Designing an Isolated Buck Converter using the LMR38020

Abstract

Fly-Buck™ converter is a multi-output converter topology implemented with a synchronous buck converter on the primary side and additional isolated outputs can be produced like in a conventional flyback converter on the secondary side of a transformer, It has been widely used in various applications due to many inherent advantages.

This article show cases a simple and cost-effective Fly-Buck™ solution using the LMR38020 device from Texas Instruments. The operating principle and step-by-step design procedures are presented, along with experimental results and some design tips for optimal design.

Trademarks

Fly-Buck™ is a trademark of Texas Instruments.

All trademarks are the property of their respective owners.

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.

2 Fly-Buck Converter

Figure 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.

3 Fly-Buck Basic Operation

3.1 Basic Intervals of Steady State Operation

Figure 3-1 shows the typical steady state waveforms of a Fly-Buck™ converter in which V_pri is the primary voltage across the coupled inductor, i_m is the magnetizing current, i_m and i_sec are primary side current and secondary side current.

The operation of the Fly-Buck™ converter basically has two modes: TON and TOFF.

Figure 3-1 Fly-Buck™ Steady State Operation Waveforms

T_ON Mode

This mode is the same as traditional synchronous buck converter when the main switch(HS) is ON. The voltage stress of the low-side (LS) switch is the input voltage(V_IN). The magnetizing inductance, L_m is charged by the input voltage minus the primary output voltage as in the regular buck converter. The secondary winding current remains zero for the diode D₂ is reverse biased according to the winding polarity configuration, and D₂ sees voltage stress of (N₂/N₁)×(V_IN-V_OUT1)+V_OUT2. The isolated output capacitor C_OUT2 is supplying the load current.

T_OFF Mode

In this mode LS is ON and HS is OFF. V_pri becomes negative, forward biasing D₂ to force a secondary current to flow to transfer part of the stored energy in the coupled inductor to the secondary output capacitor, C_OUT2 and the load, R_Load2.

Unlike the buck converter, i_pri in Fly-buck decreases at a faster rate, owing to supplying current to both loads, I_OUT1 and I_OUT2.

The secondary current waveform is determined by the load, leakage inductance, and output capacitance. The current direction of i_pri at the end of one switching cycle (positive or negative) depends on factors including the current ratio of I_OUT2: I_OUT1 and current ripple.

The primary output voltage is the same as a buck converter and is given by Equation 1.

Equation 1. $V_{O U T 1} = \frac{T_{O N}}{T_{O N} + T_{F F}} V_{I N} = D \times V_{I N}$

The secondary output voltage is given by Equation 2.

Equation 2. $V_{O U T 2} = V_{O U T 1} \times (\frac{N_{2}}{N_{1}}) - V_{F}$

where

N₁ and N₂ are the turns of the primary winding and secondary winding
V_F is the forward voltage drop of the secondary rectifier diode

3.2 Impact Of Leakage Inductor On Fly-Buck Operation

In a real circuit, the transformer has leakage inductance and other parasitic inductance or capacitance as shown in Figure 3-2, which can affect the secondary current waveform.

Figure 3-2 Fly-Buck Converter Circuit Considering Parasitic

The Figure 3-3 shows the typical current waveform under different levels of leakage inductance(L_LK).

When L_LK is low, the i_sec ramps up quickly to charge up C_OUT2. With larger L_LK, i_sec rises linearly,resulting in larger negative peak current for i_pri. If the negative peak current of i_pri reaches the negative current limit of the device, the LS will be turned off and the charging to C_OUT2 will be terminated. Consequently this would result in less energy being transferred to the output and produce lower output voltage.

Therefore, the leakage inductance should be minimized and the maximum duty cycle must be chosen carefully to mitigate these issues. When the secondary output has no load, the turn on of LS can force a small current in the secondary side, and it would gradually charge up C_OUT2. Since there is no load to discharge C_OUT2, a net charge will be accumulated on C_OUT2 and raise the V_OUT2 remarkably. In order to prevent this from happening, a preload must be added to the secondary output to help removing the net charge on C_OUT2 so as to maintain the output voltage at the setting point.

Figure 3-3 Current Waveforms Affected by Leakage Inductance