Class D Amp LPF Problem

Discussion in 'General Electronics Chat' started by MarkCSI, Feb 4, 2013.

  1. MarkCSI

    Thread Starter New Member

    Feb 4, 2013
    2
    0
    Hi,

    I am trying to use a Class D Amp to drive a non-audio application (360 - 12000Hz, load will vary), and am getting distortion on the output when no load or lighter load is present. The basic flow is:

    Class D Amp -> LPF -> Step Up Xfmr -> Output

    I am using the LPF in the Eval kit (Max9737). The LPF is designed for an 8Ω load.

    What I am seeing is not the high freq switching distortion, but much lower. I have included a scope trace . BTW, it happens with or without the XFMR. The FAE said it is because their LPF is tuned for 8Ω.

    I have a few questions:

    1. Does this make sense to anybody?

    2. He sais I should simulate the LPF to optimize. How does one create the class d signal for simulation?

    3. Does anybody have a recommendation for a free/cheap simulator?

    Thanks!

    Mark
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,003
    3,232
    Post your circuit diagram including the LPF.

    What is the minimum load where you see no distortion?

    Who is the FAE?

    You can simulate a PWM in Spice simulators by generating a ramp signal using the pulse source and then using a comparator with one input being the ramp and the other input being the modulation signal to generate the modulated PWM.

    Probably the best, free simulator, which is used by many on these electronic forums, is LTSpice from Linear Technology.
     
  3. MarkCSI

    Thread Starter New Member

    Feb 4, 2013
    2
    0
    F+ and F- are the output of the MAX9737 Eval board. I have attached the schematic. This problem shows up only with light loads. If I put the 8Ω load on the output of the Eval board, it looks fine (other than a little 300kHz remnant of the class d switching). I have not checked with loads other than 8, 4k, no load, but I suspect it gradually goes away from 4k down to 8.

    The FAE is Evan R at Maxim.

    Do Class D amps only "work" with lower loads, or is this an artifact of their LPF? He says it is the LPF.

    That sounds like a great way! I suppose I can simulate the Class D output of my sin wave with a sin generator into a comparator.

    Thanks for your help!

    Mark
     
  4. crutschow

    Expert

    Mar 14, 2008
    13,003
    3,232
    I don't see offhand why the LPF would give distortion with a high load resistance but perhaps there's some interaction between the LPF resonant frequency and the load value when driven by the PWM signal.
     
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