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Tradeoffs in LED system design - droop, under-utilization and heat management

Posted: 2013/07/23

Assuming we are bound to using one type of LED package only and the same LED light housing material, the only two primary ways to increase the brightness of an LED light is to either a) increase the number of LEDs in the light, or b) increase the current applied to the LEDs. The first option (option A) is the most obvious option to most manufacturers and consumers. We often see spotlights, for example, designated as 3x1W = 3W, meaning that the light uses three 1W LEDs. The same manufacturer most likely will have a 5x5W = 5W model, where they simply increased the number of LEDs in order to increase the light output. Understandably, the cost of the product increases, as the number of LEDs required increases.

A spotlight using three 1W LEDs that has a power consumption of...(surprise!) 3x1W = 3W.

Option B, however, is often overlooked and not considered by many manufacturers and consumers alike. Voltage and current are simply fixed by the LED driver that is selected in most consumer LED products, however, by understanding the science behind how the driver's current level was selected, we can further understand the quality and performance of an LED light. Furthermore, for those who want to custom-design an LED system, electrical current parameters can be optimized for the particular application. For either case, what must be considered are the various trade-offs involved.

Trade-off #1 - brightness vs failure risk

The first concern is LED heat management, where the additional heat generated by a higher current leads to system failure. The critical issue at hand is junction temperature, as the temperature of the LED can fatalistically determine light longevity and stability. Usually, a junction temperature of 120 degrees C is the highest recommended for stable performance and maximum lifetime. Generally speaking, the higher the temperature, the higher the risk of something going wrong. Therefore, the more light you try to squeeze out of an LED, the higher the risk of failure. However, what is often overlooked is the assumption that all lights will be used in the same location. If an LED light is to be used in refrigeration lighting in supermarkets, perhaps the current can be increased slightly without significantly affecting the longevity of the LEDs. On the contrary, if the LED light is to be used in a very hot environment, perhaps a more conservative current level should be used. By adjusting the current to a level appropriate and optimized to the application, we can achieve a higher amount of light output from the system.

Trade-off #2 - total light output capacity utilization vs luminous efficacy

At higher electric currents, a phenomenon known as the droop effect comes into play. As our in-house data show (3528 80 CRI 4000K SMD), what we see is that while when we increase the current total light output increases (the blue line), the luminous efficacy begins to decrease (the red line). In other words, the marginal increase in light output decreases as input current increases. Researchers have yet to come to definitive conclusions about the cause of this phenomenon, although one suspected reason is what is called the Auger effect.

We tested a sample 3528 SMD under different currents. We see decreasing luminous efficacy as current is increased. Test sample: 4000 K, 80 CRI.

An article explaining the phenomenon suggests that, the Auger effect is behind the high cost of the LED bulb: A 60-watt equivalent currently retails for more than US $10. As the current through an LED is increased, the Auger effect becomes stronger, meaning that there is a higher proportion of electron-hole interactions that don't end in light emission. So to keep LED efficiency from slipping to the same level as that of compact fluorescents, manufacturers pack their bulbs with more chips that operate at lower currents. Instead of being able to, in principle, generate a 75-W equivalent output with a 1-square-millimeter chip, you are obliged to use 10 chips and drive them at 10 times less current.

Trade-off #3 - lower system acquisition cost vs lower operating cost

As the article quoted above states, one of the reasons for high LED lighting acquisition costs is the fact that we do not utilize the full capacity of LEDs included in an LED light. Indeed, increasing the current used in LEDs will allow us to achieve higher light output per LED (and consequently lower $/lm). However, due to the droop effect, much of the energy savings that were originally possible are no longer as significant as before, since we are now running a less efficient (though cheaper to acquire) system.

If you are designing an LED lighting system and your greatest concern is luminous efficacy (maybe you want to save as much as possible in electricity costs or you live in an area with high electricity costs), then you might consider designing an 'under-drive' system where you utilize a lower than rated current.

If you are designing an LED lighting system and your greatest concern is system procurement cost (maybe electricity costs are not as significant of a concern), you might want to use standard or higher currents to max out the lumens output in your LEDs.

For most end consumers, operating current and voltage are fixed by the driver used in the LED light and there is little flexibility in adjusting this unless the driver itself is changed. Nonetheless, when manufacturers design LED lights, they are well aware of this phenomenon and actively make a tradeoff between luminous output, luminous efficacy and light stability. Consumers should be aware of this tradeoff and understand that a cheaper LED light might be maximizing its light output per LED, therefore cutting down on costs, but bringing into play LED failure risk and decreasing LED efficacy due to the droop effect.

Through our R&D facilities and resources, we at Beijing Yuji have a thorough understanding of how LED lighting is manufactured. By working with our various other LED manufacturers and companies, we strive to provide our customers with products that meet their specific needs. Contact us to consult with us about your LED needs!

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