UCC28180: phase current is not sinusoidal under light load

Hello Hely, 

The first waveform with no significant high-frequency ripple appears to be at no-load condition.  All of the sinusoidal current seen is reactive current through the line-filter X-caps. 

The last waveform seen is higher load, where the inductor ripple current is predominantly CCM.  THDi is low. 

The middle two waveforms are at some unspecified light load, and the ripple current is relatively high plus much of the current is X-cap current.  
I believe the inductor is operating mostly in DCM, and THDi is high when inductor current is not CCM.  The UCC28180 controller is designed for CCM-PFC. 

The sudden step in the current is caused by charge stored on Cin.  At light loads, inductor current is so low that it cannot discharge Cin to zero at the crossings, so input current through the diode bridge is zero for a time interval. 

To reduce the zero-crossing step distortion, reduce the Cin value.  To reduce the light-load reactive current, reduce the X-cap values.  However X-caps are used for Differential-Mode EMI filtering, so there is a practicable limit to reducing X-cap size. 

To improve light load THDi, increase boost inductance to reduce the amount of DCM during the line-cycle.  There is a practicable limit to inductance increase, too, so some level of DCM at some light load must be acceptable. 

Regards,
Ulrich