SNVAA87 august   2023 LMR38020

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
    1. 1.1 Micro Inverter System
    2. 1.2 Typical Power Tree and Design Requirements
  5. 2Conventional Flyback Design Challenges
    1. 2.1 SSR Design Challenges
    2. 2.2 PSR Design Challenges
  6. 3New Fly-Buck Design
    1. 3.1 LMR38020 Overview
    2. 3.2 Comparison with Conventional Flyback
    3. 3.3 Design Considerations
    4. 3.4 LMR38020 Fly-Buck Design Example
  7. 4Bench Test and Result
    1. 4.1 Start Up
    2. 4.2 Typical Switching Waveforms Under Steady State
    3. 4.3 Efficiency
    4. 4.4 Load Regulation
    5. 4.5 Short Circuit
    6. 4.6 Thermal Performance
  8. 5Summary
  9. 6References

3 New Fly-Buck Design

A Fly-Buck™ converter is one of the best design options for low power applications. The Fly-Buck™ has the merits of low component count, simplified design, high efficiency, and good transient response when compared with the conventional isolated switch mode power supply topologies.

The Fly-Buck™ converter is based on the standard buck converter topology by replacing the regular inductor with a coupled inductor or transformer to produce one or multiple isolated secondary outputs. Figure 3-1 shows the simplified Fly-Buck™ circuit schematic, to produce a non-isolated output VOUT1 and an isolated output VOUT2. By adding more secondary winding, you are able to obtain additional isolated outputs.

GUID-20230807-SS0I-P5DV-TZGD-LV9WFDMDM4ZR-low.svg Figure 3-1 Simplified Fly-Buck™ Circuit Schematic

The Fly-Buck™ closed loop control is carried out as in a regular buck converter to regulate the primary voltage. The secondary output voltage has also been regulated through the coupling of the transformer that tracks the primary voltage regulation.

Fly-Buck™ produces a tightly regulated primary output, along with one or multiple electrically isolated secondary outputs without the need of using an optocoupler to regulate the secondary. This means that the design of a Fly-Buck™ converter is relatively straight forward and can be done similarly to the design of a typical buck converter with some minor variations. In the following a design example using the LMR38020 synchronous buck converter is presented.