ISBN-10:
0750679700
ISBN-13:
9780750679701
Pub. Date:
07/06/2006
Publisher:
Elsevier Science
Switching Power Supplies A - Z

Switching Power Supplies A - Z

by Sanjaya Maniktala

Hardcover

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Product Details

ISBN-13: 9780750679701
Publisher: Elsevier Science
Publication date: 07/06/2006
Edition description: Book & CD-ROM
Pages: 528
Product dimensions: 7.60(w) x 9.30(h) x 1.40(d)

Read an Excerpt

Switching Power Supplies A–Z


By Sanjaya Maniktala

Newnes

Copyright © 2012 Elsevier Ltd.
All right reserved.

ISBN: 978-0-12-386534-2


Chapter One

The Principles of Switching Power Conversion

Introduction

Imagine we are at some busy "metro" terminus one evening at peak hour. Almost instantly, thousands of commuters swarm the station trying to make their way home. Of course, there is no train big enough to carry all of them simultaneously. So, what do we do? Simple! We split this sea of humanity into several trainloads — and move them out in rapid succession. Many of these outbound passengers will later transfer to alternative forms of transport. So, for example, trainloads may turn into bus-loads or taxi-loads, and so on. But eventually, all these "packets" will merge once again, and a throng will be seen, exiting at the destination.

Switching power conversion is remarkably similar to a mass transit system. The difference is that instead of people, it is energy that gets transferred from one level to another. So we draw energy continuously from an "input source," chop this incoming stream into packets by means of a "switch" (a transistor), and then transfer it with the help of components (inductors and capacitors) that are able to accommodate these energy packets and exchange them among themselves as required. Finally, we make all these packets merge again and thereby get a smooth and steady flow of energy into the output.

So, in either of the cases above (energy or people), from the viewpoint of an observer, a stream will be seen entering and a similar one exiting. But at an intermediate stage, the transference is accomplished by breaking up this stream into more manageable packets.

Looking more closely at the train station analogy, we also realize that to be able to transfer a given number of passengers in a given time (note that in electrical engineering, energy transferred in unit time is "power") — either we need bigger trains with departure times spaced relatively far apart OR several smaller trains leaving in rapid succession. Therefore, it should come as no surprise that in switching power conversion, we always try to switch at high frequencies. The primary purpose for that is to reduce the size of the energy packets and thereby also the size of the components required to store and transport them.

Power supplies that use this principle are called "switching power supplies" or "switching power converters."

"DC–DC converters" are the basic building blocks of modern high-frequency switching power supplies. As their name suggests, they "convert" an available DC (direct current) input voltage rail "VIN" to another more desirable or usable DC output voltage level "VO." "AC–DC converters" (see Figure 1.1), also called "off-line power supplies," typically run off the mains input (or "line input"). But they first rectify the incoming sinusoidal AC (alternating current) voltage "VAC" to a DC voltage level (often called the "HVDC rail" or "high voltage DC rail") — which then gets applied at the input of what is essentially just another DC–DC converter stage (or derivative thereof). We thus see that power conversion is, in essence, almost always a DC–DC voltage conversion process.

But it is also equally important to create a steady DC output voltage level, from what can often be a widely varying and different DC input voltage level. Therefore, a "control circuit" is used in all power converters to constantly monitor and compare the output voltage against an internal "reference voltage." Corrective action is taken if the output drifts from its set value. This process is called "output regulation" or simply "regulation." Hence, the generic term "voltage regulator" for supplies which can achieve this function, switching, or otherwise.

In a practical implementation, "application conditions" are considered to be the applied input voltage VIN (also called the "line voltage"), the current being drawn at the output, that is, IO (the "load current"), and the set output voltage VO. Temperature is also an application condition, but we will ignore it for now, since its effect on the system is usually not so dramatic. Therefore, for a given output voltage, there are two specific application conditions whose variations can cause the output voltage to be immediately impacted (were it not for the control circuit). Maintaining the output voltage steady when VIN varies over its stated operating range VINMIN to VINMAX (minimum input to maximum input) is called "line regulation," whereas maintaining regulation when IO varies over its operating range IOMIN to IOMAX (minimum-to-maximum load) is referred to as "load regulation." Of course, nothing is ever "perfect," so nor is the regulation. Therefore, despite the correction, there is a small but measurable change in the output voltage, which we call "ΔVO" here. Note that mathematically, line regulation is expressed as "ΔVO/VO × 100% (implicitly implying it is over VINMIN to VINMAX)." Load regulation is similarly "ΔVO/VO × 100%" (from IOMIN to IOMAX).

However, the rate at which the output can be corrected by the power supply (under sudden changes in line and load) is also important — since no physical process is "instantaneous." So, the property of any converter to provide quick regulation (correction) under external disturbances is referred to as its "loop response" or "AC response." Clearly, the loop response is in general, a combination of "step-load response" and "line transient response."

As we move on, we will first introduce the reader to some of the most basic terminology of power conversion and its key concerns. Later, we will progress toward understanding the behavior of the most vital component of power conversion — the inductor. It is this component that even some relatively experienced power designers still have trouble with! Clearly, real progress in any area cannot occur without a clear understanding of the components and the basic concepts involved. Therefore, only after understanding the inductor well enough, can we go on to demonstrate the fact that switching converters are not all that mysterious either — in fact they evolve quite naturally out of a keen understanding of the inductor.

Overview and Basic Terminology

Efficiency

Any regulator carries out the process of power conversion with an "efficiency," defined as

η = PO/PIN

where PO is the "output power," equal to

PO = VO × IO

and PIN is the "input power," equal to

PIN = VIN × IIN

Here, IIN is the average or DC current being drawn from the source.

Ideally we want η = 1, and that would represent a "perfect" conversion efficiency of 100%. But in a real converter, that is, with η < 1, the difference "PIN - PO" is simply the wasted power "Ploss" or "dissipation" (occurring within the converter itself). By simple manipulation, we get

Ploss = PIN - PO Ploss = PO/η - PO Ploss = PO × (1 - η/η)

This is the loss expressed in terms of the output power. In terms of the input power, we would similarly get

Ploss = PIN × (1 - η)

The loss manifests itself as heat in the converter, which in turn causes a certain measurable "temperature rise" [increment of T] over the surrounding "room temperature" (or "ambient temperature"). Note that high temperatures affect the reliability of all systems — the rule of thumb being that every 10°C rise causes the failure rate to double. Therefore, part of our skill as designers is to reduce this temperature rise and also achieve higher efficiencies.

Coming to the input current (drawn by the converter), for the hypothetical case of 100% efficiency, we get

IIN_ideal = IO × (VO/VIN)

So, in a real converter, the input current increases from its "ideal" value by the factor 1/η.

IIN_measured = 1/η × IIN_ideal

Therefore, if we can achieve high efficiency, the current drawn from the input (keeping application conditions unchanged) will decrease — but only up to a point. The input current clearly cannot fall below the "brickwall," that is, "IIN_ideal," because this current is equal to PO/VIN — that is, related only to the "useful power" PO, delivered by the power supply, which we are assuming has not changed.

Further, since

VO × IO = VIN × IIN_ideal

by simple algebra, the dissipation in the power supply (energy lost per second as heat) can also be written as

Ploss = VIN × (IIN_measured - IIN_ideal)

This form of the dissipation equation indicates a little more explicitly how additional energy (more input current for a given input voltage) is pushed into the input terminals of the power supply by the applied DC source — to compensate for the wasted energy inside the power supply — even as the converter continues to provide the useful energy PO being constantly demanded by the load.

A modern switching power supply's efficiency can typically range from 65% to 95% — that figure being considered attractive enough to have taken switchers to the level of interest they arouse today and their consequent wide application. Traditional regulators (like the "linear regulator") provide much poorer efficiencies — and that is the main reason why they are slowly but surely getting replaced by switching regulators.

Linear Regulators

"Linear regulators," equivalently called "series–pass regulators," or simply "series regulators," also produce a regulated DC output rail from an input rail. But they do this by placing a transistor in series between the input and the output. Further, this "series–pass transistor" (or "pass transistor") is operated in the linear region of its voltage–current characteristics — thus acting like a variable resistance of sorts. As shown in the uppermost schematic of Figure 1.2, this transistor is made to literally "drop" (abandon) the unwanted or "excess" voltage across itself.

The excess voltage is clearly just the difference "VIN - VO" — and this term is commonly called the "headroom" of the linear regulator. We can see that the headroom needs to be a positive number always, thus implying VO < VIN. Therefore, linear regulators are, in principle, always "step-down" in nature — that being their most obvious limitation.

In some applications (e.g., battery-powered portable electronic equipment), we may want the output rail to remain well regulated even if the input voltage dips very low — say down to within 0.6 V or less of the set output level VO. In such cases, the minimum possible headroom (or "dropout") achievable by the linear regulator stage may become an issue.

(Continues...)



Excerpted from Switching Power Supplies A–Z by Sanjaya Maniktala Copyright © 2012 by Elsevier Ltd. . Excerpted by permission of Newnes. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.

Table of Contents

Chapter 1: The Principles of Switching Power Conversion Chapter 2: DC-DC Converter Design and Magnetics Chapter 3: Off-line Converter Design and Magnetics Chapter 4: The Topology FAQ Chapter 5: Conduction and Switching Losses Chapter 6: Printed Circuit Board Layout Chapter 7: Feedback Loop analysis and Stability Chapter 8: EMI from the Ground U—Maxwell to CISPR Chapter 9: Measurements and Limits of Conducted EMI Chapter 10: Practical EMI Line Filters Chapter 11: DM and CM Noise in Switching Power Supplies

Chapter 12: Fixing EMI across the Board

Chapter 13: Input Capacitor and Stability Considerations in EMI Filters

Chapter 14: The Math behind the Electromagnetic Puzzle

Appendix 1: Focusing on Some Real-world Issues

Appendix 2: Reference Design Table

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