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PFC with 1kW+

Discussion in 'Electronic Design' started by Ban, Feb 23, 2004.

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  1. Ban

    Ban Guest

    Dear friends,
    I have been pondering of using a L4981A for a PFC pre-regulator to 405V and
    a bridge configuration to get an isolated +75V 20A for a digital amp supply.
    Are there any newer ICs which even might drive the primary FET/IGBT at
    around 48kHz directly? Or are there any ideas to do these stages in a more
    simple way?
    Which power semiconductors would you recommend?
  2. bibiphoque

    bibiphoque Guest

    Try on the International Rectifier site, they have manny drivers and
    usefull applications notes :)o)))
  3. Ban wrote...
    Have you read ST's relevant app notes, especially AN628 and AN1606?

    - Win

  4. Ban

    Ban Guest

    Shure I have all the apps printed out. The AN825 talkes about a downstream
    PFC after a conventional transformer. Maybe this is really a good way, since
    the transformer can be made a little smaller and the efficiency is higher,
    OTH a 1.6kW toroid is expensive and heavy. :-(
    I want to make the triangle generator current in the modulator of the amp
    proportional to the supply voltage and have a rather small
    capacitor(20mF/80V), so the voltage sags like an analog amplifier and the
    "music power(1600W)" is a bit higher than the continuous "rms-power(1200W)".
    I think this way the efficiency can be improved and since I want to put the
    amp into a 2or3HU rack with integrated heatsinks, I might not need forced
  5. Paul Mathews

    Paul Mathews Guest

    Most manufacturers have gone away from including a high current gate
    driver in their power switchmode controllers for several reasons,
    including these:

    1) The gate driver components should be located VERY close to the
    MOSFET, which is usually not a good place to locate the controller IC,
    with its sensitive inputs.

    2) Driving high currents on the same IC die that is used for feedback
    control poses difficult design and IC process problems.

    Gate drive for a PFC can be a very simple 2-transistor complementary
    emitter follower circuit, with a component cost of less than $.50 and
    a very small footprint, so why worry about it?

    L4981A is a good chip, but you can also consider one of the PFC/PWM
    combination chips that operate PFC in leading edge PWM and DCDC in
    trailing edge or vice versa. These can reduce component count, cost,
    and EMI. The following mfgrs offer the combo chips AFAIK:

    Champion Micro (my favorite), Fairchild, TI/Unitrode, Linear

    Some of the chips give you the option of operating the DCDC at twice
    the PFC frequency.

    Paul Mathews
  6. Ban

    Ban Guest

    yes for the power I'm aiming at it will certainly be better to use dedicated
    high- and low-side drivers. Just another isolated 12V needed. :-(
    It will be difficult to make a reliable downstream switcher for the cost of
    a primary transformer, because the volume of my application will not be that
    big, maybe 20 to 50 units/annum only. I still have to find some funding and
    marketing as well, so it will be only a few prototypes in the moment.
    Now I'm using a conventional amp(6x150Wrms) with 60 pounds weight and an
    annoying fan.
    I didn't know the champion brand, certainly very interesting.
    Thanks again for the input
  7. Paul Mathews

    Paul Mathews Guest

    You don't need another isolated 12V for gate drive: gate drive
    transformers are small and inexpensive. You'll also find that your
    actual power requirements for audio program material are a fraction of
    the peak power. Even highly compressed rock and roll music has a
    crest factor of 5 to 1, so you don't really need to rate your PFC and
    DCDC stages for as much power as you're thinking. Consider using a 2
    transistor forward converter for the DCDC, which works well with a
    dual-secondary gate drive transformer. If you use current mode
    feedback and a sense resistor in the lower transistor Source, note
    that many app notes show the gate drive winding for this transistor
    connected between the Gate and Common (i.e., the grounded end of the
    sense resistor). This is incorrect, since it introduces negative
    feedback to the gate circuit. The gate transformer should be
    connected between Gate and Source.

    If your audio amp has good power supply rejection, a better
    alternative might be to use an unregulated push-pull inverter for your
    DCDC, relying on the PFC stage for regulation and the DCDC transformer
    turns ratio for the step-down function only.

    Paul Mathews
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