Designing an Isolated Buck (Fly-BuckTM) Converter using the LMR36520

Abstract

The purpose of this application note is to lead the reader through the design of an isolated buck also known as a Fly-Buck™ converter using the LMR36520 from Texas Instruments. This application note will describe the typical operation of a Fly-Buck™ converter from a theoretical perspective, and then walk through the process of Fly-Buck™ converter design from a set of given operating conditions using design equations derived in referenced reports. Physical measurements will be compared to expected results, and design limitations will also be discussed.

Trademarks

Fly-Buck™ is a trademark of Texas Instruments.

SIMPLE SWITCHER® is a registered trademark of Texas Instruments.

All trademarks are the property of their respective owners.

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.

2 Fly-Buck Converter Device Operation

Figure 2-1 General Fly-Buck™ Converter Circuit Diagram

The simplest definition of a Fly-Buck™ converter is a synchronous buck converter with the inductor replaced by a coupled inductor and an isolated secondary output. The primary side output is still regulated just like a typical buck converter, and the ideal transfer function from input to primary output is:

Equation 1.

V_{o u t 1} = \frac{t_{o n}}{t_{o n} + t_{o f f}} \times V_{i n} = D \times V_{i n}

The secondary winding of the coupled inductor acts as the source for the secondary output which in its simplest form consists of a rectifier diode and an output capacitor. More components can be added to the secondary output to optimize its performance. See Section 3 for more details. The combination of the rectifying diode and secondary output capacitor provide a DC output voltage to the secondary load determined by Equation 2:

Equation 2.

V_{o u t 2} = (V_{o u t 1} \times \frac{N_{2}}{N_{1}}) - V_{f}

The coupled inductor or transformer provides electrical isolation between primary and secondary outputs, essentially making the secondary float with respect to the primary. Electrical isolation ensures that users are protected from dangerously high voltages which may be present on the input to the device. It is important to note that the secondary output voltage can be either positive or negative with reference to the isolated ground simply by changing the isolated ground location. This is illustrated in Figure 2-2.

Figure 2-2 Fly-Buck™ Converter with Inverting Secondary

Figure 2-3 Fly-Buck™ Converter Steady State Waveforms

Figure 2-3 shows the typical steady state waveforms expected of a Fly-Buck™ converter. During the ON-time, the HS or high-side MOSFET is ON and ideally looks like a short circuit. This means that the SW node voltage is equal to the input voltage and the voltage across the primary side of the coupled inductor is V_L = V_IN - V_OUT. Since this is a buck converter, V_IN > V_OUT so V_L is positive during the on-time. The voltage induced across the secondary winding is:

Equation 3.

V_{L 2} = V_{L 1} \times (\frac{N_{2}}{N_{1}})

Following the dot convention, the voltage at the dotted terminal of the secondary winding is positive because current enters the dotted terminal of the primary winding. This means that the rectifying diode is therefore reverse biased since the voltage seen at its anode is negative with respect to the isolated ground, and current is supplied to the secondary load by the output capacitor. This reverse bias voltage across the diode during the on-time is given by:

Equation 4.

V_{D} = V_{o u t 2} + (\frac{N_{2}}{N_{1}}) \times (V_{i n} - V_{o u t 1})

During the off-time, the SW node voltage is pulled down to GND potential due to the LS or low-side MOSFET conducting. The resulting primary winding voltage is:

Equation 5.

V_{L 1} = - V_{o u t 1}

Following the same procedure as during the ON time, the voltage induced across the secondary winding is now negative at the dotted terminal, which results in the rectifier diode being forward biased. The secondary winding now acts as a current source, as it transfers energy from the primary side to the secondary load. Similar to what happens in normal buck operation, the DC portion of this current is provided to the secondary load while the AC portion of the current charges the secondary output capacitor.