Design Of Flyback AC-DC Controller XD1568 With Constant Current/Constant Voltage Output

Portable electronic products such as smart phones, tablet computers, etc. have made great development in recent years, and their charging equipments are inevitably affected by people's higher requirements. The flyback switching mode power supply system is undoubtedly the most popular topology currently in low power applications, due to its low cost, simple structure and electrical isolation, which make it the first choice for the power adapter, LED driver and other small and medium-sized power equipment.The flyback AC-DC Controller XD1568 that has a function of constant current / constant voltage output is designed in this paper. The XD1568 isolated-flyback power-supply controller provides Constant-Voltage(CV) using an optical coupler to improve transient response to large-load steps. Constant-Current(CC) regulation is accomplished through Primary-side Regulation(PSR) techniques. This device processes information from opto-coupled feedback and an auxiliary flyback winding for precise high-performance control of output voltage and current. Discontinuous conduction mode(DCM) with valley-switching reduces switching losses. Modulation of switching frequency and primary current-peak amplitude(FM and AM) keeps the conversion efficiency high across the entire load and line ranges.The basic situation of switching power supply and the application background of flyback switching power supply are sketched in this paper firstly.Then the isolation evolution of flyback converter, the basic principles of flyback converter and several common control modes are introduced.The main functional features of XD1568 are proposed on the basis of the principle analysis above and the principle to realize constant current, constant voltage, valley-switching and line voltage compensation are derived in detail as well. After the introduction of function modules, some of key module circuits are desigened, analyzed theoretically and simulated to verify the function. The typical application circuit is given at the end, and some important peripheral circuits are described.Based on a process of 0.35 um 5V/40V/700 V BCD, XD1568 is designed and simulated with Cadence and Hspice D.The simulation results of sub modules and the whole chip show that XD1568 meets the expected specfications.