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

2 Fly-Buck Converter

GUID-20230801-SS0I-TF1N-J2JL-WFPVGZVSNBD1-low.svgFigure 2-1 General Fly-Buck Converter Circuit

The Fly-Buck™ converter is based on standard buck converter topology in which the regular inductor is replaced by a coupled inductor or transformer such that one or multiple isolated secondary outputs can be produced. Figure 2-1 shows a Fly-Buck converter with one non-isolated output and one isolated output. Additional isolated output can be easily obtained by more secondary windings coupled to the transformer core.

Basically the closed loop operation is still a buck converter and it regulates the primary output voltage. The secondary output voltage is also regulated via cross regulation by winding coupling.

Therefore the Fly-Buck converter is able to produce a tightly regulated primary output voltage, along with one or more isolated outputs without the need of an optocoupler. This means that designing a Fly-Buck™ converter is relatively straightforward and similarly to designing a typical buck converter with minor adjustments.