Why does THD and PF of a PFC boost improve as the load increases??

I have seen in many single phase PFC boost rectifiers that the PF and THD improve as the output power increases. Can anyone give a perfect explanation for it??

Its because the current sense resistor is usually kept as small as possible so it dissipates less…….this sounds good, but it also means that you get very very little current sense signal off of it…..and even less signal off of it when the load is lighter……so the control is affected as the signal from the small current sense resistors is small and deluged by noise.

I hear you argue that you could just use a current sense transformer instead…..alas no…CST’s are not good for PFC boost because the duty cycle is sometimes ridiculously high and the opportunity to reset the transformer with reasonable reset voltages is very poor.

So there it is.

There is also the point that all PFC stages actually do have some amount of DC bus capacitance…..eg 1uF for a 500W Boost PFC stage. This is for noise reduction purposes. When you are at maximum power, the DC bus voltage is pretty well a sinusoid, in spite of this bit of DC bus capacitance…however, when you are at light load, the DC bus , due to this capacitance, is not going absolutely down to zero volts, so this causes a loss of power factor…..this is because most PFC control chips actually sense the DC bus voltage and need it to be a train of half sines in order to implement a good PFC algorithm…however, at light load, the DC bus voltage is not really true half sinusoids, as it is being “held up” by the little bit of bus capacitance, so the controller is not able to implement as good a PFC algorithm.

In actual fact, in the ideal world, you’re not supposed to have the capacitance there, but in truth, you jolly well have as much capacitance as you can on the DC bus, as long as you can still pass the PFC regs.... and of course, who cares if you don't have good PFC below 75w, because below 75W, you don't actually need to be power factor corrected.

Indeed many pfc'd PSU's have output power detectors, so that the pfc stage gets completely switched out when the load has gone to light load, say 50W or so. This saves power because a pfc stage, even when operating at almost no load, typically draws around 3W of bias power, partly due to the switching losses.