Interesting. As you don't give any design details, I assume that you are busy now analyzing your circuit to find out where the distortion and insufficient PFC operation comes from.
Good luck!
By the way. Power quality regulations don't require PFC for 18 W, with the exception of lighting. It should be also obvious, that a particular PFC design has a "natural" power range. It most likely doesn't work good at a small fraction of rated power.
could you please help me if i am replacing line filtering ckt,whether it will reduce the thd,here now by replacing inductor with another i got the pf around 0.935 but the thd remains same.
so if m replacing the cap with higher value whether could it affect.
Power factor above 0.9 is simply illusional with passive filter. The circuit doesn't show the load, so a power factor can't be determined. It also has drawing errors, shorted rectifier output.
I presume the controller is BP3309? **broken link removed**
Unfortunately the datasheet has no detail information about chip operation or expectable power factor with specific load. I would look at input current and other waveforms and try to optimize the switcher operation.
If the control IC is operating properly, then you should have decent PF regardless of the input filter. Without having details on the controller's operation, we can't really say for sure. It might just be that the loop compensation is poor, leading to bad THD.
Clarifies that you should distinguish distortion and reactive power factor. Your filter can slightly reduce higher harmonics but also add reactive current if not compensated properly. As already explained, the PFC chip is expected to work without additional filters besides the input capacitor.
You need to find out how much the power factor is affected by harmonics and phase shift. Harmonics are dominated by the control IC's operation, phase shift will be affected by both the active PFC and passive filter parts.
In theory it's possible to build passive filters that yield good PF on very nonlinear loads, but in practice this is completely infeasible, hence why active PFC techniques exist. You should check that the active PFC is working before trying to "fix" the filter.