Class D Amp Schematic - A few questions

Discussion in 'The Projects Forum' started by Skeebopstop, May 20, 2009.

  1. Skeebopstop

    Thread Starter Active Member

    Jan 9, 2009
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    Hi All,

    I am following the attached app note to design my own Class D audio amp. If you go to page 50 you will find a sample schematic to which I refer. It does not require intimate knowledge of Audio, rather only to think of the speaker as an inductive load being driven by a half bridge switch.

    Please note I have a hard time believing this schematic ever worked because the upper FET has no connection to the +50V rail but in my implementation C31/C38 have their 50V connected.

    Although most everything is clear to me, there are a few tidbits of the power stage which are still unclear. I am assuming they are just 'good practice' but I haven't the experience to pick up on them.

    Here is a list of components and my guess at their intended purpose:

    R47 & C44: Current limiter and bypass cap?
    R39 & C49: Snubber? To help with bus pumping?
    C30: Snubber cap? To help with bus pumping?
    R61 & C33: Snubber to help with speaker spikes? Won't the LPF (L1 & C51) sort those out anyways?
    SD1 & VDD_1: As far as I can tell VCC, +5V and -5V are generated from regulators but I fail to see where/how/why SD1 and VDD_1 are needed. I realize they are probably off sheet, but I am struggling to determine why VCC couldn't be used and what the designer was attempting to accomplish.

    What are your thoughts on this design's current protection mechanism? It appears to me to be heavily layout dependent as it seems to be sensing voltage droops on the bus. It looks like it just toggles a relay to cut the speaker connection and thus will clear the condition and then click back in etc.. on to infinity... poor relay. I feel this protection circuit provides little benefit and a sense resistors with gate driver shutdown functionality would be better.

    Your thoughts? Thanks in advance.

    James
     
  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    When you look at the schematic it says sheet 2 of 4 on the bottom.

    R47 & C44: Current limiter and bypass cap?
    Probably current limitings, pulse damping.
    R39 & C49: Snubber? To help with bus pumping?
    Probably pulse damping.
    C30: Snubber cap? To help with bus pumping?
    Probably pulse damping, to prevent over currrent to trip at every pulse.
    R61 & C33: Snubber to help with speaker spikes? Won't the LPF (L1 & C51) sort those out anyways?
    Zobel network, to prevent the output from oscillating.
    see also : http://sound.westhost.com/lr-passive.htm
    SD1 & VDD_1: As far as I can tell VCC, +5V and -5V are generated from regulators but I fail to see where/how/why SD1 and VDD_1 are needed. I realize they are probably off sheet, but I am struggling to determine why VCC couldn't be used and what the designer was attempting to accomplish.
    the other voltaves are probably made on the other pages.

    Greetings,
    Bertus
     
  3. Skeebopstop

    Thread Starter Active Member

    Jan 9, 2009
    358
    3
    Thanks Bertus,

    Do you agree with me that such a current protection circuit toggling a relay is quite prone to nasty on/off cycling during over current situations?

    As for VCC and SD1. If you go to the next page the designer shows the finished product and boxes off all sections. There it is shown that only +-5V and 12V regulators exist so I am still confused about Vcc and SD1. Are you able to, without seeing the other sheets, help me take a guess at their function? They just appear to be pull ups/downs to me.

    Regards,

    James
     
  4. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,750
    759
    I think it might work. Class D amps are coming out now ( widely called cold amps )
    I 'd say to give it a try. I mean I would but I want a more powerfull one like 800W RMS per CH
     
  5. bertus

    Administrator

    Apr 5, 2008
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  6. Skeebopstop

    Thread Starter Active Member

    Jan 9, 2009
    358
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    Wow thanks Bertus!

    That follow up will be of great assistance to me. They even have a gate driver with over current shut down as was one of my concerns!

    I also had a bit more of a thought about the over current scenario and the relay may not be a bad idea after all if one of the FETs decides to blow and go short circuit.
     
  7. Skeebopstop

    Thread Starter Active Member

    Jan 9, 2009
    358
    3
    Hey Bertus,

    I read through that article. DIY Audio forums are down at the moment so I'll take a stab here.

    The article uses the zobel network 'in' the crossover design. Does this imply that most commercial speakers do as well? The article also places it 'after' the crossover filters, whereas any amps would place it before. Any guess at the repercussions of this?

    How does one determine the power density of audio frequencies in given ranges? It is a 200W amp but IR only used a 1W resistor for their zobel network (say >1kHz). Any idea how they might have come to that conclusion?

    Thanks,

    James
     
  8. bertus

    Administrator

    Apr 5, 2008
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