High-power LEDs are fast becoming the "darling" product in lighting and illumination. Not only does their luminous efficacy challenge the domain of incandescent and halogen light sources, but also they are available in a full range of colours and enjoy an extended operating life.
Successfully designing high-power LEDs into an application, however, differs markedly from conventional 5 mm and other low-power LEDs. It requires knowledge of a range of common pitfalls and problems if expensive mistakes are to be avoided. This includes technical issues such as thermal management.
This article will focus on the core issues in an applications context and use illustrative product examples from the most successful power LEDs to be launched to date: Luxeon from Lumileds.
After summarising the advantages of high-power LEDs, the use of these devices in applications such as industrial lighting and general lighting, automotive and mobile phones (flashes for camera phones) will be examined. Particular focus will also be given to associated electrical driving, control and power supply solutions.
High-power LEDs
Lumileds' high-power LEDs offer a long list of advantages over both conventional LEDs and competing high-power LED products. This includes an extremely high brightness (up to 160 lumens per LED) and luminosity density (lm/mm2), superior "white" performance, greater design flexibility, and enhanced thermal management and ESD protection.
Furthermore, the unobtrusive and potentially "hidden" operating nature of these devices gives designers huge flexibility; they offer a very robust resistance to vibration; there is no heat or UV light carried in the emitted light beam; they require a low voltage DC power source; they are infinitely dimmable and they come in a full range of colours.
Finally, high-power LEDs are also highly compact devices. Compared to conventional LEDs, they offer dramatically enhanced, forward constant current handling of between 350 mA and 1.4 A (for conventional LEDs this figure is just a few mA) plus a much lower thermal resistance.
Thermal management
In a regular LED, the thermal resistance (junction to pin) is typically over 300 ºC/W, corresponding to a radiometric power of less than 0.1 W and a photometric power of around 2 to 3 lm. Lumileds' high-power LEDs have a photometric power of up to 160 lm (Luxeon V family) and a typical thermal resistance (junction to case) of 8, 13 or 15 °C/W (Luxeon V, Luxeon III and Luxeon I families respectively). Also, the devices have a typical radiometric power of 700 mW at 455 nm wavelength (note this figure varies with wavelength for all LEDs).
The greater luminosity of high-power LEDs means they obviously need more power than conventional LEDs (watts instead of milliwatts) and thus produce a correspondingly higher amount of heat. For any LED, only 15 to 20 percent of the energy consumed by the device is actually emitted as light, the rest becomes heat.
This thermal energy has to be reliably conducted away from the die during operation if the die is to remain within recommended specifications for a given operational life span. Naturally, basic thermodynamics dictate that a high ambient operating temperature will require greater thermal management in the form of heatsinking.
In construction, the base of a typical Lumileds Luxeon emitter comprises a large metal heat sink (or 'slug') onto which the die is mounted (see diagram). The slug conducts heat away from the die, usually to an external heat sink. It is important to note that the standard slug can have electrical potential, so electrical isolation is required. The die itself is contained in a cup-shaped reflector to maximise light output.
Applications overview
For white light applications, the competition between power LEDs and traditional light sources is currently quite intense. The choice often boils down to design preference, but there are big differentiators in favour of high-power LEDs. This includes an extended operational life span of up to 100,000 hours compared to 1000 hours for typical incandescent bulbs (allowing domestic bulbs that could last for up to 10 years); high reliability - LEDs are solid state devices with no moving parts, glass or filaments; and reduced maintenance costs, particularly in hard-to-access or remote locations such as bridges, tall buildings, helicopter landing pads and even the tops of wind generators.
Also, LEDs consume up to 90% less energy; they contain no mercury and their longer life span means less disposal waste issues. LEDs do not produce forward infrared heat or UV radiation, and they offer increased safety in applications that need a fast response time (e.g. automotive brake lights).
An excellent example is stage lighting. This application requires not only excellent colour dynamics, but the need to be able to rebuild and move sets on a regular basis. Here dramatically smaller and light weight power LED-based rigs and lighting boxes are hugely advantageous. Power LEDs also don't emit forward heat or UV in the beam so stage and TV professionals working below them do not "feel" the heating effect associated with conventional stage and set lighting.
Electronics design implications
Before examining some sample power-LED applications in detail, it is worth noting that all applications will require a suitable electrical driving solution. This includes a constant current, along with pulse-width modulation (PWM) dimming and possibly DMX communication capabilities.
These kinds of requirements are firmly in the electronics - rather than optical or general illumination - design domain. And designers from the latter fields quite understandably did not want to have to become experts in electronics and indeed thermal design in order to take advantage of power LED technology - if only on the basis of training cost alone.
Thankfully, a number of leading semiconductor manufactures and distributors such as Future Electronics have recognised the problem and now offer dedicated single and multi-chip driver solutions and reference designs plus full technical support. This has made it relatively easy to drop the desired control electronics into a circuit for new applications.
In addition, there is a growing need for power LED-based retrofit solutions and again these will be made to be as simple to design-in as possible. However, there are a number of electronics design basics associated with power-LED power supplies that currently need to be understood before embarking on an application. This subject will be covered in the next part of this article, which will also include a detailed look at how high-power LEDs are being used in automotive lighting and mobile-phone camera flash applications.
