7.1 Overview

The UCC28742 is a flyback power-supply controller which provides high-performance voltage regulation using an optically coupled feedback signal from a secondary-side voltage regulator. The device provides accurate constant-current regulation using primary-side feedback. The controller operates in discontinuous-conduction mode (DCM) with valley-switching to minimize switching losses and allow for the use of low cost output rectifiers. The control law scheme combines frequency with primary peak-current amplitude modulation to provide high conversion efficiency across the load range. The control law provides a wide dynamic operating range of output power which allows the power-supply designer to achieve low standby power dissipation.

During low-power operating conditions, the power-management features of the controller reduce the device-operating current at switching frequencies below 25 kHz. At and above this frequency, the UCC28742 includes features in the modulator to reduce the EMI peak energy of the fundamental switching frequency and harmonics. A complete low-cost and low component-count solution is realized using a straight-forward design process.