LT3799 Active PFC FLyback controller does not give unity power factor...why?

This post relates to this closed thread, as there are further questions about the LT3799 Active PFC Flyback controller…..
https://www.edaboard.com/showthread.php?t=332473

Hello,

The LT3799 is said to be a “Flyback controller with Active PFC”.

However, when I ran the linear.com provided simulation of the LT3799 in LTspice, it is evident that the current is not really in phase with the line voltage. (as the attached waveform shows…the power factor is 0.97)
I have attached the LTspice simulation that they provided here, and have LC filtered the mains input current, so it can be easily seen. You can see that it’s not in phase with the line voltage.

Why are they saying that it is Actively Power Factor Corrected?.......when all it is doing is making the primary peak current be a scaled factor of the primary DC Bus voltage? (As you know, that provides a power factor of unity with the Boundary conduction BOOST topology…..but NOT with the Flyback topology).

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Also, the LT3799 sets the primary peak current on the basis of the "Vin_sense" pin voltage, and the lowest of the voltages voltage on the three CNTRL pins......so where in the LT3799 datasheet is the graph which shows primary peak current against Vin_sense voltage, and there should be a series of such graphs for the various CNTRL pin voltages.....i cannot understand why they dont show such a graph?

Hello,
The Error Amplifier inside the LT3799 is unusual. Usually, a “generic” error amplifier has an incoming signal_to_be_regulated coming into one of its pins, and then the other input pin of the error amplifier is a reference voltage.
The Error amplifier inside the LT3799 doesn’t seem to be like that. …It has a reference voltage on one pin, (or all three of them as it takes the lowest of the three CNTRL voltages) but the other input pin seems to be connected to the output of the scaled DC Bus voltage signal.
When a “generic” error amplifier is regulating, then the voltage on both of its input pins usually should be the same. But in the case of the LT3799 that would mean the scaled dc bus voltage being a constant voltage…..which it shouldn’t be…obviously.
It doesn’t make sense. Do you know what’s going on with the error amplifier inside the LT3799?

The attached LTspice simulation (and pdf schematic) shows two almost identical simulations of the LT3799. The only difference is that one has a 100n capacitor connected as feedback on its internal error amplifer…whereas the other one shorts this capacitor out…so the error amplifier behaves as a simple voltage follower…………………..the circuit waveforms for both simulation circuits, as you can see, are the same……so what on earth is the point of the Error Amplifier inside the LT3799?