Question about class D amplifier

[jcpu]

Dear Sir:

I am simulating output stage of class D amplifier as shown in attachment#1

-40dBc 2nd harmonic is observed at the output.
If we observe closer at the waveform (Hop) before LC filter:
(as shown in attachment #2)
overshot (>VDD=3V or <VSS=0V) are observed,
and in zommed-in pictures waveform above VDD
is not symmetric to that below VSS.

Please advise how to fix these problems?

Thanks in advance,

 

[throwaway18]

We need more information to comment on this in detail.
Signal frequency, switching frequency and the full circuit with component values, not just a block diagram.

>-40dBc 2nd harmonic is observed at the output.

Class D amps produce some harmonic distortion. That's normal. -40dB is acceptable for some audio applications. If you want very low distortion then don't use a class D amp.

>overshot (>VDD=3V or <VSS=0V) are observed,
The input of the filter will be reactive. The inductor and the capacitor will be series resonant at some frequency. Ringing is expected behaviour.

>waveform above VDD is not symmetric to that below VSS.
Is the circuit of the H-1 block symetrical?
We need to see it to comment.

 

[jcpu]

Dear Sir:

Audio signal is 1KHz tone,
with DC=1.5V and AC=0.5V

sawtooth waveform:
(1) dual slope
(2) period=8us

H_1 block simple output stage as in attachment

LC filter cutoff frequency is about 13KHz

and the load is 32ohm