UCC28500

UCC28500, UCC28501, UCC28502, UCC28503 UCC38500, UCC38501, UCC38502, UCC38503 SLUS419C − AUGUST 1999 − REVISED NOVEMBER 2001 BiCMOS PFC/PWM COMBINATION CONTROLLER FEATURES D Combines PFC and Downstream Converter Controls D Controls Boost Preregulator to Near-Unity Power Factor D Accurate Power Limiting D Improved Feedforward Line Regulation D Peak Current-Mode Control in Second Stage D Programmable Oscillator D Leading-Edge/Trailing-Edge Modulation for Reduced Output Ripple D Low Start-up Supply Current D Synchronized Second Stage Start-Up, with Programmable Soft-start D Programmable Second Stage Shutdown DESCRIPTION The UCC2850x family provides all of the control functions necessary for an active power-factorcorrected preregulator and a second-stage dc-to- dc converter. 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 dc-to-dc converter uses peak current-mode control to perform the step-down power conversion. The PFC stage is leading-edge modulated while the second stage is trailing-edge synchronized to allow for minimum overlap between the boost and PWM switches. This reduces ripple current in the bulk-output capacitor.In order to operate with over three-to-one range of input-line voltages, a line feedforward (VFF) is used to keep input power constant with varying input voltage. Generation of VFF is accomplished using IAC in conjunction with an ...