Synchronous Buck Converter Enables Multiple Bias Rails And Input Voltage Sensing For Isolated Applications

Focus:

A synchronous buck converter can be used in an isolated (Fly-Buck) configuration to create multiple power rails on both the primary and secondary sides. The isolated outputs do not need any optocoupler-based feedback for regulation. The primary output is controlled directly and the additional outputs are regulated based on the transformer turns ratio. Therefore, a Fly-Buck converter is simpler to design than the traditional flyback-based solutions and results in smaller and cheaper isolated bias power solutions. Additionally, a Fly-Buck-based design can be easily modified to transfer input voltage information to the secondary side, thereby providing a compact, relatively accurate, and cost-effective sensing and power solution. Operating principles of a Fly-Buck converter are explained; design equations are given for a three-output Fly-Buck converter; and a simple, three-output Fly-Buck converter designed example based on the LM5017 synchronous buck regulator is presented. Then, input voltage sensing schemes are discussed including those based on use of an optocoupler, a rectified secondary-winding signal, or a pulse transformer. The limitations of each approach are described. Finally, a Fly-Buck-based input-voltage-sensing scheme is introduced, its benefits are described, and an example implementation using the LM5017 is presented.

What you’ll learn:

• How to understand the benefits and operation of the isolated buck or Fly-Buck converter
• How to design a multi-output Fly-Buck converter based on the LM5017 synchronous buck regulator

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Author & Publication:

Vijay Choudhary, Applications Engineer, Texas Instruments, Phoenix, Ariz., How2Power Today, Feb 15 2013

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