8.1 Overview

The UCC28630, UCC28631, UCC28633, UCC28633 and UCC28634 family of devices are highly-integrated, primary-side-regulated (PSR) flyback controllers. The device supports magnetically-sensed output voltage regulation via the transformer bias winding. This feature eliminates the need for a secondary-side reference, error amplifier and opto-isolator. The device employs an advanced internal control algorithm that offers accurate static output voltage regulation against line and load. The fixed-point, magnetic-sampling scheme allows operation in both continuous conduction mode (CCM) and discontinuous conduction mode (DCM). Additionally, the device achieves accurate constant-current (CC) control of the output current limit using only primary-side, current sensing. Uniquely, this CC function operates seamlessly as the operating mode changes between DCM and CCM operation.

The controller includes an internal, high-voltage (HV) start-up current-source, and employs low-power sleep modes and switching frequency reduction, to improve light-load efficiency and standby power. The device typically achieves standby power levels between 0.05% and 0.1% of peak output power.

The controller operates in either DCM and CCM, using a mix of peak current-mode PWM (AM) and switching-frequency modulation (FM) schemes. The control approach improves performance (efficiency, size and cost) and can reduce transformer size and cost by allowing operation in CCM with FM during peak overload conditions. Extensive protection features are incorporated, including output overvoltage protection (OVP), bias rail overvoltage and undervoltage (OV/UV), active X-capacitor discharge, line undervoltage and brownout protection, overcurrent overload timer, open- and short-circuit pin protections, peak current adjustment with line and frequency dither for system EMI reduction. The various devices in the UCC2863x family offer a different mix of features to suit a wide range of applications and requirements.