Practical Lighting Design with LEDs

Ron Lenk, Carol Lenk

• Éditeur: Wiley-IEEE Press
• Année: 2017
• ISBN:
  9781119165323
• Édition: 2
• Sujet: Technology & Engineering
• Sous-sujet: Electrical Engineering & Telecommunications

The essential how-to guide to designing and building LED systems, revised and updated

The second edition of Practical Lighting Design with LEDs has been revised and updated to provide the most current information for developing light-emitting diodes products. The authors, noted authorities in the field, offer a review of the most relevant topics including optical performance, materials, thermal design and modeling and measurement. Comprehensive in scope, the text covers all the information needed to design LEDs into end products.

The user-friendly text also contains numerous drawings and schematics that show how things such as measurements are actually made, and show how circuits actually work.Designed to be practical, the text includes myriad notes and illustrative examples that give pointers and how-to guides on many of the book's topics. In addition, the book's equations are used only for practical calculations, and are kept at the level of high-school algebra.This thoroughly expanded second edition offers:

• New chapters on the design of an LED flashlight, USB light, automotive taillight, and LED light bulbs
• A practical and user-friendly guide with dozens of new illustrations
• The nitty-gritty, day-to-day engineering and systems used to design and build complete LED systems
• An essential resource on the cutting-edge technology of Light-Emitting Diodes

Practical Lighting Design with LEDs helps engineers and managers meet the demand for the surge in usage for products using light-emitting diodes with a practical guide that takes them through the relevant fields of light, electronic and thermal design.

Chapter 1
Practical Introduction To LEDs

Light bulbs are everywhere. There are over 20 billion light bulbs in use around the world today. That is, three for each person on the planet! We expect that within the next 10 years, the majority of these bulbs will be light-emitting diodes (LEDs). This is because LEDs can provide efficiency dozens of times higher than incandescent light bulbs. They can be as efficient as the theoretical limit for electricity to light conversion set by physics. This book is all about the practical aspects of LEDs and how you can make practical lighting designs using them.

What is an LED?

The purpose of this book is to tell you practical things about LEDs. So in this section, we're not going to regale you with jargon about “direct bandgap GaInP/GaP strained quantum wells” or such. Let's directly address the question: What is an LED?

The name “light-emitting diode” tells you a lot already. In the first place, the noun tells you that it is a diode. A diode conducts current in one direction and not the other. And that's what an LED does. While we'll explore the details of its electrical behavior in Chapter 4, the only thing to note for the moment is that it has a much higher forward voltage than the diodes usually used in electronics. While a 1N4148 has a drop of about 700 mV, an LED may drop 3.6 V. This is because LEDs are not made from silicon, but from other semiconductors. But other than that, an LED's electrical characteristics are very much like those of other diodes.

The words “light-emitting” tell you a lot more. Now all diodes emit at least a little bit of light. You can open up an integrated circuit (IC) and use a scanner to see which parts of the circuit are emitting light. This tells you which parts are conducting current. IC designers use this to help debug their ICs. However, the amount of light emitted by ICs is very small. Since the purpose of LEDs is to emit light, they have been carefully designed to optimize this performance. That's why, for example, they have a much higher forward voltage than normal, rectifier diodes. Rectifiers have been optimized to minimize their forward voltage while maximizing reverse breakdown voltage. LEDs are optimized to produce the most light of the right color at the lowest power, and things such as forward voltage (by itself) don't matter. Of course, forward voltage does enter into how much power the LED dissipates, and we'll see in Chapter 5 how to characterize the light emitted versus the power dissipated.

Small LEDs Versus Power Devices

Present-day thinking divides LEDs into two classes: small devices and power devices. Small LEDs became widely used in the 1970s. They come in all different colors, such as red, orange, green, yellow, and blue. They are the small T1¾ (5 mm) devices shown in Figure 1.1. Nowadays, there are literally tens of billions of them sold each year. They go into cell phone backlights, elevator pushbuttons, flashlights, incandescent bulb replacements, fluorescent tube replacements, road signage, truck taillights, traffic lights, automobile dashboards, and so on.

Figure 1.1 T1¾ (5 mm) LEDs.

What characterizes these small devices is their power level, or as the industry thinks of it, their drive current. The typical red small LED, for example, has a drive current of 20 mA. At a forward voltage of 2.2 V, this is only 44 mW of power. (The efficacy is so low that this is just about equal to the heat dissipation as well.) Small white LEDs have a higher forward voltage (3.6 V, corresponding to 72 mW), and some small LEDs can be run as high as 100 mA. But fundamentally, this type of LED is used as an indicator, not a real light source. It takes 14 of them to make a somewhat reasonable 1 W flashlight, and hundreds of them to make a (dim) fluorescent tube replacement.

While the information in this book is applicable to these small LEDs, the main focus is on power devices. Power devices are typically 1–3 W devices that are usually run at 350 mA. Their dice (the actual semiconductor, as opposed to its package) are substantially larger than those of small LEDs, although their footprint need not be. These devices are typically used in places requiring lighting, rather than as indicators. Applications include flashlights, incandescent bulb replacements, large-screen TVs, projector lights, automotive headlights, airstrip runway lighting, and just about everywhere lighting is used. Of course, not all of these applications have yet seen widespread adoption of power LEDs, but they will soon.

Phosphors Versus RGB

Most lighting designs are going to be made with white light (which includes incandescent “yellow” light). For this reason, this book concentrates primarily on white LEDs. However, what is described here for white LEDs can be straightforwardly applied to color LEDs. Color LEDs are very similar to white, albeit with differing forward voltage. The reason for the varying forward voltages is that the colored light (red, yellow, blue, etc.) is generated directly by the semiconductor material. The material is varied to get differing colors and the differences in material in turn cause differences in forward voltages.

However, white light cannot be directly generated by a single material (we are ignoring special types of engineered materials that are not yet in production). White light consists of a mixture of all of the colors. You already know this because white light can be separated into its constituent colors with a prism. White light thus has to be created. There are currently two main methods of generating white light with LEDs. In one method, an LED that emits blue light is used, and the blue light is converted to white by a phosphor. In the other method, a combination of different color LEDs is used.

The first method is the most common. A typical wavelength for the blue light generated by the LED is 435 nm. Why use blue light? This has to do with the physics of the way the white light is generated. The blue light is absorbed by a phosphor, and re-emitted as a broad spectrum of light approximating white. For the phosphor to be able to absorb and re-emit the light, the light coming out has to be lower in energy than the light going in. That's just like any electronic component. Energy goes in, some is dissipated as heat, and the rest comes out again, transformed. So to get all of the colors in the spectrum that humans can see, the phosphor needs to have input at a higher energy (shorter wavelength) than the shortest color's energy. For humans, this is about 450 nm, and so a 435 nm blue LED is the most energy-efficient way of generating white light using a phosphor.

Before turning to the second method of generating white light, we should say a few more words about the phosphor. There are various types of phosphors. Phosphors are designed to absorb one specific wavelength of light, and re-emit it at either one or more different wavelengths or in a band of wavelengths. LED phosphors are typically designed to do the latter. But there are limits to how broad a band of colors a phosphor can emit. So many LEDs use bi-band or tri-band phosphors to better cover the spectrum of light needed to approximate white. These phosphors are mixtures of two or three primary phosphors. These more complicated phosphors are typically used when better color rendition is needed (see the discussion of color rendering index (CRI) in Chapter 3).

As a side note, we can comment briefly on fluorescent lights. In some ways, a fluorescent light is quite similar to an LED, but its fundamental mechanism of light emission is different. It generates a high-temperature plasma inside a tube, which emits light in the ultraviolet (UV) range (254 nm) rather than in the blue range. But after that, it too uses a phosphor to absorb the light and re-emit it in the visible range. Note that since the wavelength of the light is considerably farther away from the visible spectrum than the 435 nm generated electrically by the LED die, the efficiency ultimately possible for a fluorescent is intrinsically lower than that possible for an LED. (At the moment, fluorescent lights and LEDs have roughly the same efficiency.)

But also interesting is the type of phosphor the typical fluorescent light uses. These phosphors are of the type that re-emits in just one or two narrow wavelengths, not in ...