Yes or No: Can APFC correct for AC inductive displacement phase?

[snoop33]

Hi all,

Until yesterday I was told by many an EE that correcting displacement after the bridge is not possible. Looking at APFC especially totem-pole where there's no bridge has me questioning whether my understanding is true.

I think largely this is based on PFC chips working on mains voltage where the inductive generators at power stations are phase corrected by capacitance before it reaches the consumer, so displacement correction is actually for the load and not for the supply.

Looking for verification!

[David Hess]

Yes, that seems correct.

When the active power factor correction is behind a bridge, it cannot drive current back into the supply through the bridge, so it can only correct the power factor of its load.

A totem-pole, like would be found in the output of a grid-tie inverter, can drive current in both both directions.  So in theory a grid-tie inverter can simulate capacitance, or anything else, within the limits of the power storage of its load.

[snoop33]

This is my new understanding. It doesn't seem to be widely recognised or understood among the fraternity though. 

[jbb]

As David Hess said, you can’t push reactive current out of that diode bridge. When input voltage is positive, the input current must be >= zero (likewise for negative voltage). To sink or source proper reactive current, you need positive current when the voltage is negative (and vice versa).

There is a small opportunity to get creative, though. The current consumed by the APFC can be shaped a bit to provide/consume a bit of reactive current for some of the waveform.  But you can’t do a proper job (eg source positive current into a negative voltage), so the waveform gets distorted.

The THD limits on electronic products are pretty strict, so the amount of reactive current you could produce/consume this way is really small.

[ejeffrey]

Are you talking about using an APFC power supply to supply reactive current back to the mains input to cancel an external reactive load?  Yes that is not possible. For that you  need an inverter / bidirectional converter.

But an APFC power supply can obviously supply a reactive load and present a real impedance to the grid.  For instance in a VFD.

[mtwieg]

This is my new understanding. It doesn't seem to be widely recognised or understood among the fraternity though. 

Perhaps others assume that "APFC" refers to a basic front end with a bridge rectifier, in which case they're correct.