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Slow Start Switched DC Output

Discussion in 'Electronic Design' started by [email protected], May 8, 2007.

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  1. Guest

    I want to switch DC power to an instrument "slowly". I am using a FET
    to switch the power to the instrument. Could someone direct me to a
    circuit that would slowly turn on the FET so that the voltage to the
    instrument ramps up from 0 to 28 V at a rate not to exceed 10V/
    millisecond. Thanks in advance.
     
  2. On 8 May 2007 03:36:17 -0700, in sci.electronics.design
    LT1910 from linear, check out fig 5 in the data sheet


    martin
     
  3. John Fields

    John Fields Guest

    ---
    View in Courier:

    P CHANNEL MOSFET
    /
    /
    28IN>------+----------S D---+-----28OUT>
    | G |
    [C] | |
    | | |
    +--[ZENER>]--+ [1µF]
    | |
    [R] |
    | |
    GND>-------+------------------+----GND>


    LTSPICE circuit list:

    Version 4
    SHEET 1 880 680
    WIRE -128 144 -256 144
    WIRE 32 144 -128 144
    WIRE 176 144 128 144
    WIRE 288 144 176 144
    WIRE -128 176 -128 144
    WIRE 176 256 176 144
    WIRE 288 256 288 144
    WIRE -128 288 -128 240
    WIRE -80 288 -128 288
    WIRE 48 288 48 192
    WIRE 48 288 -16 288
    WIRE -256 336 -256 144
    WIRE -128 336 -128 288
    WIRE -256 448 -256 416
    WIRE -128 448 -128 416
    WIRE -128 448 -256 448
    WIRE 176 448 176 320
    WIRE 176 448 -128 448
    WIRE 288 448 288 336
    WIRE 288 448 176 448
    WIRE -256 480 -256 448
    FLAG -256 480 0
    SYMBOL res -144 320 R0
    WINDOW 0 43 40 Left 0
    WINDOW 3 38 69 Left 0
    SYMATTR InstName R1
    SYMATTR Value 10k
    SYMBOL cap -144 176 R0
    WINDOW 0 -39 31 Left 0
    WINDOW 3 -45 60 Left 0
    SYMATTR InstName C1
    SYMATTR Value 10e-6
    SYMBOL voltage -256 320 R0
    WINDOW 3 24 104 Invisible 0
    WINDOW 123 0 0 Left 0
    WINDOW 39 0 0 Left 0
    SYMATTR InstName V1
    SYMATTR Value PULSE(0 28 0 1e-6)
    SYMBOL res 272 240 R0
    SYMATTR InstName R2
    SYMATTR Value 10
    SYMBOL pmos 128 192 M270
    SYMATTR InstName M1
    SYMATTR Value Si7469DP
    SYMBOL zener -80 304 R270
    WINDOW 0 24 90 VTop 0
    WINDOW 3 5 112 VBottom 0
    SYMATTR InstName D1
    SYMATTR Value BZX84C15L
    SYMBOL cap 160 256 R0
    WINDOW 0 43 29 Left 0
    SYMATTR InstName C2
    SYMATTR Value 1e-6
    TEXT -242 466 Left 0 !.tran .1
     
  4. MooseFET

    MooseFET Guest

    That ASCII art looks wrong to me. I assume the Zener's location is an
    error. The zener should be from the source to the gate.


    You should also put a diode across the resistor. If the 28V input
    goes to zero and then back up to 28, the output would follow without
    delay. With the diode, the slow start would repeat.
     
  5. Fred Bartoli

    Fred Bartoli Guest

    John Fields a écrit :
    This is certainly not the best way to do this (highly dependent on
    mosfet gm).

    A prefered way would be using the fet as a miller integrator:

    28V >--+------+^+-----+-------> out
    | ||| |
    | === |
    | | |
    12V z | |
    A .-. |
    | | | |
    | 100R| | |
    | '-' |
    | | || |
    +------+---||--'
    | | ||
    | .-. 10n
    - | |
    ^ | | 330K
    | '-'
    | |
    | |
    === ===
    GND GND
     
  6. John Fields

    John Fields Guest

    ---
    Shouldn't matter too much, since all he's looking for is a _minimum_
    rise time,
    ---

    ---
    Except that (Unless I did something wrong) it doesn't work:

    Version 4
    SHEET 1 1240 680
    WIRE -64 144 -192 144
    WIRE 112 144 -64 144
    WIRE 256 144 208 144
    WIRE 368 144 256 144
    WIRE -64 192 -64 144
    WIRE 128 208 128 192
    WIRE 368 288 368 144
    WIRE -64 320 -64 256
    WIRE 128 320 128 288
    WIRE 128 320 -64 320
    WIRE 192 320 128 320
    WIRE 256 320 256 144
    WIRE -192 368 -192 144
    WIRE -64 368 -64 320
    WIRE 128 368 128 320
    WIRE -192 480 -192 448
    WIRE -64 480 -64 432
    WIRE -64 480 -192 480
    WIRE 128 480 128 448
    WIRE 128 480 -64 480
    WIRE 368 480 368 368
    WIRE 368 480 128 480
    WIRE -192 512 -192 480
    FLAG -192 512 0
    SYMBOL cap 256 304 R90
    WINDOW 0 0 32 VBottom 0
    WINDOW 3 32 32 VTop 0
    SYMATTR InstName C1
    SYMATTR Value 10e-9
    SYMBOL voltage -192 352 R0
    WINDOW 3 24 104 Invisible 0
    WINDOW 123 0 0 Left 0
    WINDOW 39 0 0 Left 0
    SYMATTR Value PULSE(0 28 0 1e-6 1e-6 .05 .1)
    SYMATTR InstName V1
    SYMBOL res 352 272 R0
    SYMATTR InstName R2
    SYMATTR Value 10
    SYMBOL pmos 208 192 M270
    SYMATTR InstName M1
    SYMATTR Value Si7469DP
    SYMBOL zener -48 256 R180
    WINDOW 0 -52 30 Left 0
    WINDOW 3 -132 -8 Left 0
    SYMATTR InstName D1
    SYMATTR Value BZX84C12L
    SYMBOL res 112 192 R0
    WINDOW 0 43 40 Left 0
    WINDOW 3 38 69 Left 0
    SYMATTR InstName R5
    SYMATTR Value 100
    SYMBOL res 112 352 R0
    WINDOW 0 43 40 Left 0
    WINDOW 3 38 69 Left 0
    SYMATTR InstName R6
    SYMATTR Value 330k
    SYMBOL diode -48 432 R180
    WINDOW 0 -46 29 Left 0
    WINDOW 3 -90 -5 Left 0
    SYMATTR InstName D2
    SYMATTR Value 1N4148
    TEXT -176 496 Left 0 !.tran .2



    Also, I had some errors in my original circuit which I fixed:

    P CHANNEL MOSFET
    /
    /
    ..28IN>--+---+----------S D--------28OUT>
    .. | | G
    .. | [C] |
    .. | | |
    .. | +--[DIODE>]--+
    .. [R] | |
    .. | +--[<DIODE]--+
    .. | | |K
    .. | [R] [DIODE]
    .. | | |
    ..GND>---+---+------------+----------GND>

    and it seems to run OK now:

    Version 4
    SHEET 1 880 680
    WIRE 128 64 -16 64
    WIRE 256 64 128 64
    WIRE 416 64 256 64
    WIRE 688 64 512 64
    WIRE 256 96 256 64
    WIRE -16 176 -16 64
    WIRE 64 192 32 192
    WIRE 256 208 256 160
    WIRE 320 208 256 208
    WIRE 432 208 432 112
    WIRE 432 208 384 208
    WIRE 688 224 688 64
    WIRE 128 256 128 64
    WIRE -16 288 -16 256
    WIRE 32 288 32 240
    WIRE 32 288 -16 288
    WIRE 256 320 256 208
    WIRE 320 320 256 320
    WIRE 432 320 432 208
    WIRE 432 320 384 320
    WIRE -16 336 -16 288
    WIRE 256 368 256 320
    WIRE 432 368 432 320
    WIRE -16 480 -16 416
    WIRE 64 480 64 192
    WIRE 64 480 -16 480
    WIRE 128 480 128 336
    WIRE 128 480 64 480
    WIRE 256 480 256 448
    WIRE 256 480 128 480
    WIRE 432 480 432 432
    WIRE 432 480 256 480
    WIRE 688 480 688 304
    WIRE 688 480 432 480
    WIRE -16 560 -16 480
    FLAG -16 560 0
    SYMBOL res 240 352 R0
    WINDOW 0 43 34 Left 0
    WINDOW 3 38 61 Left 0
    SYMATTR InstName R1
    SYMATTR Value 100k
    SYMBOL cap 240 96 R0
    WINDOW 0 -39 31 Left 0
    WINDOW 3 -45 60 Left 0
    SYMATTR InstName C1
    SYMATTR Value 10e-6
    SYMBOL voltage -16 320 R0
    WINDOW 3 24 104 Invisible 0
    WINDOW 123 0 0 Left 0
    WINDOW 39 0 0 Left 0
    SYMATTR Value PULSE(0 28 0 1e-6 1e-6 1 2)
    SYMATTR InstName V1
    SYMBOL res 672 208 R0
    SYMATTR InstName R2
    SYMATTR Value 10
    SYMBOL pmos 512 112 M270
    SYMATTR InstName M1
    SYMATTR Value Si7469DP
    SYMBOL diode 448 432 R180
    WINDOW 0 -46 29 Left 0
    WINDOW 3 -75 -4 Left 0
    SYMATTR InstName D3
    SYMATTR Value 1N4148
    SYMBOL diode 320 336 R270
    WINDOW 0 36 32 VTop 0
    WINDOW 3 -4 32 VBottom 0
    SYMATTR InstName D2
    SYMATTR Value 1N4148
    SYMATTR Description Zener Diode
    SYMATTR Type zener
    SYMBOL diode 384 192 R90
    WINDOW 0 0 32 VBottom 0
    WINDOW 3 32 32 VTop 0
    SYMATTR InstName D4
    SYMATTR Value 1N4148
    SYMATTR Description Zener Diode
    SYMATTR Type zener
    SYMBOL sw -16 160 M0
    WINDOW 0 32 15 Left 0
    WINDOW 3 32 44 Left 0
    SYMATTR InstName S1
    SYMBOL res 112 240 R0
    SYMATTR InstName R3
    SYMATTR Value 1000
    TEXT -8 536 Left 0 !.tran 3
    TEXT -8 504 Left 0 !.model SW SW(Ron=1e-3 Roff=10Meg Vt=0.5Vh=0)
     
  7. Fred Bartoli

    Fred Bartoli Guest

    John Fields a écrit :
    You're right: in a hurry I got it wrong and the integrator reference
    should have been GND, not the 28V input :-(

    So:

    .-------------.
    | |
    28V >--+-------+-----+^+-----)----+--> out
    | | ||| | |
    | | === | |
    | | | | |
    12V z | | 330K.-. |
    A 330K.-. .-. | | --- 10n
    | | | | | | | ---
    | | | | | '-' |
    | '-' '-'100R | |
    | | | | |
    '-------+-----+ .---+ |
    | | | |
    \| | |/ |
    |-+-| |
    <| | |> |
    | - | |
    | ^ | |
    | | | |
    +---+--------+
    | | |
    .-. .-. -
    | | | | ^
    10K| | 10K| | |
    '-' '-' |
    | | |
    === === ===
    GND GND GND

    But you're still in error: your circuit is a current source, which means
    that the dv/dt depends on the load. Now make the output load 100R or 1K
    or 10K and see what happens.
     
  8. John Fields

    John Fields Guest

    It still doesn't work:


    Version 4
    SHEET 1 880 804
    WIRE -32 -80 -304 -80
    WIRE 400 -80 -32 -80
    WIRE -32 64 -32 -80
    WIRE 64 64 -32 64
    WIRE 160 64 64 64
    WIRE 576 64 256 64
    WIRE 688 64 576 64
    WIRE -32 144 -32 64
    WIRE 64 144 64 64
    WIRE 176 144 176 112
    WIRE 400 144 400 -80
    WIRE 576 240 576 64
    WIRE -32 256 -32 208
    WIRE 64 256 64 224
    WIRE 64 256 -32 256
    WIRE 176 256 176 224
    WIRE 176 256 64 256
    WIRE 176 288 176 256
    WIRE 400 288 400 224
    WIRE 288 336 240 336
    WIRE 336 336 288 336
    WIRE 688 336 688 64
    WIRE 288 400 288 336
    WIRE 176 512 176 384
    WIRE 288 512 288 464
    WIRE 288 512 176 512
    WIRE 576 512 576 304
    WIRE 576 512 288 512
    WIRE -304 560 -304 -80
    WIRE 176 576 176 512
    WIRE 400 576 400 384
    WIRE 576 576 576 512
    WIRE -304 704 -304 640
    WIRE 176 704 176 656
    WIRE 176 704 -304 704
    WIRE 400 704 400 656
    WIRE 400 704 176 704
    WIRE 576 704 576 640
    WIRE 576 704 400 704
    WIRE 688 704 688 416
    WIRE 688 704 576 704
    WIRE -304 784 -304 704
    FLAG -304 784 0
    SYMBOL voltage -304 544 R0
    WINDOW 3 24 104 Invisible 0
    WINDOW 123 0 0 Left 0
    WINDOW 39 0 0 Left 0
    SYMATTR Value PULSE(0 28 0 1e-6 1e-6 1 2)
    SYMATTR InstName V1
    SYMBOL pmos 256 112 M270
    SYMATTR InstName M1
    SYMATTR Value Si7469DP
    SYMBOL cap 560 240 R0
    SYMATTR InstName C2
    SYMATTR Value 10e-9
    SYMBOL res 672 320 R0
    SYMATTR InstName R2
    SYMATTR Value 100
    SYMBOL npn 240 288 M0
    SYMATTR InstName Q1
    SYMATTR Value 2N3904
    SYMBOL npn 336 288 R0
    SYMATTR InstName Q2
    SYMATTR Value 2N3904
    SYMBOL diode 304 464 R180
    WINDOW 0 24 72 Left 0
    WINDOW 3 24 0 Left 0
    SYMATTR InstName D1
    SYMATTR Value 1N4148
    SYMBOL res 384 560 R0
    SYMATTR InstName R1
    SYMATTR Value 10k
    SYMBOL res 160 560 R0
    SYMATTR InstName R3
    SYMATTR Value 10k
    SYMBOL diode 592 640 R180
    WINDOW 0 24 72 Left 0
    WINDOW 3 24 0 Left 0
    SYMATTR InstName D2
    SYMATTR Value 1N4148
    SYMBOL res 160 128 R0
    SYMATTR InstName R4
    SYMATTR Value 100
    SYMBOL res 384 128 R0
    SYMATTR InstName R5
    SYMATTR Value 330k
    SYMBOL zener -16 208 R180
    WINDOW 0 24 72 Left 0
    WINDOW 3 24 0 Left 0
    SYMATTR InstName D3
    SYMATTR Value BZX84C12L
    SYMBOL res 48 128 R0
    SYMATTR InstName R6
    SYMATTR Value 330k
    TEXT -280 768 Left 0 !.tran 10

     
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