UCC3858

The UCC3858 provides all of the control functions necessary for active ower factor corrected preregulators which require high efficiency at low ower operation. The controller achieves near unity power factor by haping the AC input line current waveform to correspond to the AC input ine voltage using average current mode control.

The operation of the UCC3858 closely resembles that of previously designed nitrode PFC parts with additional features to allow higher efficiency oost converter operation at light loads. This is accomplished by inearly scaling back the PWM frequency when the output of the voltage rror amplifier drops below a predetermined user programmable level indicating  light load condition. The frequency is scaled back by reducing he charging current for the CT ramp (in proportion to the output power), nd increasing the dead time. There is also an instantaneous reset input o pull the IC out of foldback mode quickly when the load comes back up.

The PWM technique used in the UCC3858 is leading edge modulation. hen combined with the more conventional trailing edge modulation on he downstream converter, this scheme offers the benefit of reduced ripple urrent on the bulk storage capacitor. The oscillator is designed for asy synchronization to the downstream converter. A simple synchronization cheme can be implemented by connecting the PWM output of he downstream converter to the SYNC pin.