UCC1858

UCC2858

UCC3858

PRELIMINARY

DESCRIPTION

The UCC3858 provides all of the control functions necessary for active

power factor corrected preregulators which require high efficiency at low

power operation. The controller achieves near unity power factor by

shaping the AC input line current waveform to correspond to the AC input

line voltage using average current mode control.

The operation of the UCC3858 closely resembles that of previously de-

signed Unitrode PFC parts with additional features to allow higher effi-

ciency boost converter operation at light loads. This is accomplished by

linearly scaling back the PWM frequency when the output of the voltage

error amplifier drops below a predetermined user programmable level in-

dicating a light load condition. The frequency is scaled back by reducing

the charging current for the CT ramp (in proportion to the output power),

and increasing the dead time. There is also an instantaneous reset input

to 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.

When combined with the more conventional trailing edge modulation on

the downstream converter, this scheme offers the benefit of reduced rip-

ple current on the bulk storage capacitor. The oscillator is designed for

easy synchronization to the downstream converter. A simple synchroni-

zation scheme can be implemented by connecting the PWM output of

the downstream converter to the SYNC pin.

(continued)

High Efficiency, High Power Factor Preregulator

BLOCK DIAGRAM

FEATURES

• Programmable PWM Frequency

Foldback for Higher Efficiency at Light

Loads

• Leading Edge PWM for Reduced

Output Capacitor Ripple Current

• Controls Boost PWM to Near Unity

Power Factor

• World Wide Operation without

Switches

• Accurate Power Limiting

• Synchronizable Oscillator

• 100µA Startup Supply Current

• Low Power BCDMOS

• 12V to 18V Operation

03/99

UDG-96191-1