SNVA790A October   2020  – July 2022 LMR36520

 

  1.   Abstract
  2.   Trademarks
  3. 1Introduction
  4. 2 Fly-Buck Converter Device Operation
    1. 2.1 Output Current Equations and Considerations
  5. 3LMR36520 Fly-Buck Converter Design
    1. 3.1 Coupled Inductor
    2. 3.2 Primary Output Capacitor
    3. 3.3 Rectifying Diode
    4. 3.4 Secondary Output Capacitor
    5. 3.5 Preload Resistor
    6. 3.6 Zener Diode
    7. 3.7 Snubber Circuit
  6. 4Experimental Results
    1. 4.1 Steady State
    2. 4.2 Secondary Output Voltage
    3. 4.3 Load Transient
    4. 4.4 Start-up
    5. 4.5 Output Current
  7. 5Conclusion
  8. 6References
  9. 7Revision History

1 Introduction

Many applications require various power rails to supply power to different devices within a system. It is not uncommon for these different power rails to each have differing load requirements. Some rails may need to be very tightly regulated, offering a minimum of peak-to-peak variation to meet their respective load requirement. Some rails on the other hand, can power loads that are more robust to variations on the input supply and, therefore, do not require such tight regulation.

Rather than having a different DC/DC regulator for each different power line, some systems can benefit from using a single IC to produce multiple power rails. The Flyback topology offers the possibility of producing multiple power lines from a single input, but that comes with a cost of using more complex control circuitry to regulate the secondary. This is where the isolated buck, or Fly-Buck™ converter, comes in. The Fly-Buck™ converter is a modification to the standard buck converter topology that replaces the standard inductor with a coupled inductor or transformer to produce one or multiple isolated secondary outputs.

The Fly-Buck™ converter topology is beneficial because it 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.

This application note will detail the process of designing a Fly-Buck™ converter using the LMR36520. The LMR36520 is a 4.2-V to 65-V 2-A synchronous buck converter that is well-suited for industrial power applications. The HSOIC-8 package can handle PCB strain better than other leadless packages and also makes debugging in the field much easier by enabling visual inspection of the device leads. Internal compensation reduces external component count and simplifies pinout making the LMR36520 ideal for Fly-Buck™ converter applications.