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Pspice transfer function

Discussion in 'CAD' started by Robert Baer, May 25, 2005.

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  1. Robert Baer

    Robert Baer Guest

    When i test the actual circuit, the output has a transition "hump"
    between the linear I/O part and the regulation part; it is about 900mV
    in amplitude.
    The Spice does not show it; why?
    ** Regulator.asc
    Version 4
    SHEET 1 880 680
    WIRE -400 208 -400 -32
    WIRE -368 -32 -400 -32
    WIRE -272 -32 -288 -32
    WIRE -272 144 -272 -32
    WIRE -240 144 -272 144
    WIRE -32 144 -160 144
    WIRE -32 144 -32 64
    WIRE -32 176 -32 144
    WIRE -32 336 -32 272
    WIRE 64 -32 -272 -32
    WIRE 64 144 -32 144
    WIRE 64 224 32 224
    WIRE 64 224 64 208
    WIRE 112 64 -32 64
    WIRE 112 64 112 32
    WIRE 256 -32 160 -32
    WIRE 256 64 256 -32
    WIRE 256 224 64 224
    WIRE 256 224 256 144
    WIRE 256 272 256 224
    WIRE 256 384 256 352
    WIRE 336 384 256 384
    WIRE 336 384 336 240
    WIRE 368 -32 256 -32
    WIRE 368 128 368 -32
    WIRE 368 240 336 240
    WIRE 368 240 368 192
    WIRE 368 272 368 240
    WIRE 480 -32 368 -32
    WIRE 480 128 480 -32
    WIRE 480 240 368 240
    WIRE 480 240 480 208
    FLAG -400 288 0
    FLAG -32 400 0
    FLAG 368 272 0
    SYMBOL npn 64 32 R270
    WINDOW 0 136 33 VRight 0
    WINDOW 3 100 7 VRight 0
    SYMATTR InstName Q1
    SYMATTR Value ZTX849
    SYMBOL Misc\\battery -400 192 R0
    WINDOW 123 0 0 Left 0
    WINDOW 39 0 0 Left 0
    SYMATTR InstName V1
    SYMATTR Value 38V
    SYMBOL res -272 -48 R90
    WINDOW 0 0 56 VBottom 0
    WINDOW 3 32 56 VTop 0
    SYMATTR InstName R1
    SYMATTR Value 1e-3
    SYMBOL npn 32 176 M0
    WINDOW 0 103 58 Left 0
    WINDOW 3 77 24 Left 0
    SYMATTR InstName Q3
    SYMATTR Value FMMT459
    SYMBOL zener -16 400 R180
    WINDOW 0 24 72 Left 0
    WINDOW 3 62 38 Left 0
    SYMATTR InstName D1
    SYMATTR Value BZX84C12L
    SYMBOL cap 48 144 R0
    SYMATTR InstName C1
    SYMATTR Value 150pF
    SYMBOL current -240 144 R270
    WINDOW 0 -32 40 VBottom 0
    WINDOW 3 32 40 VTop 0
    WINDOW 123 0 0 Left 0
    WINDOW 39 0 0 Left 0
    SYMATTR InstName I1
    SYMATTR Value 1mA
    SYMBOL cap 352 128 R0
    SYMATTR InstName C2
    SYMATTR Value .01µF
    SYMBOL res 240 48 R0
    SYMATTR InstName R2
    SYMATTR Value 175K
    SYMBOL res 240 256 R0
    SYMATTR InstName R3
    SYMATTR Value 125K
    SYMBOL res 464 112 R0
    SYMATTR InstName R4
    SYMATTR Value 10K
    TEXT 304 -72 Left 0 ;N006
    TEXT -96 -64 Left 0 ;N002
    TEXT 144 248 Left 0 ;N005
    TEXT -128 120 Left 0 ;N003
    TEXT -328 -176 Left 0 !.dc V1 31 33 0.01
    TEXT -328 -144 Left 0 !.LIB D:\\SPICE\\ZMODELS.LIB
     

  2. Hello Robert,
    Where did you uy the 1mA-current source?
    Type:?
    Manufacturer:?

    Is it just a schematic simplification?
    Then please provide the real circuit(schematic) for this current source.

    Is this Z-diode in the real circuit or is this also some kind of
    simplification for another device?

    Have you tried to replace this crrent source with a simple 33k resistor.
    It would be very interesting how your real circuit behaves then
    compared to the simulation.

    Best regards,
    Helmut
     
  3. Robert Baer

    Robert Baer Guest

    The current source is a DMOSFET; the DN3545 and a 2.2K resistor.
    Do not know how to model that.
    This was tested using an E-B transistor junction (in reverse) for the
    zener; the transistor type was selected to have no negative R effects
    and have low noize at all currents from nanoamps to milliamps.
    The "hump" mentioned was measured not only with meters, but also seen
    on a curve tracer.
     
  4. Hello Robert,

    You could try to find a SPICE model of a similar device
    from another manufacturer.
    That's it.
    If you replace this "bad" device with a real Zener diode, the
    problem should go away. It's a very very bad idea to use the
    B-E breakdown as a Zener-diode for any serious application.

    I rate the whole thing not as a problem to find a SPICE
    model for the B-E breakdown. It's simply a bad design
    practice to use this B-E breakdown as a reference voltage.

    Best regards,
    Helmut
     
  5. Robert Baer

    Robert Baer Guest

    Sorry, this particular transistor E-B junction is superior to any
    zener diode i have seen. Furthermore, if i add in the B-C junction in
    forward mode, i get a voltage reference good to 180C.
    Furthermore, when i replace it with a UCC2801 (zeners near 13.5V), i
    see exactly the same thing.
     

  6. Hello Robert,

    Is this "bump" a dynamic behaviour when the inut voltage is ramped up
    with a certain speed or is it also visible when you manually
    increase the input voltage very slowly?

    Means this "bump" a higher or a lower voltage?

    Best regards,
    Helmut
     
  7. Robert Baer

    Robert Baer Guest

    ----------- SNIPped for brevity -----------
    The "bump" is seen at any speed - curve tracer or manual input
    voltage increments.
    The "bump" is seen "at" the transition between the linear I-V from
    zero to the regulation at the output.
    I have experimented with various voltage dividers, the E-B-C
    transistor, the UCC2801 (which is what will be used) and a 15V zener.
    Depending on what i use, i see two different kinds of curves.
    1) I-V slope abruptly changes to a lower value (higher R) during turn-on
    of the depletion mode MOSFET, then abruptly flat ("infinite" R) at
    regulation; no "bump".
    2) I-V slope abruptly changes to a higher value (lower R) during turn-on
    of the depletion mode MOSFET, then abruptly flat ("infinite" R) at
    regulation; has "bump".
    I fiddled around with a few FETS in the library (there are no models
    for a depletion mode FET) to replace the current source.
    The result is similar to case #1 above, except the first transition
    is not abrupt (enhancement mode FETS are log Id VS Vgs, and maybe
    depletion mode FETS are loglog?).
    --------------
    Version 4
    SHEET 1 880 680
    WIRE -400 208 -400 -32
    WIRE -368 -32 -400 -32
    WIRE -272 -32 -288 -32
    WIRE -272 144 -272 -32
    WIRE -80 144 -96 144
    WIRE -80 192 -112 192
    WIRE -80 192 -80 144
    WIRE -32 144 -80 144
    WIRE -32 144 -32 64
    WIRE -32 176 -32 144
    WIRE -32 336 -32 272
    WIRE 64 -32 -272 -32
    WIRE 64 144 -32 144
    WIRE 64 224 32 224
    WIRE 64 224 64 208
    WIRE 112 64 -32 64
    WIRE 112 64 112 32
    WIRE 256 -32 160 -32
    WIRE 256 64 256 -32
    WIRE 256 224 64 224
    WIRE 256 224 256 144
    WIRE 256 272 256 224
    WIRE 256 384 256 352
    WIRE 336 384 256 384
    WIRE 336 384 336 240
    WIRE 368 -32 256 -32
    WIRE 368 128 368 -32
    WIRE 368 240 336 240
    WIRE 368 240 368 192
    WIRE 368 272 368 240
    WIRE 480 -32 368 -32
    WIRE 480 128 480 -32
    WIRE 480 240 368 240
    WIRE 480 240 480 208
    FLAG -400 288 0
    FLAG -32 400 0
    FLAG 368 272 0
    SYMBOL npn 64 32 R270
    WINDOW 0 136 33 VRight 0
    WINDOW 3 100 7 VRight 0
    SYMATTR InstName Q1
    SYMATTR Value ZTX849
    SYMBOL Misc\\battery -400 192 R0
    WINDOW 123 0 0 Left 0
    WINDOW 39 0 0 Left 0
    SYMATTR InstName V1
    SYMATTR Value 38V
    SYMBOL res -272 -48 R90
    WINDOW 0 0 56 VBottom 0
    WINDOW 3 32 56 VTop 0
    SYMATTR InstName R1
    SYMATTR Value 1e-3
    SYMBOL npn 32 176 M0
    WINDOW 0 3 106 Left 0
    WINDOW 3 -80 133 Left 0
    SYMATTR InstName Q3
    SYMATTR Value FMMT459
    SYMBOL zener -16 400 R180
    WINDOW 0 24 72 Left 0
    WINDOW 3 62 38 Left 0
    SYMATTR InstName D1
    SYMATTR Value BZX84C6V2L
    SYMBOL cap 48 144 R0
    SYMATTR InstName C1
    SYMATTR Value 150pF
    SYMBOL cap 352 128 R0
    SYMATTR InstName C2
    SYMATTR Value .01µF
    SYMBOL res 240 48 R0
    SYMATTR InstName R2
    SYMATTR Value 175K
    SYMBOL res 240 256 R0
    SYMATTR InstName R3
    SYMATTR Value 125K
    SYMBOL res 464 112 R0
    SYMATTR InstName R4
    SYMATTR Value 10K
    SYMBOL nmos -272 192 R270
    WINDOW 0 69 11 VRight 0
    WINDOW 3 -16 -37 VRight 0
    SYMATTR InstName M1
    SYMATTR Value IRL3915
    SYMBOL Misc\\battery -208 192 R270
    WINDOW 123 0 0 Left 0
    WINDOW 39 0 0 Left 0
    WINDOW 0 -53 3 VRight 0
    WINDOW 3 -54 42 VRight 0
    SYMATTR InstName V2
    SYMATTR Value 4.25V
    SYMBOL res -80 128 R90
    WINDOW 0 0 56 VBottom 0
    WINDOW 3 -56 57 VTop 0
    SYMATTR InstName R5
    SYMATTR Value 2.2K
    TEXT 304 -72 Left 0 ;N006
    TEXT -96 -64 Left 0 ;N002
    TEXT 144 248 Left 0 ;N005
    TEXT -56 40 Left 0 ;N003
    TEXT -328 -176 Left 0 !.dc V1 10 25 0.001
    TEXT -328 -144 Left 0 !.LIB D:\\SPICE\\ZMODELS.LIB
     
  8. Robert Baer

    Robert Baer Guest

    ----------- SNIPped for brevity -----------

    Offending part: the UCC2801 has a *large* negative resistance "kink";
    that is what caused the wierd curves seen.
    The compensation capacitor used was large enough, in combination to
    the gain loss in the feedback divider, allowed the regulator to operate
    smoothly at all measured points.

    And that brings up the question as to how to model such a beastie...
     
  9. Hello Robert,

    You could add a resistor in series with a negative
    resistance, e.g. -200 Ohm.
    A more complex behaviour can be modeled with B-sources.
    Be warned, dealing with negative resistance always causes
    a lot of headache with convergence.

    Btw, the DN3545 model is on the website of Supertex.
    It's a 4-pin subcircuit. This requires to change the Prefix
    in the instance of your "nmos4" to X. Don't change the
    original symbol!!! An instance is the already placed
    symbol in your schematic.

    I have send my LTspice file to your email address.
    I can't attach it here because it contains too long
    lines. Only very experienced LTspice users could restruct
    these splitted lines. If somebody eles wants this file,
    then send me an email so I can send it to you.

    Best regards,
    Helmut


    PS: We are simulating here with LTspice and not wit PSPICE.
    LTspice is free and unlimited SPICE with a GUI. It's
    also named SwitcherCADIII.
    Download it from here: www.linear.com

    There is also an independent user group. It's a moderated
    group and it requires to register for access.
    There are over 6000 messages now there and hundreds of
    examples, links and other stuff.
    http://groups.yahoo.com/group/LTspice

    I am one of the two moderators of this group.



    Supertex SPICE model
    --------------------

    *
    * MARCH 07, 2000
    *
    * COMPOSITE MODEL FOR DN3545
    *
    ..SUBCKT DN3545 1 2 3 4
    *
    * NODE 1 = DRAIN
    * NODE 2 = GATE
    * NODE 3 = SOURCE
    * NODE 4 = BODY
    *
    MOS1 11 2 3 4 ND_DMOS L=2.5E-06 W=58E-3
    JFET 1 3 11 JMOD 1
    DBODY 4 1 DMOS
    R 1 11 1E+6
    *
    ..MODEL ND_DMOS NMOS
    + LEVEL=3 UO=307 VTO=-1.829 NFS=5.0E+11
    + TOX=5E-08 NSUB=3.59E+15 NSS=0 VMAX=5E+04
    + RS=1E-06 RD=1E-06 RSH=5000 CGDO=1.96E-9
    + CGSO=1.5E-09 CGBO=0 CBD=4.0E-11 CBS=1.0E-15
    + MJ=0.5003 MJSW=0.33 IS=5E-13 PB=0.4507
    + FC=0.5 XJ=1.2E-05 LD=0 DELTA=0
    + THETA=0 ETA=1.0E-6 KAPPA=1.0E-6
    *
    ..MODEL DMOS D
    + IS=281.0E-15 N=0.950 RS=2.5
    + BV=450 IBV=1.0E-3 TT=1.0E-6
    *
    ..MODEL JMOD NJF
    + VTO=-3.5 BETA=0.100 IS=281E-15
    + RD=9.0 LAMBDA=0
    ..ENDS
     
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