Design Con 2015

Basic concepts of linear regulator and switching mode power supplies, Part one

-August 28, 2013

Editor's note: Designers at all levels of power management design will find this application article both educational for those not very familiar with these basic concepts as well as an excellent refresher for the seasoned power designer. Part one will give an introduction as well as basics of the operation of a linear regulator and a switching power supply. Part two will cover design considerations of the switching power components, discuss the feedback loop, PCB layout and other important aspects and topologies of switching supply design.

Abstract

This article explains the basic concepts of linear regulators and switching mode power supplies (SMPS). It is aimed at system engineers who may not be very familiar with power supply designs and selection. The basic operating principles of linear regulators and SMPS are explained and the advantages and disadvantages of each solution are discussed. The buck step-down converter is used as an example to further explain the design considerations of a switching regulator.

 

INTRODUCTION

 

Today’s designs require an increasing number of power rails and supply solutions in electronics systems, with loads ranging from a few mA for standby supplies to over 100A for ASIC voltage regulators. It is important to choose the appropriate solution for the targeted application and to meet specified performance requirements, such as high efficiency, tight printed circuit board (PCB) space, accurate output regulation, fast transient response, low solution cost, etc. Power management design is becoming a more frequent and challenging task for system designers, many of who may not have strong power backgrounds.

 

A power converter generates output voltage and current for the load from a given input power source. It needs to meet the load voltage or current regulation requirement during steady-state and transient conditions. It also must protect the load and system in case of a component failure. Depending on the specific application, a designer can choose either a linear regulator (LR) or a switching mode power supply (SMPS) solution. To make the best choice of a solution, it is essential for designers to be familiar with the merits, drawbacks and design concerns of each approach.

 

This article focuses on non-isolated power supply applications and provides an introduction to their operation and design basics.

 

LINEAR REGULATORS

How a Linear Regulator Works

 

Let’s start with a simple example. In an embedded system, a 12V bus rail is available from the front-end power supply. On the system board, a 3.3V voltage is needed to power an operational amplifier (op amp). The simplest approach to generate the 3.3V is to use a resistor divider from the 12V bus, as shown in Figure 1. Does it work well? The answer is usually no. The op amp’s VCC pin current may vary under different operating conditions. If a fixed resistor divider is used, the IC VCC voltage varies with load. Besides, the 12V bus input may not be well regulated. There may be many other loads in the same system sharing the 12V rail. Because of the bus impedance, the 12V bus voltage varies with the bus loading conditions. As a result, a resistor divider cannot provide a regulated 3.3V to the op amp to ensure its proper operation. Therefore, a dedicated voltage regulation loop is needed. As shown in Figure 2, the feedback loop needs to adjust the top resistor R1 value to dynamically regulate the 3.3V on VCC.

 


 

Figure 1. Resistor Divider Generates 3.3VDC from 12V Bus Input

     

Figure 2. Feedback Loop Adjusts Series Resistor R1 Value to Regulate 3.3V

 

This kind of variable resistor can be implemented with a linear regulator, as shown in Figure 3. A linear regulator operates a bipolar or field effect power transistor (FET) in its linear mode. So the transistor works as a variable resistor in series with the output load. To establish the feedback loop, conceptually, an error amplifier senses the DC output voltage via a sampling resistor network RA and RB, and then compares the feedback voltage VFB with a reference voltage VREF. The error amplifier output voltage drives the base of the series power transistor via a current amplifier. When either the input VBUS voltage decreases or the load current increases, the VCC output voltage goes down. The feedback voltage VFB decreases as well. As a result, the feedback error amplifier and current amplifier generate more current into the base of the transistor Q1. This reduces the voltage drop VCE and hence brings back the VCC output voltage, so that VFB equals VREF. On the other hand, if the VCC output voltage goes up, in a similar way, the negative feedback circuit increases VCE to ensure the accurate regulation of the 3.3V output. In summary, any variation of VO is absorbed by the linear regulator transistor’s VCE voltage. So the output voltage VCC is always constant and well regulated.

   

 

Figure 3. A Linear Regulator Implements a Variable Resistor to Regulate Output Voltage

 


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