Reference Design: Isolated 50 Watt Flyback Converter Using the UCC3809 Primary Side Controller
Lisa Dinwoodie · 2001
The flyback power stage is a popular choice for single and multiple output dc-to-dc converters at power levels of 150 Watts or less. Without the output inductor required in buck derived topologies, such as the forward or push-pull converter, the component count and cost are reduced. This application note will review the design procedure for the power stage and control electronics of a flyback converter. In these isolated converters, the error signal from the secondary still needs to cross the isolation boundary to achieve regulation. By using the UC3965 Precision Reference with Low Offset Error Amplifier on the secondary side to drive an optocoupler and the UCC3809 Economy Primary Side Controller on the primary side, a simple and low cost 50 Watt isolated power supply is realized. INTRODUCTION The flyback converter reviewed in this application note is specifically designed to interface with the voltage ranges used in the telecommunications industry. The primary goal of this 5V, 50 Watt power supply is an efficient design which meets all the specifications while maintaining low cost. This goal is achieved by using the UCC3809 on the primary side for fixed frequency current mode control and using the error amplifier and precision reference of the UC3965 on the secondary side. Each of these 8-pin integrated circuits requires minimal external parts resulting in an economical yet effective design. The schematic is shown in Figure 1 and the list of materials is tabulated on page 16.