I require help in controlling 3rd & 5th harmonic distortion in 15W cfl circuit.
What are the factors that control these factors. Can any one help, We are fcl ckt . manufacturer

 

You forgot to attach your schematic.
Negative feedback is used to reduce distortion in audio amplifiers. Good amplifiers have a total harmonic distortion of 0.003% at full output.

Some opamps like the LM4562 have a total harmonic distortion of only 0.00003% (typ). It could be used to drive darlington power transistors included in the negative feedback loop and produce 15W at extremely low distortion. Two amplifiers could be bridged to provide 55W.

3rd and 5th harmonics are odd (not even) harmonics. They are usually caused by symmetrical clipping or symmetrical compression of the peaks of the waveform. If a resistor is used to turn on each output transistor then the base current becomes less as the output voltage (and resulting output current) increases. This causes compression of the peak of the waveform. A constant current circuit replacing the resistor helps reduce the distortion.
If the volume control is turned up too high then the amplifier output clips and makes square-waves. Square-waves are full of odd-harmonics distortion.