Surface Mount LEDs – Part 4: Applications

This is Part 4 of a 4 Part Series

  1. Surface Mount LEDs – Part 1: The Basics
  2. Surface Mount LEDs – Part 2: How They are Made
  3. Surface Mount LEDs – Part 3: Optical Properties
  4. Surface Mount LEDs – Part 4: Applications (this article)

Hey designers and engineers!  Do you need a light for your application that is tiny, inexpensive, bright, long lasting, efficient, low power and just about any color you can imagine, including variable color and dimmable?  Then welcome to the world of LEDs! Because of their performance characteristics, light emitting diodes have become ubiquitous…they are everywhere, and for good reason! Whether you need indicator lights, or mood lighting, or you are developing a projector or other high intensity lighting source, or a display, or a sanitizing device, etc., LEDs are more and more becoming the clear choice for almost all lighting applications, as they are often the lowest cost, most efficient, and overall best solution.

When packaged as surface mount technology (SMT) devices, LEDs are very easily used in circuits, enabling a great deal of design flexibility.  SMT LEDs are currently available in packages as small as 0201 size (0.020” x 0.010”). They are also easily arrayed, allowing design flexibility for indicator lighting, general lighting, mood lighting, light engines, displays and the like, especially when they are used in combination with flex circuit technology.  

When designing an LED into your application, please remember that LEDs are polar devices and reverse bias can ruin them; meaning that placement orientation is critical, and to not design them into AC circuits if there is any significant negative voltage portion to the AC source.  Additionally, remember that the current-voltage characteristics of LEDs enable high currents at voltages just beyond the forward voltage, and that current will change from device to device, and even within a device as the temperature of the LED changes; so better to put a resistor in series with the LED so as to limit the current and avoid burning it out.  When placed in series with the LED, the resistor value selected should be at least the circuit voltage divided by the rated current of the LED (calculated from the rated wattage of the LED). Also, the rated wattage of the resistor should at least match that of the LED. While this approach wastes some energy due to resistive heating and limits the brightness of the LED, it prevents premature failure of the LED and is a common method used for simple indicator LED circuits and the like.

Another, better way to avoid premature failure of your LED is to use an LED driver circuit.  Typically, these are constant current DC-DC converters, which enable consistent LED output with about 90% efficiency.  A further advantage is that by providing constant current to the LED, brightness does not diminish as the voltage of the power source changes (e.g., battery discharge or the like).   This method of powering your LED is highly recommended if your budget allows. Constant current driver circuits for LEDs are available in small SMT devices, such as the small outline transistor size SOT-23-6 (illustrated below) that is ~3mm square.

An even better way to provide power to your LED is to use a driver circuit that employs pulsed width modulation (PWM).  This is usually accomplished by way of a microcontroller that uses either MOSFET (metal oxide semiconductor field effect transistor) or IGBT (insulated gate bipolar transistor) technology.  PWM enables dimming of an LED in an efficient manner by adjusting the percent duty cycle of power delivery.

PWM also enables higher brightness in an LED, as you can drive to higher voltages (and thus higher current) without burning out the LED.  This is typically done using a relatively high drive frequency (i.e., short duration cycles) of relatively high peak voltage, and adjusting the % duty cycle, so that the LED is effectively overdriven, yet does not overheat and burnout, since the intense pulses are relatively short in duration.  To do this effectively, the designer needs to know the peak forward current rating of the LED, as well as the conditions under which this peak current rating is specified, at least in terms of pulse width and duty cycle.

Properly employed, PWM can be an effective way to increase LED brightness in an efficient manner that does not adversely affect device longevity.  Further, SMT LEDs mounted on flex circuitry offer a form factor that is useful for a broad range of applications. Two examples of this configuration are shown below.

In summary, LEDs offer myriad options to the designer.  SMT LEDs are very small, presenting a useful form for designers, especially when flex circuit technology is employed.  The drive circuitry can be anywhere from a simple battery (with high internal resistance), through an included series resistor, through use of a DC-DC convertor, to more exotic programmable systems employing PWM technology. Simply put, SMT LEDs and associated circuit technologies are enabling more designers and engineers to realize their design dreams every day…welcome to the new world!

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