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7812 using MOSFET for more current.

Discussion in 'Electronic Design' started by [email protected], Sep 7, 2005.

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  1. Guest

    Hi all. :D
    I have seen alot of circuits with a 7812 (12V regulator) wit
    a PNP transistor for current bypass (or how you explain it)

    I would really like MOSFET transistors
    instead om bipolar transistors

    How??? :?

    Tn
    Johan in Karlskoga, Sweden
    SM4YZ
     
  2. On Wed, 07 Sep 2005 11:36:20 -0500, the renowned
    Why? There's no particular advantage, in fact there's a big
    disadvantage- the dropout voltage will be quite a bit higher because
    of the higher "Vbe". That means more wasted heat.

    Best regards,
    Spehro Pefhany
     
  3. Pooh Bear

    Pooh Bear Guest

    What do you 'like' about them then ?

    It's not a suitable applcation for a mosfet actually for the reasons
    Spehro mentions. A bipolar is the way to go.

    Graham
     
  4. Genome

    Genome Guest

    Well, if you really want to do that sort of thing then instead of a 7812 use
    a low power 78L05 or whatever the latest surface mount SOT-23 thing would
    be.

    Here's a bodge in LTSpice, hope you can figure out what's going on.....

    I'm not going to even think about what I might say about stability and such
    stuff. You might want to ask though........

    DNA

    http://ltspice.linear.com/software/swcadiii.exe

    Version 4
    SHEET 1 880 840
    WIRE -176 368 -176 160
    WIRE -176 464 -176 368
    WIRE -176 624 -176 464
    WIRE -176 736 -176 704
    WIRE -176 768 -176 736
    WIRE -144 368 -176 368
    WIRE -48 160 -176 160
    WIRE -32 368 -64 368
    WIRE -32 368 -32 208
    WIRE 64 368 -32 368
    WIRE 64 464 -176 464
    WIRE 208 736 -176 736
    WIRE 208 736 208 528
    WIRE 432 160 48 160
    WIRE 432 224 432 160
    WIRE 432 368 352 368
    WIRE 432 368 432 304
    WIRE 432 624 432 368
    WIRE 432 736 208 736
    WIRE 432 736 432 704
    WIRE 464 160 432 160
    FLAG 464 160 VOUT
    IOPIN 464 160 Out
    FLAG -176 768 0
    SYMBOL pmos 48 208 M270
    WINDOW 0 102 78 VLeft 0
    WINDOW 3 77 78 VLeft 0
    SYMATTR InstName M1
    SYMATTR Value FDC638P
    SYMBOL res -48 352 R90
    WINDOW 0 0 56 VBottom 0
    WINDOW 3 32 56 VTop 0
    SYMATTR InstName R1
    SYMATTR Value 10K
    SYMBOL res 416 208 R0
    SYMATTR InstName R2
    SYMATTR Value 1K4
    SYMBOL res 416 608 R0
    SYMATTR InstName R3
    SYMATTR Value 1K
    SYMBOL voltage -176 608 R0
    WINDOW 0 -84 46 Left 0
    WINDOW 3 -82 68 Left 0
    WINDOW 123 0 0 Left 0
    WINDOW 39 0 0 Left 0
    SYMATTR InstName V1
    SYMATTR Value 13V
    SYMBOL PowerProducts\\LT1121-5 208 416 R0
    WINDOW 0 -127 -141 Center 0
    WINDOW 3 -140 -113 Left 0
    SYMATTR InstName U1
    TEXT -176 824 Left 0 !.op
     
  5. Fred Bloggs

    Fred Bloggs Guest

    View in a fixed-width font such as Courier.
     
  6. Genome

    Genome Guest

    That looks like it might be positive feedback.

    DNA
     
  7. Genome

    Genome Guest

    No..... I give up

    DNA
     
  8. kell

    kell Guest

    Compare Fred's circuit to this:
    http://www.rason.org/Projects/vreg/vreg.htm
     
  9. kell

    kell Guest

    Of course, if you replace Q2 with a mosfet you still have the dropout
    voltage problem...
     
  10. kell wrote...
    That needn't be so bad, at least on account of the MOSFET. A 7812 and
    PNP transistor want at least 2.2 volts to operate (at 0.25A), and all
    of this voltage is available as gate-source voltage for the FET. Some
    power MOSFETs are happy with such a modest gate voltage, when used as
    linear pass elements in their subthreshold region. This is especially
    true for logic-level MOSFETs. But if the 7812 was replaced with an LDO
    regulator, for reduced dropout voltage, that'd be another matter! Then
    MOSFET (even a logic-level type) would become the limiting element.

    One other issue for this circuit. Stabilizing the feedback loop with
    all its multiple gain elements will be a bit tricky. Many folks would
    be tempted to use Spice to evaluate the circuit. Let's assume you find
    a good 7812 model. There's still one nasty problem, which is to create
    an accurate subthreshold FET model. The sad truth is the manufacturer's
    models are disasters in this region, and completely fail to work properly.
    There is a solution, which I've written about here before in detail. A
    Google usenet search should turn up several useful threads.
     
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