Three phase PFC.....DCM near zero cross

[cupoftea]

Hi,
Woudl you agree the drive of the upper fets will need care at the phase zero crossings, otherwise reverse current flow could flow in the PFC inductors, when the inductor current goes DCM?

In this thread, #3...

https://www.edaboard.com/threads/three-phase-2-level-pfc-inductor-design.401709/

...the suggestion is that alls thats needed is a sinusoidal PWM...no attention needs paying to the DCM.....so i suppose it never actually happens?
Though one would wonder what happens in light load?

 

[FvM]

Don't understand which problem you are considering. Active front end is generally operated with synchronous switching (CCM), inductor current is bidirectional. High- and low side switching pattern is symmetrical.

 

[cupoftea]

Thanks, so when in light load, you just keep the sinusoidal PWM going, and the average inductor current just automatically goes to zero...ie, the inductor current just shuttles back and forth?, (just like in a sync Buck with continuous PWM) since there's no load.

 

[FvM]

Yes.

 

[cupoftea]

Thanks, sounds easier than single phase PFC, which seems counter intuitive.

 

[FvM]

A synchronous switching phase H-bridge implements similar behaviour for single phase PFC.

 

[dick_freebird]

Is the DCM @ zero crossing perhaps just "short pulse limit" flicker?