This application note provides information on how to select FETs for primary side switches in 48V power converters. Basic topologies of different 48V power converters are introduced and detailed guidelines for selecting FETs are provided.
This document addresses the design of voltage converters powered by the nominal 48-V telecom supply, which typically ranges from 36V to 72V. It highlights the need for input to output isolation of up to 1500V and discusses output voltages frequently used in processors, such as 5V, 3.3V, and as low as 1.3V. The note introduces a family of high voltage MOSFETs ranging from 80V to 200V drain voltage specifications to assist in proper FET selection.
The text details several power converter topologies suitable for 48-V conversion, including flyback, single transistor forward, and two transistor forward converters. It analyzes the advantages and limitations of each, such as power levels and efficiency, and recommends specific Fairchild MOSFET models like the FDS3670, FDS2570, FDS2670, and FDS3570 based on voltage requirements and power levels.
The maximum useful power level for a flyback design will be around 20 to 30 watts.
The maximum output power that can be obtained with a single primary-side FET is approximately 80W.
Using the FDS3670, the maximum output power is 180 watts.