Designing LED Lighting Systems For Optimal Light Output

Jan 1, 2011 12:00 PM
Jeff Perry Senior Development Manager, National Semiconductor

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With the rising costs of energy and increasing governmental requirements for high efficiency lighting, the high brightness LED market is expanding rapidly. LEDs promise high efficiency, but the cost can be high, so how can a designer get more light out of an LED to reduce the number of LEDs required in an array? The most direct way is to increase the current, but that in turn increases temperature, which can degrade the light output, decrease efficiency and reduce lifetime. Thus, heat sinking is necessary. A proper LED lighting design will take into account all these factors and also the LED driver and LED string sizing. New tools have been developed to ease the LED lighting design process and allow users to make tradeoffs between high efficiency, small footprint and low cost.

LED BEHAVIOR IS DYNAMIC

The first goal a lighting designer needs to set is the light output of the system. This is typically specified using luminous flux, in units of lumens, which is a measure of visible light from a given source. Traditionally, the designer would go to an LED datasheet and look at the specifications for luminous flux and use those parameters to choose the number of LEDs required for the system. But as anyone who has studied an LED datasheet knows, LED behavior is dynamic based on the current being used to drive the LEDs and also the LED temperature.

Typically, the luminous flux is specified at a constant 25°C temperature using a short burst of current in the lab. But in reality, LEDs run hot and the temperature is considerably above the ambient. The best production LEDs on the market today are only about 25% to 30% efficient, which may be considered surprising given the energy efficiency hoopla surrounding them. In fact, this is great compared to the efficiency of a tungsten filament bulb, which may run around 2.2% for a 100W bulb giving off 1500 lumens. But the question is where does that 70% power loss in an LED go? Unlike a tungsten bulb which emits a significant amount of infrared radiation to give off heat, LEDs must get rid of heat through conduction. And that means heat sinks and temperature control are a must. What specific parameters should a designer be concerned with which vary with the LED current and temperature? The important ones include luminous flux, Vf (LED forward voltage drop) and luminous efficacy (luminous flux divided by the power consumed in units of lumens/watt) which is a measure of the efficiency of the LED.

The luminous flux of LEDs goes up with LED current, which can be useful if a designer wants to reduce the number of LEDs in the array to lower the cost (Fig. 1A). In fact, LEDs can often be driven with up to 2x or 3x the nominal current (check the datasheet for maximum current) to get more light output. But the tradeoff is high temperature which increases with increasing current for a fixed heat sink size (Fig. 1B). Higher temperatures mean decreased lifetime and reliability for the LEDs. This also lowers the light output of the LED, perhaps significantly (Fig. 1C). To lower the temperature, a larger heat sink can be used, but this will increase the cost and footprint of the design.

In contrast to the luminous flux, the luminous efficacy goes down with increasing current (Fig. 1D). This drop in efficacy may cause the loss of a governmental efficiency standard approval and certainly make the product less appealing from an energy conservation standpoint. In addition, the forward voltage of the LEDs increases with current and decreases with temperature which may affect the driver design. For example, in a series string of LEDs, the total forward voltage must be kept below the minimum input voltage for a simple buck design, otherwise a boost or buck-boost topology may be required. Thus, we see that the lighting designer must make compromises between cost, footprint, reliability and efficiency when designing an LED system. It's not as simple as just raising the current.

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