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Half-bridge MOSFET drivers for class D audio?

Discussion in 'Electronic Design' started by Tony, Apr 14, 2004.

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  1. Tony

    Tony Guest

    Hi,

    I'm heading down the class D power amp road, and I intended to use IR's dual
    half bridge drivers. But the critical dead-time tradeoff (distortion vs
    destruction) depends on device selection, temperature, load current, etc, and
    therefore seems to require either:
    - turn-on only after the opposite side drive has dropped to <1V
    (complex driver cct and still slightly more than optimum dead time), or
    - Crown's BCA technology which isolates the two MOSFETs
    (patented, more expensive output filter).

    Now LT's LT1336 driver seems to have the driver interlocking internal and fast,
    but it only works to 60V. Does anyone know of a similar part (even if just in
    the works) that works to, say, 200V?

    Tony (remove the "_" to reply by email)
     
  2. Tony wrote...
    My favorite part for this application is Intersil's HIP4081A, because it's
    faster than the alternate parts (much faster than the LT1336). Yes, it's
    limited to a 80V supply, but in the usual H-bridge configuration that gives
    you 160V peak-to-peak, which should be more than enough! 500W rms, Sheesh!

    Thanks,
    - Win

    whill_at_picovolt-dot-com
     
  3. R.Legg

    R.Legg Guest

    You seem to be confusing drive method with topology and modulation
    method.

    The BCA topology claims a reduction in delay-induced distortion due to
    the fact that equal opposing delays are cancelled, as is ripple
    current under certain conditions, due to the bipolar supply and
    grounded load.

    The same modulation scheme can be applied to four active switches,
    though there is no natural guarantee that opposing delays will be
    matched in either arrangement. It is potentially achievable in the
    BCA, with great care, at zero load current.

    The cancellation reduces feedback requirements and simplifies them; it
    does not eliminate them. The BCA modulator is a ternary modulator, and
    is not restricted to use in the BCA arrangement.

    The asymmetric switch can reduce delay and is obviously simpler to
    drive, but it does not eliminate the delay or reverse recovery
    problems that you've mentioned.

    RL
     
  4. Tony

    Tony Guest

    Sorry if my post appeared confusing (or confused). I understand the various
    BCA-related issues that you mentioned (better now, since your explanation), but
    my post really was only concerned with the drive issues. It mentioned BCA only
    because its output inductors inherently eliminated shoot-through the two totem
    pole MOSFETs by separating them (as does the Crest design, apparently, but in a
    much less dramatic way). I considered that 30ns or so of diode reverse recovery
    time could be easily handled with any practical MOSFET turn-on time. I haven't
    decided on a modulation scheme, but at the moment a 2nd order delta modulator
    interests me, as its inherent "immediate feedback" is far better able to
    compensate for variations in delays, supply voltage and switch resistance.

    Tony (remove the "_" to reply by email)
     
  5. Tony

    Tony Guest

    Sorry to hear the LT is so slow. Actually I need around a 120V nominal supply to
    properly drive the intended loads (pro audio speakers). I understood that it
    would be unwise to operate the HIP4080A or HIP4081A even near their 80V rating,
    not to mention power supply droop and tolerances (perhaps that warning was due
    to spikes on bad PCB layouts?).

    Tony (remove the "_" to reply by email)
     
  6. Tony wrote...
    You should use a well-regulated supply, which reduces distortion.
    Spikes can be controlled, get with the program! Bottom line, you
    can use the 4081 at 80V, or even higher (!) for personal equipment.
    But of course it's not advisable in production.

    WRT your 120V spec, do you need to drive your speakers to +/-120V?
    For an 8-ohm load that's 1800 peak watts without clipping, more
    with some distortion. Seems a bit much.

    Thanks,
    - Win

    whill_at_picovolt-dot-com
     
  7. Tony wrote...
    Another issue to watch out for is MOSFET turn-off-time delay.

    Thanks,
    - Win

    whill_at_picovolt-dot-com
     
  8. Tam/WB2TT

    Tam/WB2TT Guest

    Can't you transformer couple the gate drive to the FETs, the way you would
    with an offline switcher? Then the max voltage is determined by the
    breakdown voltage of the transformers, typically 1500V.

    Tam
     
  9. Hardly. Been to a pop concert lately? Even small clubs are pumping that
    much these days. (I say this as a musician and a professional sound man,
    not as a curmudgeon.) I personally own two class-D power amps with output
    at least that much. Partly it's because everyone (but me) evidently wants
    things louder. I have a theory that partly it's because over the last
    couple decades amplified pop music has pushed about an octave lower in the
    frequency spectrum. People are building and playing instruments intended to
    emphasize fundamental frequencies in the range of 28 to 32Hz, almost
    subaudible. Loudspeakers are less efficient and ears less sensitive down
    there.

    I suppose this is not actually a new thing, but rather a reversion to an
    older thing: the last time we had high acoustic energy at very low
    frequencies in pop music was with big pipe organs in churches.
     
  10. I read in sci.electronics.design that Walter Harley
    about 'Half-bridge MOSFET drivers for class D audio?', on Thu, 15 Apr
    2004:
    There aren't many pipe organs that will go below 32 Hz fundamental. You
    can simulate C in the lowest (64 ft) octave by sounding C and G in the
    next octave together (which make the second and third harmonics of the
    low C), but it takes an awful lot of wind.
     
  11. Tony

    Tony Guest

    Hi Win,

    Yes, it was the MOSFET turn-off times (delay and transition) that I was
    concerned about in the original post; I'm still trying the weigh up the options.
    The later comment was in answer to Rob's concerns.

    Tony (remove the "_" to reply by email)
     
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