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active PFC input filter

Discussion in 'Electronic Design' started by Jamie Morken, Apr 10, 2008.

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  1. Jamie Morken

    Jamie Morken Guest


    I have a simulation of multiphase active power factor correction
    for a 3.6kW battery charger, that generates a 380VDC bus from
    either 120VAC single phase or 240VAC split phase. At 120VAC input
    the output power is limited to 1.8kW to keep the required inductor
    sizes lower.

    There are three phases with each phase having a 75uH/25Amp inductor.

    The base PWM frequency for the inductor boost switches is 200kHz,
    with the pulses to each switch being offset by 1/3rd of the PWM

    Here is the schematic:

    The only shunt being used for the PFC algorithm is the lowside shunt.

    The input filter is just an LC lowpass on the output of the bridge
    rectifier. The rectified current ripple is here:

    I haven't been able to really reduce this current ripple to make a nice
    signal showing good power factor. What is a better way to hook up a
    filter for this type of circuit rather than using the LC filter on the
    output of the bridge rectifier?

  2. Jamie Morken

    Jamie Morken Guest

    Also here is a pic of the input current to the PFC stage,
    without using an input filter before the PFC stage:

    I think the PFC algorithm is possibly incorrect as there
    is a large variance in the current ripple.

    Any idea how to smooth this current out or what part of the
    algorithm could use adjustment?

  3. MooseFET

    MooseFET Guest

    More stages of filtering may be the way to go. You are going to need
    an EMI filter before the bridge to kill the harmonics the diodes
    create so why not use a PI filter there to help the ripple reduction.
  4. legg

    legg Guest

    If you want to examine ripple at the conversion frequency, you're
    going to have to look at much shorter duration periods at different
    input LF phase angles.

    This looks like a model's output. If so, it's only sensible to talk
    about the model, and not about an actual circuit. They will have
    widely varying causes for perceived problems.

  5. Jamie Morken

    Jamie Morken Guest

    Would a common mode choke across the two AC inputs before the bridge
    rectifier be good to use as well?

  6. Jamie Morken

    Jamie Morken Guest


    I made a test PI filter in ltspice, it lets about 60mA of 300kHz current
    ripple back to the AC source (V1 and V2 are 240VAC splitphase), any
    suggestions on how to improve this filter, or is this a good
    configuration to use?

    The cap before the first inductor, L1, doesn't seem to really do much in
    the simulation, what is the reason for using a PI filter on the AC lines
    rather than just an LC?

    The right side of the schematic leads to the multiphase PFC stage.
    The ripple is 300kHz as there are 3 100kHz boost circuits in parallel,
    offset by 1/3rd.

  7. Jamie Morken

    Jamie Morken Guest


    What type of filter would be good for a 3phase input source (rather than
    120VAC single phase or 240VAC split phase).

    I was thinking just series inductors with caps like this:

    I don't have the 3phase common available so I can't put caps to that.

  8. MooseFET

    MooseFET Guest

    The leakage inductance of a common mode choke may help reduce the
    switcher ripple. Other than that they would be of little help for
    this situation.
  9. MooseFET

    MooseFET Guest

    LTSpice assumes a zero impedance voltage source. That isn't the case
    in real life. Power wiring looks like a messy RLC circuit. At low
    frequencies it has impedance levels in the 0.1 to 10 Ohm range.

    That sounds like it should be fairly easy to stomp the ripple from
    down to reasonable levels. You may need film capacitors to get enough
    capacitance in real parts but a few stages of a few uF combined with a
    few uH and some lossiness should do the trick.

    Remember once you make RF power it has to go somewhere.
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