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current multiplier

Discussion in 'Electronic Design' started by Winfield Hill, Feb 1, 2006.

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  1. Here's an interesting problem, with what I hope you'll find
    to be an interesting solution. One can find nice, modest
    TEC controllers with current source outputs, attractive
    units with digital readouts and IEEE-488 interfaces, etc.,
    available for low prices on eBay. I say modest, because
    these controllers might be able to provide only up to 4A
    max with a 4V compliance. It would be appealing to beef
    up these controllers to say 100W, or 10A at 10V, etc.

    If a simple 2.5x current multiplier were available, then
    an ordinary power supply could provide the power, while
    the low-power unit provided computer control of the TEC-
    driven temperature and digital readouts.

    Here's my candidate 2.5x current multiplier / booster:

    .. INPUT x2.5 current booster, 10A max out
    .. CURRENT
    .. o--, IRL2505 ,------o -- TEC ---o
    .. | Q4 |
    .. | |--' |
    .. +-----+-----+-------||<-, POWER
    .. | | | |--+ SUPPLY
    .. | R3 R4 | 10V at 10V
    .. | 150 100 | |
    .. V\| | | |
    .. Q1 |---+ +-----, |
    .. /| | | | |
    .. | \| | |/ |
    .. | |---+---| |
    .. | V/| |\V | Iout = Iin R1/R2
    .. | | Q2 Q3 | |
    .. +-----' '-----+ Q1 - TIP145 Darlington
    .. | | Q2, Q3 - matched 10mA NPN
    .. R1 R2 Q4 - IRL2505 logic-level
    .. 0.25 0.10 R1, R2 - 10W stable WW 1%
    .. | |
    .. o--+-----------------------+-------o

    This circuit is a power current-mirror, although perhaps not
    like most you've seen.

    Q2 and Q3 are a common-base differential amplifier amplifying
    the voltage difference across R1 and R2. Q2 drives Darlington
    power transistor Q1, which aborbs all but 10mA of the master
    current-source input and passes it along to input current-sense
    resistor R1. The diff amp drives Q1 and thence output FET Q4,
    so the voltage across output current-sense resistor R2 matches
    the voltage across R1. The MOSFET must be a logic-level type.
    The IRL2505 is a 200W TO-220 FET, with Vgs = 2.8 volts at 10A.
    Its gate voltage biases Q3 at about 11mA, set by R4, the value
    isn't critical. R3 sets a similar current through Q2. With
    the values shown, the current booster drops 3.8V at 4A input.

    The result is a small box simply marked x2.5, with 4 terminals,
    marked IN and OUT. It has a BIG heat sink, true, but no power
    cord and no ON/OFF switch.
     
  2. John  Larkin

    John Larkin Guest

    Bet you could also do it with an opamp powered off the input. Or,
    given that it's probably inside a pid loop, why not just...

    +-------------
    |
    d
    --------+-----------g fet or bipolar
    | s
    | |
    r r
    | |
    gnd gnd



    Which raises the idea of making a current mirror that *adds* to the
    input current, rather than wasting it in a resistor and replicating a
    copy of it. Yes, there is the compliance issue in this case.

    John
     
  3. Phil Hobbs

    Phil Hobbs Guest

    Interesting idea--I like the common-base diff pair. I'm a little
    puzzled, though: Since your TEC controller is always going to be used
    closed-loop, why worry so much about accuracy in the booster?

    For lower-current applications I usually use an unbalanced current
    mirror--connect the output transistor's collector to the supply, come in
    the input, and come out via the common emitters (the TL014 was my fave,
    *sniff*). You could do this here too--a couple of 2N3055s, with 0.15
    ohm and 0.10 ohm emitter resistors, and save a lot of power, with under
    a volt headroom loss.


    Cheers,

    Phil Hobbs
     
  4. Phil Hobbs wrote...
    It's not very accurate, 2 to 3%, just enough so you can label it
    x 2.5, instead of "booster," without saying how much. A primary
    motivation for a semblance of accuracy is so the controller's
    digital meter still has meaning.
    I agree, I used the TL014 in several of my low-power products,
    buying a few thousand, but still not enough to persuade TI to
    keep it. I think part of the problem was where to put it in
    the catalog so engineers would find it. I still have a few
    left, but they're too precious to use.
    Yes, that was my first idea, but I figured a few more parts
    would give me that "semblance of accuracy," and also let me
    use a MOSFET, which is better suited for delivering power to
    a heatsink.

    There's probably a good low-V opamp for the job. Its lower
    offset voltage would mean smaller resistors could be used.

    .. INPUT x2.5 current booster, 10A max out
    .. CURRENT
    .. o--, IRL2505 ,------o -- TEC ---o
    .. | Q4 |
    .. | |--' |
    .. +--------+------||<-, POWER
    .. | | |--+ SUPPLY
    .. | _| | |
    .. V\| / -|-----, | 10V at 10V
    .. Q1 |---< | | | |
    .. /| \_+|--, | |
    .. | | | | | Iout = Iin R1/R2
    .. +------- |---' '---+ Q1 - TIP145 Darlington
    .. | | | Q4 - IRL2505 logic-level
    .. R1 | R2 R1, R2 - 3W stable WW 1%
    .. 0.10 | 0.04
    .. | | |
    .. o--+--------+----------+-------o
     
  5. Joerg

    Joerg Guest

    Hello Win,
    There are quite a few of those "too precious to use" products from
    unobtanium land. When the last order bell was clanging for the LH0063
    high speed buffer I rushed to stock up on those. Plus some crosspoint
    switches that were vanishing around that time as well, never to be
    replaced by anything else. That prevented a lot of grief when I had to
    create a hi-Z test fixture for the HP3577.

    In the same way I treasure a little stash of Ge diodes and transistors.
    It's just too easy to build sensitive passive RF detectors with these.
    No batteries that can leak.

    Regards, Joerg
     
  6. Winfield Hill wrote...
    I don't know which is better, a PNP or an NPN Darlington...
    (Note the change in opamp sign.)

    .. INPUT x2.5 current booster, 10A max out
    .. CURRENT
    .. o--, IRL2505 ,------o -- TEC ---o
    .. | Q4 |
    .. | |--' |
    .. +--------+------||<-, POWER
    .. | | |--+ SUPPLY
    .. | _| | |
    .. \| / +|-----, | 10V at 10V
    .. Q1 |---< | | | |
    .. V/| \_-|--, | |
    .. | | | | | Iout = Iin R1/R2
    .. +------- |---' '---+ Q1 - TIP145 Darlington
    .. | | | Q4 - IRL2505 logic-level
    .. R1 | R2 R1, R2 - 3W stable WW 1%
    .. 0.10 | 0.04
    .. | | |
    .. o--+--------+----------+-------o

    It may have a bearing on feedback-loop stability. There's
    a question of what the input 4A current source looks like.
     
  7. Jim Thompson

    Jim Thompson Guest

    Yep, It'll be unstable.

    Since the ratio is not mission critical, why not just do an NPN
    Darlington mirror?

    ...Jim Thompson
    --
    | James E.Thompson, P.E. | mens |
    | Analog Innovations, Inc. | et |
    | Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
    | Phoenix, Arizona Voice:(480)460-2350 | |
    | E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
    | http://www.analog-innovations.com | 1962 |

    The harder you work the luckier you get.
     
  8. Jim Thompson wrote...
    Hmm, matched Darlington transistors...

    .. INPUT x2.5 current booster,
    .. CURRENT 10A max out
    .. o--, ,----+---o
    .. | | |
    .. +-----, | |
    .. | | | |
    .. \| | |/ |/
    .. |---+---|----| TIP145
    .. V/| |\V |\V
    .. | Q2 Q3 | | Q4 Iout = Iin 2 R1/R2
    .. | | |
    .. R1 R2 R3 Q2-Q4 - matched Darlington
    .. 0.125 0.1 0.1 R1-R3 - 5W stable WW 1%
    .. | | |
    .. o--+-----------+----+---o

    I wonder if matched power NPNs wouldn't be better?

    .. INPUT x2.5 current booster,
    .. CURRENT 10A max out
    .. o--, ,-----+----+---o
    .. | | | |
    .. Rx | | |
    .. | |/ | |
    .. +---| Q1 | |
    .. | |\V | |
    .. \| | |/ |/
    .. |---+---|----| TIP35
    .. V/| |\V |\V Iout = Iin 2 R1/R2
    .. | Q2 Q3 | | Q4
    .. | | | Q1 - 100mA NPN
    .. R1 R2 R3 Q2-Q4 - matched 5A NPN
    .. 0.125 0.1 0.1 R1-R3 - 5W stable WW 1%
    .. | | |
    .. o--+-----------+----+---o

    Perhaps resistor Rx would help stabilize the circuit,
    without bothering the input current source.
     
  9. Jim Thompson

    Jim Thompson Guest

    There you go, Win, you're learning to be an I/C designer... 3 matched
    transistors and one "helper" ;-)

    Split your stabilizing resistor and put some of it in the emitter of
    Q1.

    ...Jim Thompson
     
  10. Phil Hobbs

    Phil Hobbs Guest

    They made a pretty nice 5x current multiplier as well as a 4x mirror.
    I even have some LH4009s--a very short-lived improved LH0063, sold in
    1989-1990. Anyone remember the 1989 National Linear book that suddenly had
    all these hybrid devices that lasted a year or so before being discontinued?
    I'd love to know how that happened there--pretty counterintuitive to be
    bringing out a lot of hybrids at that late date.

    Cheers,

    Phil Hobbs
     
  11. Joerg

    Joerg Guest

    Hello Phil,
    Maybe a management shake-up? Anyway, I learned a lesson in the 80's. If
    a component is almost too good to be true, it usually is. Haven't really
    been burned by it but seen enough others.

    I do miss hybrids though. Designed quite a few.

    Regards, Joerg
     
  12. Jim Thompson wrote...
    Matched transistors maybe, but with severely mismatched
    operating temperatures, due to Vce mismatch. More parts
    are needed to complete the task and insure matched Vbe,
    allowing small lower-power Rs. There, this should do it.

    .. INPUT x3 current booster,
    .. CURRENT 12A max out, 1.8V overhead
    .. o-----, ,-----+----+----+---o
    .. to 4A | | | | |
    .. --> Rx | | | |
    .. | |/ Q10 | | |
    .. +-------| | | |
    .. | |\V | | |
    .. \| | |/ |/ |/
    .. |--+----+---|----|----| TIP35
    .. V/| | | |\V |\V |\V
    .. Q6 | | |/ | | | Q9
    .. | '--| Q1 | | |
    .. | |\V | | |
    .. \| | |/ |/ |/
    .. |-------+---|----|----| TIP35
    .. V/| |\V |\V |\V Iout = Iin 3 R1/R2
    .. Q2 | Q3 | | Q4 | Q5
    .. | | | | Q1, Q10 - 400mA NPN
    .. R1 R2 R3 R4 Q2-Q5 - matched 5A NPN
    .. 0.10 0.1 0.1 0.1 Q6-Q9 - more 5A NPN
    .. | | | | R1-R3 - 3W stable WW 1%
    .. o------+---------------+----+----+---o

    Hmm, it's not clear to me what would be going on there, why
    would that help?
     
  13. Jim Thompson

    Jim Thompson Guest

    I've seen them oscillate due to capacitance on the emitter of the
    helper.

    ...Jim Thompson
     
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