't exceed 0.6v when it's conducting. The drop across
    the load can be whatever you want, as long as you don't exceed the
    other parameters of the mosfet (like max off-state Vds).

    If you have a 3v motor with 1a current, the *motor* dissipates 3
    watts, but that doesn't mean the *mosfet* does. Let's say the on
    resistance of the mosfet is 1.5 ohms. At 1A, it's a 1.5 volt drop.
    That's 1.5 watts of power dissipated by the mosfet itself.
     
  3. To add to what DJ said:

    The basic idea of switching control is that the MOSFET is either turned on
    all the way or turned off all the way. Power = current * voltage. So when
    it's turned off, no current is flowing, so power is zero; when it's turned
    on, current can flow but the voltage drop across the MOSFET is small (= Rds
    * I) so power is still low.

    So average dissipation in the MOSFET will be (Rds(on) * I * duty-cycle),
    where duty-cycle is the fraction of time that the MOSFET is turned on for.

    When you look at power ratings, keep in mind that the power has to get
    dissipated as heat, and that heat has to go somewhere. So you need to
    consider "thermal resistance", which is expressed in degrees C per watt.
    The datasheet will tell you the junction-to-ambient resistance. For
    instance, a TO92 case might have 200 C/W of junction-to-ambient thermal
    resistance. That means that if the MOSFET is dissipating 0.5W, the junction
    temperature (which is what matters) will be 100C hotter than the air around
    the transistor. If the air around the transistor is 50C (and remember, if
    you have your electronics inside a case, it will be hotter than room
    temperature), that means the junction is 150C, which is probably its rated
    maximum.

    So you can see that it is usually thermal resistance that is the real power
    limitation - the rated power maximum of the device is not the first thing
    you run into, unless it is mounted to a very good heat sink.
     
  4. ErikBaluba

    ErikBaluba Guest

    ....
    Ok, but I assume you mean power dissipation is Rds(on)*I*I*duty-cycle?

    ....
    Thanks a lot, that was a very useful. I tend to skip pass those C/W
    parameters in the datasheet :) I would think those ratings will not vary
    much for different SO-8 MOSFETs, I will take a look around again.

    As for Rds(on) ratings, it seems this follows the same principle as
    resistance in a copper-wire, where tick wires have less resistance and thus
    lower voltage drop and power dissipation? I noticed that only physically
    large mosfets in TO-92 casing etc provide really small Rds(on) in the
    milli-ohm range. The smallest Rds(on) ratings I found for MOSFETs in SO-8
    casing was several Ohms, and with a much higher power dissipation as a
    result.

    erik
     
  5. Correct. Sorry, my mistake.
    You're right; for a given package type you won't find a whole lot of
    variation. Laws of physics, and all that.

    Note that for surface-mount components, free-air thermal resistance is
    usually very high; but in practice, they dissipate their heat through the
    leads, to the circuit board traces. So you will typically see a thermal
    resistance spec for that, too; and it will often specify how big and thick
    the traces need to be.
    Roughly speaking, yes, it has to do with the size of the channel. Of
    course, they play all kinds of interesting tricks to improve Rds(on) while
    keeping the die small. But the more channel area, the higher the
    gate-to-source capacitance is (for a given breakdown voltage, anyway), which
    introduces other problems; so there are a bunch of tradeoffs.
     
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