PFC and THD Design (urgent)

[SurekhaIris]

I have to make the design for High PFC and Low THD,Please suggest me.how to do calculation.

Right now my circuit output wattage 18 W output, i have given 230v input,

when i chekced the design i got pF around 0.8632 and THD is 18.00

Please i want to reduce thd below 10 and PF>=0.936 please suggest.

Do the needful.

 

[FvM]

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.

 

[SurekhaIris]

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.

 

[mtwieg]

No one can help you unless you post details on the design, like a schematic. Is this a passive PFC scheme, or does it use active PFC?

 

[SurekhaIris]

the solution is for led driver for 18W, and ckt have passive filter design.

I will post the schematic FYI,

- - - Updated - - -

Attached is Schematic for filter ckt

 

[FvM]

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.

 

[SurekhaIris]

Here is the attached full schematic

 

[FvM]

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.

 

[SurekhaIris]

u r right,they didnt provide any design operation.m checking with various filter design but still it remains same

 

[FvM]

I assume, that the insufficient PF is caused by BP3309 operation itself and hasn't much to do with the passive filters.

The common mode filter doesn't have an effect on power factor, the 1 mH inductor has a neglectible effect related to the input current with 18W load.

 

[mtwieg]

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.

 

[SurekhaIris]

That means,any filter design could not affect on THD as bz the IC is not operating properly.plz explain....

i have added filter it reduces the thd but the problem is that it will also affect the PF it is also decerese.

Their is no way to solve issue

 

[FvM]

i have added filter it reduces the thd but the problem is that it will also affect the PF it is also decerese.

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.

 

[mtwieg]

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.