Need help with a PFC controller

[Jolle]

Hi
I am designing a 120W PFC using a Texas instument UCC28051 controller. The converter works as it should when I look at the output voltage, which is about 396 V, but when I look at the current through the inductor it looks like the controller enters some kind of burst mode?!

You can see a plot of the inductor current (green) and gate signal (yellow) below, during a 50 Hz cycle, the converter was delivering 60 watt to a load when the plot was made:


Has anyone seen a PFC behave this way before and know why it behaves as it does?

Regards

[muvideo]

I would check these:
- core saturation (linear voltage ramp on CS pin)
- spikes on CS pin
- device supply voltage Vcc
- ZCD pin

Does the output voltage fall or is regulated?

Fabio.

[Kremmen]

But isn't it supposed to behave something like that? A PFC is doing 2 things: maximize cos(phi) i.e. make voltage and current equal in phase, and secondly minimize current spiking due to capacitive load. The ideal would be a resistance-like V/I curve. The PFC cannot do anything to the instantaneous line voltage but by modulating it can distribute the current more evenly over the half-wave.
Just by looking at the picture i cannot say if that is what it is doing, or if it is some fault condition.

[muvideo]

But isn't it supposed to behave something like that? A PFC is doing 2 things: maximize cos(phi) i.e. make voltage and

As far as I know to do so that current envelope should be like a sinusoid semiperiod,
the control loop then changes (acts on time scales slower than a semiperiod) 
the current value to regulate the output voltage on the capacitor bank. 

Fabio.

[Kremmen]

Well yes, but i am not familiar with that chip and its capabilities. In case it just does a rough approximation this could be the result. Or then it is just malfunctioning.

[Jolle]

Kremmen hit the nail on the head with the comment about my IC being malfunctioning. I hadn't thought of that since the controller was regulating, but when I soldered on a new IC everything worked fine and my PF is now very close to 1 

So thanks to all of you who helped me with my relative simple problem.