Hi, I have some questions for the "Current Sense Offset and PWM Ramp" which is mentioned in UCC28070 datasheet 8.2.2.8. Could you please help to answer them?
1, As the datasheet said, the purpose of this offset and ramp circuit is to add some offset at light load, when inductor current is in DCM. But i did not understand that, during PFC mosfet turned off period, UCC28070 will have current synthesizer to simulate the down slope current signal. That is say even there is current signal ringing when current is DCM, the chip should not be false triggered, because it is using the simulated current signal
2, could you please explain the figure 28 the false current ? In this figure, Rsa voltage has some offest, so it look like the waveform of offset and ramp circuit impelemented. But why when Mosfet is off, Rsa has no offset voltage? And if the ramp and offset circuit has already be implemented, why it is called "Discontinours current cause false current signal".
Thanks
Tao Wu