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PID CONTROL FOR BOOST CONVERTER

Discussion in 'Electronic Design' started by manish, Nov 3, 2006.

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  1. manish

    manish Guest

    Hi everybody

    I am currently working on my final year project which consists of
     
  2. Tim Wescott

    Tim Wescott Guest

    Like many control problems, this is one that should be simple, and can
    be simple, but will have subtle little effects hiding in the corners to
    make your life difficult, at least temporarily.

    I don't know how much control systems background you have, so forgive me
    if I go into too much detail.

    I assume that when you say 'boost' converter you mean 'flyback'
    converter with an output stage like this one (view in fixed-width font):


    V in ___ HV out
    o---------UUU--o-->|---o----------o
    | |
    | |
    ||-+ ---
    PWM ||<- ---
    o-----------||-+ |
    | |
    | |
    === ===
    GND GND
    (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

    If I remember correctly, if you use a flyback converter to drive a
    resistive load your system will be stable, but the converter will have a
    couple of 'interesting' (read 'hard to deal with') properties. If you
    drive something other than a resistive load, your converter will tend to
    have even more 'interesting' properties.

    The first of these interesting properties stems from the nature of a
    flyback converter that uses a single pass transistor and a diode. The
    operation of the converter can either be such that the coil always has
    current flowing through it ("continuous conduction") or that the coil
    current stops every cycle after discharging through the diode
    ("discontinuous conduction"). As the duty cycle changes, and if the
    converter goes from continuous mode to discontinuous mode, the dynamics
    of the system changes in a way that impacts the allowable tuning of your
    controller.

    The second interesting property is that when the converter is in
    continuous conduction mode the converter's transfer function from PWM to
    voltage out has an unstable zero. This means that if you suddenly
    increase the duty cycle your converter's output will first drop (because
    the output capacitor is getting a smaller share of the current in the
    coil) then rise (because the coil current has increased due to the
    longer on time).

    In combination you get a system that is hard to stabilize (because of
    the unstable zero) in the first place, and this difficulty is compounded
    by the fact that the zero is crawling all over the place as you change
    your command to the drive section.

    These are all things that can be overcome, however.

    If you are going at this with the sort of background assumed in my "PID
    Without a PhD" article, then I suggest you do the following:

    * Set up your software so that you can command a constant PWM, and
    verify that your power stage is stable and well-behaved when you drive a
    resistor.
    * Spend a bit of time with an oscilloscope, stepping the PWM command and
    seeing how the output behaves.
    * Try this with different values of resistance on the output, so you can
    see what happens with different baseline duty cycles, and with different
    conduction modes.
    * Now close your loop with a _very little_ bit of proportional gain, and
    use the procedures in that article to tune the loop. Continue to
    monitor the output with your o-scope, and make steps in your target
    output voltage to see how the system responds.

    If you are going at this from a more sophisticated control systems
    perspective, do this instead:

    * Set up the system as before, and familiarize yourself with it.
    * Read my article about characterizing systems in the frequency domain
    (http://www.wescottdesign.com/articles/FreqMeas/freq_meas.html). If it
    doesn't make sense, read about z transforms at
    http://www.wescottdesign.com/articles/zTransform/z-transforms.html.
    * Write the code to characterize your drive, and do so. Collect
    information with a number of different loads.
    * Use formal frequency-domain design procedures (Bode plot, Nyquist
    plot) to design a controller that will be stable for all of the
    different loads that you tried.

    If you _do_ want to use the more formal methods, but _don't_ have the
    control systems background, then I have a book for you:
    http://www.wescottdesign.com/actfes/actfes.html. I don't know if it'll
    fit in your budget, but if not you may be able to convince your
    university library to get a copy.

    --

    Tim Wescott
    Wescott Design Services
    http://www.wescottdesign.com

    Posting from Google? See http://cfaj.freeshell.org/google/

    "Applied Control Theory for Embedded Systems" came out in April.
    See details at http://www.wescottdesign.com/actfes/actfes.html
     
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