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3phase PFC

Discussion in 'Electronic Design' started by Jamie Morken, May 8, 2008.

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  1. Jamie Morken

    Jamie Morken Guest

    Hi,

    I'm trying to make a 3phase PFC circuit based on this schematic:

    "http://www.rocketresearch.org/new/3phase PFC/3-phase vienna rectifier.jpg"

    more info about this circuit:

    "http://www.rocketresearch.org/new/3phase PFC/3phase PFC using vienna rectifier.doc"


    I'm doing a test in ltspice with this circuit and three single phase PFC
    controller IC's (ie. LT1249) but am not sure how to wire up these IC's
    to detect the input phases bipolar voltages and currents, as for single
    phase the input voltages and currents are just positive.

    Here is the ltspice circuit so far:

    "http://www.rocketresearch.org/new/3phase PFC/3phase ltspice test.zip"

    Are there any 3phase PFC controller IC's out there?

    cheers,
    Jamie
     
  2. MooseFET

    MooseFET Guest

    Circuits with the word "fuzzy" in them rarely work as well as ones
    without it.
    IR may have one.


    This may be a case where "use a PIC" really is the right answer. In a
    converter of this size, the cost of a slightly more complex controller
    section may be worth it. If you have a part that can remember what
    happened a full cycle ago, you can do a much better job on bringing
    the out of phase and harmonic content down to zero.
     
  3. Jamie Morken

    Jamie Morken Guest

    Hi,

    My three phase source (a brushless generator) has a wide output voltage
    range, rectified from 60VDC all the way up to 360VDC depending on the
    RPM, the power output scales fairly linearly over this range, from only
    60watts at 60VDC, all the way up to about 1300watts at 360VDC.

    I would like the output of the 3phase PFC boost to be about 400VDC or so
    over the whole input voltage range. Is it possible to maintain a high
    efficiency (96%+ ideally) over this whole range using the same boost
    inductors, or would it be better to switch boost inductors depending on
    the input voltage.


    I am trying to size the PFC boost inductors for this using this tool:

    http://www.daycounter.com/Calculators/Switching-Converter-Calculator.phtml

    For the same output voltages, it suggests a high inductance inductor at
    the low voltage input, low power output, and then a smaller inductance
    inductor at higher input voltage, higher power output.

    I'm not sure how to size this inductor for the whole voltage range.

    cheers,
    Jamie
     
  4. Tony

    Tony Guest

    If you're trying to convert a dedicated source like a car alternator that already has a
    rectifier, why go to the trouble of a 3 phase PFC? In fact, why worry about PFC at all? A
    simple single step-up converter should be fine for those power levels (with
    current-controlled inner loop and slower outer voltage loop).
     
  5. Jamie Morken

    Jamie Morken Guest

    Hi,

    It is more efficient to use a PFC stage rather than a boost stage as the
    generators RMS current is lower with PFC. This means more energy can be
    extracted from the generator for the same work input and the heat in the
    generator will be lower for the same output power level.

    cheers,
    Jamie
     
  6. Jamie Morken

    Jamie Morken Guest

    Hi,

    Since only small output current is required at low input voltages,
    but this requires a large inductance, and then large output current
    is required at higher input voltages, which requires a smaller
    inductance, can these two inductors be put in series, and the high
    inductance one will saturate when the input voltage is high, or would
    it require a bypass current path (ie. relay) to short out this inductor?

    Here's a boost circuit with what I am thinking of:

    "http://www.rocketresearch.org/new/3phase PFC/series boost inductors.jpg"

    If the second inductor doesn't saturate then the inductance will be too
    large for high input voltages, so maybe the switching frequency could
    be decreased.

    Here's another circuit I was thinking of, it uses two switches, S2 runs
    at low input voltages to get the large inductance, and then S1 runs at
    higher input voltages with only the 300uH inductor.

    "http://www.rocketresearch.org/new/3phase PFC/series boost inductors dual switches.jpg"

    Here's one more using two mosfets to short out the large inductor for
    decreasing the inductance at high input voltages:

    "http://www.rocketresearch.org/new/3phase PFC/series boost inductors with short.jpg"

    cheers,
    Jamie
     
  7. legg

    legg Guest

    You might want to crunch some numbers on this for the 3ph case, while
    examining other methods of power factor correction that are available.

    What you should really be considering is deliverable power vs your
    requirement and permissible temperature rise.

    PFC requirements are primarily driven by legislation based on
    infrastructure requirements, not by end-use economics or concerns
    about efficiency, as such. Your source is free from that major
    influence.

    RL
     
  8. legg

    legg Guest

    This is intuitively backwards from your real application requirement.
    For a constant power drain, current will increase as voltage reduces.
    This argues for an inductor that handles higher current at low voltage
    input, while still having an inductance that is still high enough to
    be practically functional (not discontinuous) at higher input
    voltages.

    For the same power transfer, the low voltage inductor would have a
    lower inductance, if you were actually switching between optimum part
    types. The optimum parts have energy storage requirements that
    increase at lower input, but this energy storage is related to the
    square the current.

    If you are considering altering RPM to adjust for power drain
    (suggested by your lower current lower voltage reference), you might
    just as well do so to crudely regulate the generator output voltage
    instead, considerably reducing the work of downstream regulators.

    RL
     
  9. MooseFET

    MooseFET Guest

    Being able to work over that range pushes the cost of the inductors up
    a lot. It seems to me that you really don't need a good PFC so much
    as a good booster design.

    You may be better off with a two range design. With low input
    voltages you are beyond where the simple booster works well and into
    where a transformer looks better. Perhaps you could share the
    inductive elements. I'm thinking of something like this:


    ========================
    -----))))))----+-----))))))-----+------>!--- To load
    ! !
    !!- V
    ----!! Q1 --- D1
    !!- !
    ! !!-
    GND ---!! Q2
    !!-
    !
    GND

    Q1 is only used when the voltage is low. At high voltages Q2 is
    used. D1
    is needed to keep reverse current out of Q2 when Q1 is on.
     
  10. MooseFET

    MooseFET Guest

    Try changing the frequency with the input voltage. This reduces the
    range of the inductors.
     
  11. legg

    legg Guest

    - All the controller outputs were labeled for ph1 - effectively
    shorted together

    - All modulator references had the same phase angle. (After
    correction, your model will not work with alt solver, so you'll have
    to switch back to the normal solver.)

    - There was no current feedback. This chip expects to see
    negative-going current sensing signals on the Mout pin of less than
    1V1 peak.

    - Once running, you have to load it to get current waveforms.

    - Don't expect to see attempted regulation prior to the 6.5mSec
    timestamp, as the inrush and overshoot doesn't clear prior to that
    time, even with a 1KW load. Also don't expect 60Hz performance at
    600Hz.

    The asc file is reworked (and still needs much more attention to run
    properly as a simulation) at:

    http://www.magma.ca/~legg/TVS/1249-3phase-2.zip

    With simulations that take as long to run as this one does,
    doublechecking your work, your ideas and your aims becomes important,
    prior to pressing the GUI's 'run' button. Make sure you've got plenty
    of drive space. The raw files mount up at the rate of about 150Meg per
    millisecond of active converter runtime.I'm always surprised that they
    run at all.

    The Vienna rectifier is about the most expensive,complicated and lossy
    version of PFC correction that a body could have picked.

    RL
     
  12. Jamie Morken

    Jamie Morken Guest

    Thank you for modifying the sim to work! :)

    What circuits would be better than the vienna rectifier for 3phase
    PFC? I found two other ones:

    http://www.rocketresearch.org/new/3phase PFC/3 phase PFC stage.png
    http://www.rocketresearch.org/new/3phase PFC/3 phase PFC stage2.jpg

    I would like to generate +220V and -220V rails, and use them to generate
    splitphase 120VAC sine waves using halfbridges and two LC filters. What
    would be the best 3phase PFC circuit to generate these two rails, or if
    not using 3phase PFC, could a boost circuit off the 3phase 6diode
    rectifier generate a positive and negative rail?

    cheers,
    Jamie
     
  13. Jamie Morken

    Jamie Morken Guest

    Hi,

    Can the inductors also be on separate cores? For a 60VDC to 380VDC
    input voltage range, with linear power increase with voltage up to
    2kW at 380VDC, and 60watts at 60VDC input, what inductances would be
    a good starting point for these two inductors?

    cheers,
    Jamie
     
  14. Jamie Morken

    Jamie Morken Guest

    I'm using an FPGA, so can control the PWM frequency (increase it for
    lower input voltage), With this method and the linear graph of input
    voltage over output power (required output power is low with low input
    voltage) would it be possible to use a single stage boost design and
    still have high efficiency over the whole range? Or is it a good idea
    to use the two stage boost as well as variable frequency? Easy to do
    in an FPGA, so the only real variable is finding good inductances to use
    I think!

    cheers,
    Jamie
     
  15. JosephKK

    JosephKK Guest

    Yes and no. Energy efficiency improvements do have economic payback.
     
  16. Jamie Morken

    Jamie Morken Guest

    I guess this is like an autotransformer?

    cheers,
    Jamie
     
  17. legg

    legg Guest

    Depends on whether you're designing hardware to cook for ten thousand,
    or just two.... Or maybe even just planning a menu.

    RL
     
  18. MooseFET

    MooseFET Guest

    Yes that is the winding polarity I was suggesting. You still need a
    physically larger inductor to be able to do it.

    Not combining the inductors may be better from the point of view of
    needing two smaller cores in place of one large one.
     
  19. Jamie Morken

    Jamie Morken Guest

    Hi Terry,

    Thanks for the dual rail boost! ;)
    Is this an alternate circuit to the vienna rectifier for 3phase PFC?
    Do you have a schematic of it?

    Here's an ltspice sim from Larry Carroll on the ltspice yahoo
    group, it implements 3phase PFC with one cycle PFC control!

    "http://www.rocketresearch.org/new/3phase PFC/lbcarroll backup.zip"

    cheers,
    Jamie
     
  20. MooseFET

    MooseFET Guest

    You can get a wide range and good efficiency but you end up with a
    mechanically large inductor and switch device.
    Each stage should be limited to about a 4:1 voltage range.
     
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