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Trying to develop MOSFET intuition

Discussion in 'Electronic Design' started by Peter S. May, Oct 11, 2007.

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  1. Peter S. May

    Peter S. May Guest

    I'm trying to rebuild parts of the Microchip PICDEM FS USB board
    schematic partly as an experiment but mostly as a learning experience.
    There's a part of the circuit that appears to allow the device to
    auto-select between USB bus power and wall-brick power, that looks sort
    of like this:

    NDS352P 1N5817
    [VBUS>---+--+--[S G D]---->|--+-[+5V>
    | | | |
    | | | |
    \ | \ 1N5817
    100k / =.1uF / 33k
    \ | \
    | | |
    +--+-----+--| GND

    From what I read, NDS352P is a P-channel MOSFET. I also can't seem to
    locate this part on Allied, and didn't have any better luck with a
    couple of cross-references.

    I'm thinking that this is an opportunity for me to learn about MOSFETs
    since I've never used one before. I only recently really learned about
    BJTs, and can only really keep it straight because, in general, only a
    small selection of the figures are relevant: hFE (the current gain Ib
    to Ic), which isn't crucial when used as a switch instead of an analog
    amplifier, Vce (voltage drop collector to emitter, i.e., how much less
    than Vcc is dropped across the output load), and max Ic (how much
    current can be passed through the load).

    What are the important (analogous?) figures for a MOSFET? What features
    should I be looking for in a replacement for NDS352P?


    I could write about MOSFETS for hours, but somebody already did a good job
    in the above link.

    Also: Buy some of different type and experiment.
  3. Peter S. May

    Peter S. May Guest

    I've actually read quite a bit about how they work, but it is
    information overload--much like I don't have to think about PN
    junctions, electrons, and holes to know the high-level functionality of
    BJTs, surely I don't have to have every word of the Wikipedia article on
    MOSFETs down to be able to use the things.

    In other words, I humbly beg for a summary of, say, the three to five
    most relevant stats to look at, because I lack the talent to pick out
    the important parts of a wordy article. :)

  4. Yes, sure, but if you just bought some, and / or looked up the datasheet,
    you would find out!!
    What is important depends on what you want to do.
    Speed (Ton / Toff) may not be important, input capacitance may not be
    important, gain (A/V) may not be important, cut off point (Vgs for Id zero)
    may not be important, drain source voltage (Vds max) may not be important,
    Hell in the time I typed this you could look up XXXFET.pdf and study it.

    I cannot drink water for you, and I cannot give you hands on experience.
  5. And I forgot Ron, case type, thermal resistance, max junction temp,
    price, second source, Pmax, Id max, and a few more.

    All of these may or may not be important.
  6. Peter S. May

    Peter S. May Guest

    Fair enough--let's get some hands-on experience.

    In reading so far, I get the impression that the 2N3904 and the 2N2222
    are generally universal representations of the NPN transistor (and
    2N3906 for PNP), and I've found this out for myself by using a few. If
    there is a handful of similarly ubiquitous MOSFETs, which do you think
    they'd be? I ask this primarily because there is a wide variety and
    can't tell the difference between the common and the extreme.

    Also, here's a question I have about another specific application that
    just came to mind: I have an arbitrary power-hungry 5VDC circuit (let's
    say 500mA) that I want to be able to switch on and off using HC logic.
    The Vce on a bipolar transistor for that kind of load is too steep, but
    MOSFETs seem to act a bit more like normal switches. Would one be
    appropriate here?

    Thanks, and I apologize if I'm trying your patience...
  7. Yes, but those are very old.
    My TUN and TUP (Transistor Universal NPN, Transistor Universal PNP)
    are in Europe more like BC548, BC558.
    Those are small signal Si.
    Universal is perhaps also 2N3055....., the BD series.
    Well, I have a lot of IRFZ44A laying about, high current low voltage switch.
    Also some dual gate MOSFETS BF191 BF192, very nice for RF circuits.
    Dual gates are magic!
    Used a lot of BUZ44A for higher voltage (no longer made I think).

    I have some IRLZ34N, those will switch on from 5V logic, max treshold
    voltage is 2V, it carries max 30A, and allows 50V.

    It is easier than making an animation in Blender.
  8. D from BC

    D from BC Guest

    A mosfet is like a driving a complicated capacitor and it conducts
    like a complicated resistor.

    D from BC
  9. Ecnerwal

    Ecnerwal Guest

    Go to any of the parts house that have stock numbers listed. Search for
    mosfets. Now sort by "Number in stock" (Newark does this easily, I don't
    recall if there others typically allow you to sort by it in results or
    not). Buy some from the "several thousand in stock" class, those are
    probably pretty ubiquitous.

    Heavy duty, old-school, probably terrible numbers per more up to date
    parts, still work, cheapish to buy - IRF540 (IRF9540 for P-channel)

    Slightly more modern, find some on a junk hard drive board near you for
    free (not every one, of course, but they seem common there), 2 per
    package - IRF7101 (not all that common per the above test, however -
    probably obsoleting faster, while the 540 has decades of staying power)
  10. D from BC

    D from BC Guest

    For fun..
    The most expense P mosfet I can find on Digikey is $12.07Can each

    The most expensive N mosfet I found is $94.23Can each.
    D from BC
  11. Fred Bloggs

    Fred Bloggs Guest

    That doesn't look like an auto-select circuit to me.
  12. Paul Mathews

    Paul Mathews Guest

    The Schottky diodes in the circuit allow whichever voltage is higher
    to supply power to the circuit. It's not clear why anyone thought they
    needed the PFET in this case. At most, it might reduce the magnitude
    of a power up surge, since the 33K slows turn on slightly. The C is
    simply for bypass. The 100K discharges C when VBUS goes away.

    Simplest mental model of MOSFET: VOLTAGE Gate to source reduces
    RESISTANCE between Drain and Source (Rdson) when Vgs is of correct
    polarity for type of MOSFET. For Vgs > threshold, Rdson doesn't
    decrease much further. A BODY DIODE shunts Drain and Source, such that
    Drain and Source act like a diode for reverse polarity Vds. Dynamic
    capacitances, gate spreading resistance, gate oxide breakdown, and
    thermal characteristics complicate the model.
    Paul Mathews
  13. Jim Thompson

    Jim Thompson Guest

    Posted originally in 2004....

    ...Jim Thompson
  14. Peter S. May

    Peter S. May Guest

    I think I accidentally put a cross where there was supposed to be a
    joint. The correction is where VREG crosses between the gate and the
    33k resistor:
    The selector appears in context on page 54 of the board's datasheet:

  15. A safer way to draw is to avoid crossing wires altogether in case of a connection:
    NDS352P 1N5817
    [VBUS>---------[S G D]---->|----[+5V>
    | | | |
    | | |------->|--
    \ | \ 1N5817
    100k / =.1uF / 33k
    \ | \
    | | |
    ------------| GND
  16. Similar concept:

    It's a bit expensive though.

  17. Fred Bloggs

    Fred Bloggs Guest

    That makes much more sense, the presence of Vreg turns off the Vbus
    feed. You will find that the majority of enhancement mode MOSFET
    applications are as "load switches" and, as the name implies, they are
    used as a simple switch. The foremost parameters are the VGS,th,
    gate-to-source threshold voltage, and RDS,on, the on state
    drain-to-source channel resistance. Apply a gate-source bias well in
    excess of VGS,th and the MOSFET state changes from a leaky drain-source
    open circuit to a low resistance channel. N-channel devices require a
    positive VGS, to attract negatively charged electrons into the N type
    channel, and P-channel devices require a negative VGS, to attract
    positively charged holes into the P type channel. A zero or reverse VGS
    puts the MOSFET in a high impedance state.
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