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Voltage controlled 10V->100V DC-DC converter

Discussion in 'Electronic Design' started by [email protected], May 26, 2005.

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  1. Guest

    Is there an easy way to make a voltage controlled DC-DC converter with
    say a 0-10V input and a 0-100V output? MAXIM has an app note for
    something like this, though it's controlled by resistors as far as I
    can tell.

    http://www.maxim-ic.com/appnotes.cfm/appnote_number/1751

    I might be able to use a FET as a voltage controlled resistor (this may
    show my complete lack of knowledge about circuits...) and therefor
    control the output voltage though. I need to make ~100 of these, so I
    need it to be kinda small -- a chip with some R/C/L/diodes would be
    preferable.


    Jesse
     
  2. John Fields

    John Fields Guest

     
  3. Guest

    Output current is going to be in the low mA range, though I don't have
    an exact number (the load is almost entirely capacitive, with a very
    high R in parallel). I was hoping that the physical size would be
    something like 1"x1" on a PC board, if possible. And as far as
    efficiency, I assume you mean

    Power out / Power in ?

    If so, I can deal with very low efficiency. I'd have no problem having
    a standard rack HP power supply, or a few of them, powering the whole
    thing. As far as a control input voltage, I'll be controlling these
    DC-DC convters via CPU (with a National Instruments analog I/O board)
    that can supply -10V-10V analog (I can do digital too, but sounds more
    complicated).
     
  4. wrote...
    I think you want a linear high-voltage opamp, such as those
    made by Apex, powered from a small 150V dc-dc switching power
    supply. Setup the opamp for a gain of 10x. Yawn.
     
  5. John Fields

    John Fields Guest


    I don't know much about switching supplies, but how about something
    like this:


    +12V>--+-----------------------------------+
    | | Vc Vout
    [L1] | 0-10 0-100
    | | | /
    +---[CR1>]--------------------------|---|---+--->>---+
    | | | | |
    | | | | |
    C A---+ /+|--+ [R3] |
    Q1 B--[R1]-+-Y NAND | A-------< | | +---+
    E | B---+-+-Y NAND \-|------+ | |
    | | | B---+ | | [C] [R]
    | | | | | | | |
    | | +--[R2]--+ | [R4] +---+
    | | | | | |
    | +--------[C1]-------+ | | |
    | | | |
    GND>---+-----------------------------------+-------+--->>---+

    Basically a boost converter where the two NANDs form a gated
    oscillator which turns Q1 on and off, forcing current through L1 and
    then shutting it off abruptly which makes a spike that gets integrated
    by C (your capacitor). As the stream of spikes charges C, the voltage
    at the junction of R3 and R4 (a 10:1 voltage divider) rises until it
    equals the voltage on the comparator's + input, at which time the
    output of the comparator goes low, stopping the oscillator and turning
    Q1 off. Then, when R (your load resistance) discharges C to the point
    where the voltage on the - input of the comparator goes lower than the
    voltage on the + input, the comparator's output will go high again,
    starting the oscillator and recharging C.
     
  6. Fred Bloggs

    Fred Bloggs Guest

    A few gotchas to watch for in that boost converter are that 1) the 12V
    feeds through L+CR1 to output when the transistor is off so that output
    is never less than 12V and 2) the steady state Vout/Vin= 1/(1-Duty)
    where Duty is transistor switch duty cycle- so you would want that to be
    about 0.1 or so when Vin=12V. A 50% duty cycle gives you a maximum of
    24V for Vout.
     
  7. Fred Bloggs

    Fred Bloggs Guest

    Voops->(1-Duty) is about 0.1 which makes Duty=0.9 for a x10 boost...
     
  8. Genome

    Genome Guest

    Not if it's operating discontinuously.

    How is Barry these days?

    DNA
     
  9. Mebart

    Mebart Guest

    An interesting circuit, and one that I had not seen. Thanks for
    pointing it out. But, it appears you need slightly more output current
    than the Maxim circuit can supply. You can add an outboard totem pole
    driver to beef up the Maxim circuit, it should work well for you.

    I might use paralleled cmos bus driver gates in a free running gated
    oscillator and feed that input into the Maxim step up circuit. Use a
    micropower comparator with a built in reference voltage to gate the
    oscillator on or off as needed to maintain the desired output. Sample
    the output and feed the stepped down voltage into the comparator to be
    the input for the gating.

    Also consider an EL (not ccfl) inverter chip to make your high
    voltage. They run with a simple inductor and produce constant current
    AC up to the clamp voltage that you select with resistors. One of the
    nicer ones is below and they do have an evaluation module available.
    Read on.

    If you want a small chip with few components, 1cap, 1 inductor and a
    Rogers D355B chip give you 200v p-p and is designed to drive
    capacitive loads (EL backlight displays). There is a press release at:

    http://www.rogers-corp.com/durel/new_el_driver.htm

    The complete specs and design notes are at:

    http://www.rogers-corp.com/durel/pdf/lit-i9034a07.pdf

    Rogers also sells a module with all components included, not sure
    about the module price. But, it's worth a look to see. The pdf file
    has many design notes including add on's for additional features.
    There's a PWM input in case you want to regulate the output voltage.

    Regards,

    Mebart
     
  10. Mebart

    Mebart Guest

    Rogers also has a complete chip assembly, including the inductor all
    in a tiny smt package. Sorry, forget to mention that before i hit the
    send button on the last message.

    Look at:

    http://www.rogers-corp.com/durel/pdf/LIT-I9052A03.pdf

    for information on the turn key module.

    M
     
  11. Winfield Hill wrote...
    I didn't notice you were going to make 100 of these, powered from a
    single power supply. The APEX amplifiers are easy to use, but they
    are expensive. I still think a linear solution bests a switching
    supply solution, but with 100 units something simple and cheap is in
    order. I suggest you use a circuit like fig 3.75 in our book, except
    with transistors instead of FETs. Here's one that should work well:

    .. ---+-------+-- 130 to 150V HV supply
    .. | | 100 amplifiers requires
    .. R4 10k R5 70mA maximum (all at 0V)
    .. Simple, Cheap 220k |
    .. Precision Slow | |/ Q2
    .. HV Amplifier +-----| mpsA42
    .. | |\v
    .. G = 1 + R2/R1 | | R3
    .. +--|<|--+--+--- 2.2k ---- out
    .. | 1n4148 | 0 to 110V
    .. |/ | +/- 4mA
    .. gnd --| mpsA42 |
    .. |\v Q1 |
    .. 0-10V __quad opamp | |
    .. in -----|+ \ R6 | R2
    .. | >--+-- 3.3k --' 1.00M 1%
    .. ,--|-_/ | |
    .. | === 330pF |
    .. | | C1 |
    .. '---------+---------------------+
    .. | R1
    .. +/-15v opamp supply 100k 1%
    .. |
    .. gnd

    I've used a common-base non-inverting high-voltage stage to allow
    simple one-cap C1 feedback-loop compensation. R5 and R6 provide
    short-circuit limiting. R3 isolates the feedback loop from large
    capacitive loads, but can be eliminated if you increase C1. Note,
    the mpsA42 transistors can handle 300 volts, so the output range
    could easily be increased.

    You haven't mentioned the accuracy or speed you need. If these are
    very relaxed specs, you can eliminate the opamp and drive the level-
    shifting stage directly from your D-A stage, using the -10 to 0V
    part of your D-A range. Although a regulated HV supply is required,
    this is a dramatically simplified circuit!

    .. ---+-------+------- Vcc = 120V
    .. | | regulated HV supply
    .. R4 27k R5 60mA max for 100 amps
    .. 200k 1% |
    .. Super Simple | |/ Q2
    .. no-feedback +-----| mpsA42
    .. HV Amplifier | |\v
    .. | |
    .. +--|<|--+---+------ out
    .. | 1n4148 | 0 to 110V
    .. |/ 220k +/- 1mA
    .. gnd --| mpsA42 |
    .. |\v Q1 gnd
    .. |
    .. R6 | R4
    .. -10 to 0V in --- 12.0k --' Vout = Vcc - -- (-Vin - 0.65V) - 0.65V
    .. 1% R6

    There's a slight nonlinearity, especially near the maximum output
    voltage, where the D-A converter signal approaches Q1's 0.65V Vbe.
    But you could easily calibrate it out, because it'll be the same
    for all 100 amps and it won't change much at room temperature. If
    the 1st 0.65V term in the equation is too much of a pain, you could
    create a +0.65V point and tie all 100 Q1 bases to that.

    Or you can add 25 quad opamps to eliminate the Q1 offsets:

    .. ---+-------+------- Vcc = 120V +/-0.5V
    .. | | regulated HV supply
    .. R4 27k R5 60mA max for 100 amps
    .. 200k 1% |
    .. Simple 2% Precise | |/ Q2
    .. HV Amplifier +-----| mpsA42
    .. | |\v
    .. | |
    .. 1/4 LM324 +--|<|--+---+------ out
    .. __ | 1n4148 | 0 to 110V
    .. gnd ----|+ \ |/ 220k +/- 1mA
    .. | >-----| mpsA42 |
    .. ,--|-_/ |\v Q1 gnd
    .. | |
    .. '--------------+
    .. R6 | R4
    .. -10 to 0V in --- 15.0k --' Vout = Vcc - -- ( -Vin ) - 0.65V
    .. 1% R6

    The last 0.65V term comes from the Vbe drop of Q2. There's still
    a small missing contribution to the output equation coming from
    Q1's base current, but that should be under 1% for typical parts.
    If that concerns you, make Q1 a Darlington with two mpsA42 parts.

    I think if you add it all up, this'll beat 100 switching circuits.
     
  12. Guest

    Okay, thanks, gotta sort all this info out.
     
  13. Guest

    And sorry, what is "our book"?
     
  14. Mebart

    Mebart Guest

    Jesse,

    Are you going to have HV generators with 100 separate outputs and 100
    separate (0-10v) inputs, all controlled independently?

    I thought you needed 100 high voltage supplies running independently
    and that you needed to control the outputs based on an independent
    analog input, hence my suggestion of the low power EL inverter chip
    which could be controlled with through the dimming input.

    I'm wondering, can you have a single (relatively large and powerful)
    100 volt source with 100 outputs that are (effectively) electronic
    variable resistors???? I don't think there are any cheap low power
    high voltage op amps out there, they usually tend to be high voltage
    and high power (and expensive).

    If you have a single big 100 volt power supply, I'd think that a PWM
    chip hooked to simple switching transistor would do the job. There are
    some low cost low power pwm chips available.

    M
     
  15. Fred Bloggs

    Fred Bloggs Guest

    Oh yeah- he said:"Output current is going to be in the low mA range,
    though I don't have an exact number (the load is almost entirely
    capacitive, with a very high R in parallel)"- so discontinuous it is.
    I suppose he's out there ripping off some gullible idiot with flash and
    dazzle techno babble that will amount to nothing eventually...
     
  16. Guest

    Um, I'm sorry Mr. Hill, I had no idea that you were actually of
    "Horowitz and Hill" fame (gulp).
     
  17. John Fields

    John Fields Guest

     
  18. Mebart

    Mebart Guest

    It looks like an absolutely outstanding book, but an expensive one.
    Are there any plans for an updated version??

    M
     
  19. Mebart wrote...
    Its cheap for its size and content. We'll have a new edition in a
    few years, if we can pick up the writing pace. But, respectfully,
    it looks like Jesse needs help now. He should get a copy. You can
    get one too, Mebart. :>)
     
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