SNVAA82 august   2023 LMR38020

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
  5. 2Fly-Buck Converter
  6. 3Fly-Buck Basic Operation
    1. 3.1 Basic Intervals of Steady State Operation
    2. 3.2 Impact Of Leakage Inductor On Fly-Buck Operation
  7. 4Design A Fly-Buck Converter with LMR38020
    1. 4.1 IC Select
    2. 4.2 Switching Frequency Set
    3. 4.3 Transformer Design
      1. 4.3.1 Turns Ratio
      2. 4.3.2 Magnetic Inductance
      3. 4.3.3 Check Ipk
    4. 4.4 Output Capacitor Selection
      1. 4.4.1 Primary Output Capacitor
      2. 4.4.2 Secondary Output Capacitor
    5. 4.5 Secondary Output Diode
    6. 4.6 Preload Resistor
  8. 5Bench Test Results
    1. 5.1 Typical Switching Waveforms Under Steady State
    2. 5.2 Start Up
    3. 5.3 Efficiency
    4. 5.4 Load Regulation
    5. 5.5 Short Circuit
    6. 5.6 Thermal Performance
  9. 6Design Considerations
  10. 7Summary
  11. 8References

1 Introduction

Conventionally, the flyback converter topology has been a very popular solution for applications that need multi isolated output voltages. However, Flyback converter design has to employ either an opto-coupler or an auxiliary winding as the feedback circuit for output regulation. The loop compensation becomes difficult and sometimes tricky. And the use of optocoupler not only increases the solution cost but also reduces the circuit reliability. To overcome these drawbacks, the Fly-Buck™ converter topology, also called isolated buck, are introduced.

A Fly-Buck™ converter is one of the most suitable options for low power applications in industrial automation, communication power supplies, intelligent electric meters, and so on. The Fly-Buck™ has the merits of low component count, simple design, high efficiency, and good transient response when compared with the conventional flyback converters.

The LMR38020 is a 4.2 V to 80 V, 2-A synchronous buck converter in the HSOIC-8 package. It's internal compensation saves external component and simplifies the IC pin out, making the LMR38020 ideal for Fly-Buck™ converter applications.

This article presents the basic operating principles of a Fly-Buck™ converter by going over key waveforms and design equations. The step-by-step design procedure is given through an example of one non-isolated and two isolated outputs.