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Class D amps with ext clock over 1MHz?

Discussion in 'Electronic Design' started by Joerg, Oct 29, 2004.

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  1. Joerg

    Joerg Guest

    Hello All,

    Is there a fairly comprehensive list on the web about class D audio amps
    that run above 1MHz, synchronized or externally clocked?

    Ideally I am looking for something from the mainstream suppliers such as
    National, TI, ST, Philips. Their web sites aren't all that helpful here.
    Their product selectors are sometimes incomplete and don't list clock
    speeds so you have to trundle through every single datasheet to pry out
    that info. Done that for several dozen and the only one I found was the
    MAX9712 but nothing for the big suppliers.

    Regards, Joerg
     
  2. Pooh Bear

    Pooh Bear Guest

    None that I know off offhand. Tripath's 'spread spectrum' design maxes out
    at about 600kHz IIRC.

    Haven't seen any device featuring sync either.

    Any special reason to go that high ?


    Graham
     
  3. Joerg

    Joerg Guest

    Hi Graham,
    I wanted to see if they can be used as precision pulse width modulators
    up to 1-2MHz. With the THD levels they claim, these amps should be quite
    precise. Most of all they would cost a whole lot less than rolling your
    own from CMOS chips and discretes.

    Regards, Joerg
     
  4. Pooh Bear wrote...
    One serious issue that's not often talked about is an asymmetric
    power MOSFET turn-on and turn-off time. Moreover, FET turnoff
    time has a slow recovery tail, and is memory dependent for short
    time intervals. The distortions from this issue are exacerbated
    at high PWM frequencies, and lead to a degraded performance.
     
  5. Pooh Bear

    Pooh Bear Guest

    Absolutely. Just noticed the same today looking at IGBTs.

    Tripath get 'better than most' results by taking feedback from the output
    to counter this. They have a programmable dead-time too. Needless to say -
    the shortest dead-time gives the highest performance with the greatest
    risk of cross-conduction.

    Driving gates at high frequencies takes some current too !


    Graham
     
  6. Ken Smith

    Ken Smith Guest

    This sounds like a job for a feedback loop. You could measure the
    difference between the turn on delay and the turn off delay and apply that
    correction on the next cycle.
     
  7. Ken Smith wrote...
    Right, but then you're analog at a critical spot, and no longer a
    cool completely-digital-to-the-speakers system.
     
  8. Joerg

    Joerg Guest

    Hi Winfield,
    With a monolithic solution the chip designer would probably compensate
    at least for some of these transition differences. One of the Maxim
    class D chips (MAX9712) is claimed to run at 0.01% THD when clocking at
    1.1MHz. It doesn't go much above 0.02% for most of the output power
    range. It might get worse when running it up to its 2MHz clock maximum
    but probably not by a lot.

    Regards, Joerg
     
  9. Tony

    Tony Guest

    Yes, dead time remains the critical missing block in good class D
    design. Even a few ns can cause serious (to an audiophile) crossover
    distortion, and a new ns of overlap can start to heat things up quite
    a bit. Plus it all varies with load impedance, current, temperature
    and devices, so there's really no practical option but to control it
    dynamically (and quickly) to some criterion (peak punch-through
    current?). LT did a neat bias IC for class B linear output stages
    decades ago. Wouldn't it be great if IR could integrate dynamic dead
    time control into one of their drivers? Then the differential
    propagation delay specs wouldn't be so critical. If someone doesn't do
    it I guess I'll need to before I finally get around to that big class
    D monster I've been planning for years; either that or steal Crown's
    method (which itself is not without complications).

    Tony
    Tony (remove the "_" to reply by email)
     
  10. Ken Smith

    Ken Smith Guest

    I won't tell if you don't.

    You could run the switching stuff into the timing circuit running at many
    GHZ and then feed that number back into a DSP thus closing the loop
    digitally.


    Or, you could just enclose the whole thing in an analog servo loop and get
    better linearity at the cost of some marketing boost.
     
  11. Ken Smith

    Ken Smith Guest

    [... Class D and MOSFET switching ..]
    If you want real big fun, try it with bipolars.

    Many years ago, I made a class D with bipolars as the switches. Even with
    very complex Baker clamping circuits, the switching times were way
    different. I ended up wrapping the switches with Schottkys and enclosing
    the whole thing in a servo loop. The result sound quite good. AM radios
    didn't like working near the circuit however.
     
  12. N. Thornton

    N. Thornton Guest

    I assumed that rather than implementing audio feedback, he meant just
    measure the device turn on and turn off times, and feedforwardly tweak
    the on and off times by that to cancel the problem. Thus all digital.

    NT
     
  13. classd101

    classd101 Guest


    Hi,

    Tripath's hysteretic modulators, or "spread spectrum frequency
    modulator" can go up to over 1Mhz during idle/low signal conditions.
    It's an analog modulator, with adaptive filtering to learn and
    digitally control the timing of the output stage, I'm not sure it
    qualifies as taking feedback from after the filter, don't think it
    does though.

    What is it that we don't like about fully analog, self oscillating
    designs? There's a number of them out there, some of which are of the
    highest quality out of any class of amp, considering the power levels
    that is. Non switch near 1Mhz either.

    Thanks,
    Chris
     
  14. Pooh Bear

    Pooh Bear Guest

    This is a 'yeah but' argument.

    Switching times alter with load current and temperature. You can never fix the problem reliably that
    way.

    Graham
     
  15. Pooh Bear

    Pooh Bear Guest

    Maybe on newer versions. It topped out at about 600-800 kHz a few yrs back.
    It takes feedback pre the output filter IIRC.

    Which were you thinking of ?

    Agreed that none switch at 1 MHz or near that.


    Graham
     
  16. Ken Smith

    Ken Smith Guest

    [/QUOTE]

    Yes this is what I was arguing and I believe Win understood the point.
    Win is quite right that the system would not be truely completely digital.
    The measuement would contain at least a small analog section.

    Assuming that, I will now disagree with Win (or maybe this is just a
    misunderstanding):
    If we assume that the switching frequency is well above the highest signal
    frequency and that the amplifier is never driven to clipping, I think we
    can make the following simplifications:

    (1)
    The die temperature does not change much from on cycle to the next.

    (2)
    The current in the MOS-FET, the next time its on, will be predictable
    based on the previous input data and the newest point.

    (3)
    The delay characteristics of a MOSFET at a certain temperature and current
    changes only very slowly as it ages.


    Based on this, I suggest that a clever enough circuit that contains the
    following would work:

    You will need some circuit to convert the large amplitude signal that
    indicates whether the MOS-FET is conducting or not into a digital logic
    level.

    This logic signal would be connected to some sort of counter clocked at,
    lets say, 5GHz. This counter would measure the time between the ideal
    turn on time and the true turn on time and the ideal turn off to true turn
    off.

    either:

    (A)
    The numbers from this counter are fed back to the PWM circuit which is
    also clocked at 5GHz. In the PWM circuit, the delay times are subtracted
    from the ideal times and these corrected times are then used to drive the
    outputs.

    or:

    (B)
    The numbers from the counter are used to keep a table discribing the
    MOS-FETS characteristics up to date. Values from this table are then used
    to adjust the numbers before they are presented to the PWM output section.


    The weakness in this idea is that it assumes that the MOSFET delays and
    then switches perfectly after that delay. This circuit can improve
    matters but without correcting for the actual turn on and turn off shapes,
    the results will still be less than ideal.
     
  17. Genome

    Genome Guest

    | Pooh Bear wrote...
    | >
    | > Joerg wrote:
    | >
    | >> Is there a fairly comprehensive list on the web about class D audio
    | >> amps that run above 1MHz, synchronized or externally clocked?
    | >
    | > None that I know off offhand. Tripath's 'spread spectrum' design
    | > maxes out at about 600kHz IIRC.
    |
    | One serious issue that's not often talked about is an asymmetric
    | power MOSFET turn-on and turn-off time. Moreover, FET turnoff
    | time has a slow recovery tail, and is memory dependent for short
    | time intervals. The distortions from this issue are exacerbated
    | at high PWM frequencies, and lead to a degraded performance.
    |
    |
    | --
    | Thanks,
    | - Win
    |
    | (email: use hill_at_rowland-dotties-org for now)

    I'd like to know the basis for that one?

    DNA
     
  18. Genome

    Genome Guest

    |
    |
    | Winfield Hill wrote:
    |
    | > Pooh Bear wrote...
    | > >
    | > > Joerg wrote:
    | > >
    | > >> Is there a fairly comprehensive list on the web about class D audio
    | > >> amps that run above 1MHz, synchronized or externally clocked?
    | > >
    | > > None that I know off offhand. Tripath's 'spread spectrum' design
    | > > maxes out at about 600kHz IIRC.
    | >
    | > One serious issue that's not often talked about is an asymmetric
    | > power MOSFET turn-on and turn-off time. Moreover, FET turnoff
    | > time has a slow recovery tail, and is memory dependent for short
    | > time intervals. The distortions from this issue are exacerbated
    | > at high PWM frequencies, and lead to a degraded performance.
    |
    | Absolutely. Just noticed the same today looking at IGBTs.
    |
    | Tripath get 'better than most' results by taking feedback from the output
    | to counter this. They have a programmable dead-time too. Needless to say -
    | the shortest dead-time gives the highest performance with the greatest
    | risk of cross-conduction.
    |
    | Driving gates at high frequencies takes some current too !
    |
    |
    | Graham
    |

    IGBTs are a different class of beasty.

    DNA
     
  19. Ken Smith

    Ken Smith Guest

    They are not really that different. They look a lot like a N-MOSFET
    driving a PNP. I think we should be developing class D audio amplifiers
    using mercury ignitrons. Using the capacitively coupled quenching
    circuits, you'd have to have some very tricky DSP code to get the PWM
    right. You would have lots of power and that nice "warm tube sound" so
    there would be a market.
     
  20. N. Thornton

    N. Thornton Guest

    yeah but...
    temp is slow changing compared to the 600kHz etc switching speed, so
    thats not a problem
    load current ditto, though its faster moving than theta.

    NT
     
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