Chapter 1

Introduction

Abstract

This chapter introduces the book and outlines the content of each chapter. A brief introduction is given to the types of lighting application that can benefit from using light-emitting diodes.

Keywords

lighting
applications
light emitting diode
LED
dimming
color
As a Field Applications Engineer for many years, for one of the pioneering developers of integrated circuits (ICs) for driving power light-emitting diodes (LEDs), I have helped many potential customers in solving design problems. Some have had little or no experience about how to properly drive an LED. Others have had experience with traditional constant voltage power supplies, but LEDs need to be driven with a constant current.
The datasheet of a particular LED will give its current rating, which if exceeded will shorten the expected lifetime. Low power LEDs rated at 20 mA or so can be abused to some extent. However, the power requirements have been increasing; current ratings of 30, 50, 100, 350 mA, and higher are becoming common. If a high power LED is abused, its lifetime will be shortened. Now there are several manufacturers and the power levels are up to 20 W and rising; but these higher powers use LED chip arrays. The names high-bright (HB)-LEDs and ultra-bright (UB)-LEDs are becoming meaningless as the power levels continue to rise. This book will cover all types of LED drivers, from low power to UB-LEDs and beyond.
The unique advantage of LEDs over older styles of lighting is that the color is precise and can be selected for the application. Old filament lamps producing white light are filtered to produce a color, but this is very inefficient because most of the light is blocked by the filter. Obvious applications are traffic lights, using red, amber, and green LEDs. Less obvious applications are lamps for plant growing, where the color affects the type of growth (foliage or fruit). Color control is also used in some alarm clocks, so they wake the user in a controlled manner; color affects mood. A similar application is lighting for seasonally affected disorder (SAD), particularly for people living in the far north or far south, where long periods of darkness during the winter months can lead to depression. Also, LEDs are being used in increasing numbers; in channel lighting (signage), street lights, automotive lighting, mood/atmosphere lighting (color changing “wall wash”), theater lighting for stairs, and emergency exits. More details of these and other applications will be given in Chapter 17.
Is power LED driving simple? No, not usually. In a few cases a linear regulator can be used, which is simple, but most of the cases require a switching power supply with a constant current output. Linear driving is inefficient and generates far too much heat, although for low current applications they can be a good low-cost solution. Fortunately, many IC suppliers provide calculation tools for switching supplies, to help the designer. With a switching supply, the main issues are electromagnetic interference (EMI) and efficiency, and of course cost. The problem is to produce a design that meets legal requirements and is efficient, while costing the least.

1.1. Objectives and General Approach

The approach of this book will be very practical, although some theory is introduced when necessary for understanding of later chapters. It is important to understand the characteristics of components before they can be used effectively. It is also important to understand circuit limitations, to decide whether a particular circuit type is suitable or not to meet the end-equipment’s specification. In some cases, costs can be reduced by asking for a change in specification, which can make the circuit simpler. For example, if the power supply voltage is just slightly higher than the LED forward voltage, a linear regulator is cheap and efficient.
In most chapters, I will include a section called “Common Errors.” This section will highlight errors that engineers have made, and how these can be avoided, with the hope that readers will not make the same mistakes. It is said that people learn from their mistakes, but it is also true that we can learn from the mistakes of others. Our own mistakes are more memorable, but also more costly!
Usually the first problem for a designer is to choose between different topologies. When is a buck preferred to a buck–boost or a boost? Why is a Cuk boost–buck better than a fly-back type? These choices will be discussed in Chapter 10.
Power supply design equations will be given and example designs of practical supplies will be worked through. With switching power supplies, equations are needed to make the correct component choice; a wrong component can make a poor power supply and require a lot of corrective action. Power LEDs generate a lot of heat in a small area, which makes thermal management difficult, so a colocated power supply should be efficient and will not add too much heating effect.
The implications of changing the calculated component values into standard values, which is more practical, will be discussed. In many cases, customers want to use standard off-the-shelf parts because of ease of purchase and cost. Calculations rarely produce a standard value, so a compromise has to be made. In some cases the difference is negligible. In some cases it may be better to choose a higher (or lower) value. All changes in component value will introduce some “error” in the final result.
Having proven, worked examples in the book will help the reader to understand the design process: the order in which the design progresses. It will also show how the calculated component value compares with the actual value used and will include a description of why the choice was made.

1.2. Description of Contents

In Chapter 2, LED characteristics are described. It is also important to understand the characteristics of LEDs to understand how to drive them properly. One of the characteristics is color; an LED emits a very narrow band of wavelengths so the color is fairly pure. The LED color is determined by the semiconductor materials, which also affect the voltage drop across the LED while it is conducting, so a red LED (a low energy color) has a low forward voltage drop and a blue LED (a high energy color) has a high forward voltage drop. The voltage drop also varies with the current level because there is internal resistance that drops part of the voltage. But the current level determines the light output level: higher current gives higher luminosity from a given LED. The light output from an LED is characterized by both intensity and the angle of beam spreading.
But LED development was driven by the requirements of the end applications in many cases, so Chapter 2 also describes these, but not in great detail. More details of applications will be given at the end of this book, in Chapter 17. The description of some LED applications will show the breadth of the LED driving subject and how LED’s physical characteristics can be used as an advantage.
Chapter 3 will show that there are several ways to drive LEDs. As most electronic circuits have traditionally been driven by a voltage source, it is natural for designers to continue this custom when driving an LED. The trouble is that this is not a good match for the LED power requirement. A constant current load needs a constant voltage source, but a constant voltage load (which is what an LED is) needs a constant current supply.
So, if we have a constant voltage supply, we need to have some form of current control in series with the LED. By using a passive series resistor, or active current regulator circuit, we are trying to create a constant current supply. In fact, a short circuit in any part of the circuit could lead to a catastrophic failure, so we may have to provide some protection. Detecting an LED failure is possible using a current monitoring circuit. This could also be used to detect an open circuit. Instead of having a constant voltage supply, followed by a current limiter, it seems sensible to just use a constant current supply! There are some merits of using both constant voltage supply and a current regulator, which will be described in Chapter 4.
Chapter 3 continues, describing features of constant current circuit. If we have a constant current source, we may have to provide some voltage limiting arrangement, just in case the load is disconnected. For example, in a switching boost converter, the output voltage could rise to high levels and damage components in the circuit. Open circuit protection can take many forms. If the circuit failed open, the output voltage would rise up to the level of the open circuit protection limit, which could also be detected.
A short circuit at the load of a linear regulator would make no difference to the current level, so voltage monitoring would be a preferred failure detection mechanism. In some case, the heating effect will be the greatest problem because all of the supply voltage will be across the regulator. However, in a switching buck converter, a short circuit will cause problems because the switching duty cycle is unable to reach zero. This topic is discussed further in Chapter 5.
Another fault detection method, used in switching regulators, is to monitor the switching duty cycle. A short circuit would result in a very short duty cycle and an open circuit would result in a very long duty cycle, so monitoring the duty cycle is an indirect means of fault detection.
Chapter 4 describes linear power supplies, which can be as simple as a voltage regulator configured for constant current. Advantages include no EMI generation, so no filtering is required. The main disadvantage is heat dissipation and the limitation of having to ensure that the load voltage is lower than the supply voltage; this leads to a further disadvantage of only allowing a limited supply voltage range.
Switched linear regulators, where several linear regulators are used in combination, are used in AC mains–powered applications. The regulators are turned on and off as the AC voltage rises and falls. These types of regulators produce low levels of EMI and generally have a good power factor (PF), which are highly desired characteristics. With careful design, good efficiency and good performance are possible. Chapter 4 will discuss the design of such circuits.
Chapter 5 describes the most basic switching LED driver: the buck converter. The buck converter drives an output that has a lower voltage than the input; it is a step-down topology. This type of topology is quite efficient and there are a number of current control methods that are commonly used, such as hysteretic control, synchronous switching, peak current control, and average current control. A number of example driver ICs will be described and compared. The reader will be taken through design processes, followed by example designs.
Chapter 6 describes boost converters. These are used in many applications including LCD backlights for television, computer, and satellite navigation display screens. The boost converter drives an output that has a higher voltage than the input; it is a step-up topology. Battery-powered systems use either inductive boost or charge pump topologies. Higher input voltage systems are usually based on an inductive boost topology. Driver ICs from a number of semiconductor manufacturers will be described. The reader will be taken through the design process, followed by example designs, for both continuous mode and discontinuous mode drivers.
Chapter 7 describes boost–buck converters. These have the ability to drive a load that is either higher or lower voltage compared to the input. However, this type of converter is less efficient than a simple buck or boost converter. These types of topologies are well known as “Ćuk” or “SEPIC.” Again, a number of driver ICs are available for these topologies and they will be described. Design examples of Ćuk and SEPIC will be given.
The boost–buck is popular in automotive applications and battery-operated handheld equipment because the load voltage can be higher or lower than the supply voltage. In automotive applications, the battery voltage varies a lot; low when the starter motor is being operated and high when the alternator is charging the battery. In handheld equipment, the battery voltage starts high, but can fall to low levels when the battery is fully discharged.
Chapter 8 describes nonisolated circuits that have power factor correction (PFC) incorporated into the design. I will start with a typical PFC boost circuit, before moving on to more specialist converters: boost–buck, boost–linear, buck–boost–buck (BBB), and Bi-Bred. These converters are intended for AC input applications, such as traffic lights, streetlights, and general lighting.
The topologies described in Chapter 8 combine PFC with constant current output. In some cases the circuits can be designed without electrolytic capacitors, which are useful for high reliability applications. The efficiency of a circuit with PFC is lower than a standard offline buck converter, but government regulations worldwide require LED lighting to have a good PF if the power level is 25 W or more. This power limit is being reduced and in future could be as low as 5 W. Another reason to have a good PF is cost; utilities charge commercial and industrial customer more if the PF is low.
Chapter 9 describes fly-back converters and isolated PFC circuits. This chapter describes simple switching circuits that can be used for constant voltage or constant current output. A fly-back circuit using two or three windings in the power inductor permits isolation of the output. PFC can be achieved by controlling the input power dynamically over the low frequency AC mains input cycle. But a fly-back using a single winding inductor is actually a nonisolated buck–boost circuit and this is sometimes used for driving LEDs.
Chapter 10 covers topics that are essential when considering a switch mode power supply. The most suitable topology for an application will be discussed. The advantages, disadvantages, and limitations of each type will be analyzed in terms of supply voltage range and the ability to perform PFC. Discussion will include snubber techniques for reducing EMI and improving efficiency and limiting switch-on surges using either inrush current limiters or soft-start techniques.
Chapter 11 describes electronic components for power supplies. The best component is not always an obvious choice. There are so many different types of switching elements: MOSFETs, power bipolar transistors, and diodes, each with characteristics that affect overall power supply performance. Current sensing can be achieved using resistors or transformers, but the type of resistor or transformer is important; similarly with the choice of capacitors and filter components. The performance of operational amplifiers (op-amps), comparators, and high-side current sensors will also be discussed here.
Magnetic components are often a mystery for many electronic engineers and these will be briefly described in Chapter 12. One of the most important physical characteristics from a power supply design point of view, whether designing your own inductors or buying off-the-shelf parts, is magnetization and avoiding magnetic saturation, which will be discussed.
Chapter 12 will also be useful for those designing their own inductors and fly-back transformers. There are different materials: ferrite cores, iron dust cores, and special material cores. Then there are different core shapes and sizes. Some cores need air gaps, but others do not. All these topics will be discussed.
EMI and electromagnetic compatibility (EMC) issues are the subjects of Chapter 13. It is a legally binding requirement in most parts of the world that equipment should meet EMI standards. Good EMI design techniques can reduce the need for filtering and shielding, so it makes sense to carefully consider this to reduce the cost and size of the power supply. Meeting EMC standards is also a legal requirement in many cases. It is of no use having an otherwise excellent circuit that is destroyed by externally produced interference, such as a voltage surge on an AC line or a voltage spike across an automotive DC supply. In many areas, EMC practices are compatible with EMI practices; so fixing one often helps to fix the other.
Chapter 13 also covers MOSFET-driving techniques that, while reducing EMI, will also reduce switching losses. This will increase the efficiency and reliability of the LED driver.
Chapter 14 discusses thermal issues for both the LEDs and the LED driver. The LED driver has issues of efficiency and power loss. The LED itself dissipates most of the energy it receives (voltage drop multiplied by current) as heat: very little energy is radiated as light, although manufacturers are improving products all the time. Handling the heat by using cooling techniques is a largely mechanical process, using a metal heat sink and sometimes airflow to remove the heat energy. Calculating the temperature is important because there are operating temperature limits for all semiconductors.
Another legal requirement is safety, which is covered in Chapter 15. The product must not injure people when it is operating. This is related to the operating voltage and some designers try to keep below safety extra low voltage (SELV) limits for this reason. When the equipment is powered from the AC mains supply, the issues of isolation, circuit breakers, and creepage distance (the space between high and low voltage points on the PCB) must be considered. Some applications, such as swimming pool lighting, have very strict rules for safety (as you would expect).
Chapter 16 covers control systems. These include traditional control like 1–10 V linear dimming and triac-controlled phase-cut dimmers. A description of triac-controlled dimmers will be given to explain why they are so difficult to use with LED lighting. Newer digital control techniques, such as DALI and DMX will be described. Automotive and industrial applications often use the LIN bus or the CAN bus, so some description of these will be given.
Chapter 17 returns to the topic of applications. Applications were briefly described in Chapter 2 to help explain the development of LEDs. In this chapter we describe more applications and in greater detail, while referring to the various LED driver circuits that are suitable. The reasons why some circuit topologies are better than others in a particular application will be discussed.
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