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need pwm controller

Discussion in 'Electronic Design' started by Yzordderrex, Apr 28, 2005.

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  1. Yzordderrex

    Yzordderrex Guest

    I'm looking for a very simple pwm controller. I would prefer just 8
    pins and an absolute minimum of functions. Maybe soft start and cycle
    by cycle Ilimit. I don't even need an error amp. I actually thought
    about doing discreet, and may end up that way.

    Application is for a forward off line controller to be used as a
    modulator for a rf deck. 375w carrier 1500wpep. Voltage mode control
    0-100vdc output.

    I looked at the TI UCC35701 part, but looks like I will have some
    difficulty dealing with their fb scheme.

    regards,
    Bob
     
  2. Pooh Bear

    Pooh Bear Guest

    How do you plan to implement *voltage mode control* with no error amp ?


    Graham
     
  3. Pooh Bear (Graham) wrote...
    The ucc35701 is a nice chip. I imagine he's surprised by its
    "voltage feedforward" capability and uncertain what to do with
    it. He could ignore it by setting it to a fixed value, but it's
    too valuable a capability to ignore. The Vff voltage sets Irt,
    and is used to instantaneously set the Ct capacitor charge rate,
    thereby proportionally setting the pulse width without using the
    feedback loop, which is useful for offline applications (in the
    longer term it has no effect, since Vrt/Irt cancel each other).
    A little power-line sag or dip comes along, no problem. Note,
    older-style non-PFC smps are especially susceptible to the line-
    dip issue, because they work off the very peak-ety-peak voltage,
    and respond to any lack of pure power-line sine-wave integrity.

    While Yzordderrex could use the rf output as a voltage-feedback
    term, he may be wise to add a driver output voltage-limit path.
    If he wants more applications info, he can check out Unitrode's
    lower-frequency ucc3570 predecessor.
     
  4. Yzordderrex

    Yzordderrex Guest

    Thanks for replies.

    I would use an outboard error amp. Hell, I would settle for a nice
    triangle generator in 8 pins.

    As far as the feedforward pin goes I know what to do with it, and give
    it the respect it deserves. There is a group of... well, here, A
    thread I am involved in on another board. I am N9NEO. Guys there
    build these modulators that are totally open loop and require massive
    capacitor banks in order to get the 120Hz hum out of carrier. They DO
    NOT use the feedforward pin. I have already made the suggestion they
    use that pin.

    http://classe.monkeypuppet.com/viewtopic.php?t=443


    Ok, so maybe I will take another look at the 35701 or 3570. I have
    already built a full bridge modulator that does full legal power and
    fits into a small cigar box. This forward switching converter is going
    to feed into an analog output section. Just keep a few volts headroom
    above the bipolar to run it. I plan to run the bipolar output section
    with enough bandwith to get rid of the 140kHz switching noise from the
    forward converter.

    regards,
    Bob
     
  5. Genome

    Genome Guest

    Hmmmm I once had a troll for 8 pin voltage mode off-line controllers. They
    are few and far between..... like I found two. One was the TI part you
    mention.... but it needs an external driver.

    I was going to suggest the Fairchild FAN7556 but when I went to their
    website I couldn't find it no more. Bastards.

    DNA
     
  6. I read in sci.electronics.design that Yzordderrex
    Good name for a non-humanoid autonomous robot.
    How does this hum problem arise? Keeping hum out of audio amplifiers
    (which I suppose is what the modulator is, in a sense) isn't rocket
    science, at any power level.
     
  7. Yzordderrex

    Yzordderrex Guest

    John,

    No Hum, but they pay big money to eliminate it. The hum would be
    noticable if there were 120Hz on the bus caps.

    They run a big 60Hz transformer into an even bigger bank of caps. Some
    nice optos to drive the transistors fast in order to try and keep it
    all linear. Big butterworth filters on the output to get rid of the
    85kHz switching garbage. Nice big signals with Hi-Fi audio. Can be
    found pretty much coast to coast near 3880-3990kHz

    I suspect they just like to build these big transmitters reminescent
    of the old boatanchor days. They are a very nice bunch, but a little
    set in their ways. I am going to try and give them a modulator which
    isn't so far removed from the present technology.

    regards,
    Bob
     
  8. I read in sci.electronics.design that Yzordderrex
    Have they ever heard of a smoothing choke? Or maybe even better, a
    choke-input filter?
    Low PSRR in the output stage? that can be fixed.
    I'm in UK and I don't have an HF receiver any more.
     
  9. colin

    colin Guest

    the uc3843 is 8 pin curent mode controler but its easy to run it in voltage
    mode, you just feed the triangle waveform from the oscillator into the
    curent sense pin, its shown how in the datasheet, although if u want curent
    limit too you would have to combine the two signals.

    Colin =^.^=
     
  10. colin

    colin Guest

    just wondered why you would be using forward conversion rather than flyback,
    is it posible the power output of the transmiter can be controled with
    current
    as efectivly as with voltage ?

    current mode control would provide a hum free output as it is not influenced
    by line voltage.

    obviously you would need to control the maximum output voltage, but this
    would not be dificult.

    Colin =^.^=
     
  11. Yzordderrex

    Yzordderrex Guest

    Colin,

    Going to use forward mode.

    Flyback mode trying to get to 1500w pep would be unmanagable. Would
    need to store too much energy. Forward mode is probably a leap as
    well. I've already done it the right way (bridge) now I feel this
    perverse need to do it again. (It's actually the audience that I'm
    playing to.)

    This is not just a power supply. This converter has to control the
    voltage at an audio rate. Need gain well beyond 5kHz - It's a
    modulator.

    regards,
    Bob
     
  12. colin

    colin Guest

    yes i apreciated its a modulator, assuming the transmiter apears resistive
    modulating the current would efectivly modulate the voltage, so you could
    feed the input directly into the curent comparator. (with some limit on the
    peak voltage)

    i agree 1500w probably is rather a lot for flyback, would mean a large
    transformer, although im not sure how feasable it would be to have a
    seperate inductor for energy storage in paralell with an ordinary
    transformer.

    you can use a bridge with flyback too, just the use diodes in place of
    diagonaly oposite switches.

    It was just an alternative idea, just for thought, as I read your link out
    of interest and noticed the need for very large capacitors to get rid of
    hum, but output power with curent mode control is unafected by line voltage
    so does away with the need for large caps.

    Colin =^.^=
     
  13. Ken Smith

    Ken Smith Guest

    The LT1246 can be made to do a forward converter. The datasheet examples
    are done as a flyback but the basic PWM action is not dependant on the
    type of converter you are doing.

    The "slope compensation" circuit can be copied to make a circuit with
    normal (non-current mode) operation if you want.

    Using an extra part or two you can make it into a "constant off time" or
    "constant dead time" controller if you want.

    It works ok up to about 750KHz and can drive the gate of a largish MOSFET.


    Doing 1500W in any single ended topology converter is likely to be
    trouble. You may want to consider a multiphase design to reduce the size
    of the inductive elements.
     
  14. Ken Smith

    Ken Smith Guest

    Yes, in designs like this, the supply input impedance of the output
    section is nearly resistive. Its value determined mostly by the load
    impedance connected to the output stage.

    For this reason the OP will want to feedback both the voltage and current
    to the modulator section. This way the system can be made well behaved
    for modest mismatches and self defending for larger ones.
    Breaking the inductor and transformer action apart makes for more troubles
    not less. Making 1500W out of, lets say 5 300W switchers would be what I
    would suggest if I have to do this with flyback designs
     
  15. Yzordderrex

    Yzordderrex Guest

    Thanks for the input Ken (and others)

    Yes, the LT part could probably do a good job. I like the idea of
    constant off-time. I did some thumbnail simulations and could probably
    get the thing to soft switch over most of the modulation envelope.
    Looks like I would have problems at the bottom of the envelope at the
    low currents. Probably not a big deal down there anyway.

    Multiphase might be trouble coordinating without current mode control,
    and probably harder to keep resonant. I might like the challenge of
    the 1500w peak with the one converter. No fun doing things the easy
    way. Heheh.

    thanks
    Bob
     
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