26 August 2010
By Tony Armstrong, Director of Product Marketing, Power Products, Linear Technology Corporation
LEDs & Autos
LEDs continue to penetrate automotive lighting applications due to their long life and flexibility of use for body styling and
interior design. It might come as a surprise to some people that this trend continues to gain momentum despite the fact that a LED’s cost of implementation is greater than that of the incandescent light bulb. So, why would this be the case? The answer is surprisingly simple; the front of any vehicle looks like a face and how it looks has a profound effect on a potential buyer. LED lighting, used for the headlights, daytime running lights and turn signal indicators, allow for much greater design flexibility than that of either Xenon or incandescent light bulbs. This permits the body designers of an automobile’s front end to do things that they could never do before. A good example of this design flexibility is readily illustrated by Audi’s A-series automobiles that have their daytime running lights in a linear array underneath the headlights and turn signal indicators. If you have ever seen one in your rear view mirror, you’ll know exactly what I am talking about.
Nevertheless, it is the headlight itself, which has been the elusive goal of LED adoption in an automotive environment. The main reason for this has been the thermal design aspects of the LEDs and their associated driver circuits. Unlike a traditional light bulb, which is basically a ‘heater’ and operates at high temperature, a LED and its driver circuit need heatsinking incorporated as part of their enclosure so that heat can be taken away from the LEDs. Clearly a LED driver’s efficiency of conversion along with its associated power loss can have a significant impact on the thermal design aspects of the LED housing used in an automobile headlight configuration. All of these precautions are necessary because a LED’s light output and operational lifetime are negatively impacted by exposure to high temperatures.
LEDs & Capabilities
A high power, or high brightness LED’s light output has already exceeded the critical milestone of 100 lumens per Watt (lm/W). In fact, some manufactures are already claiming 200 lm/W in the laboratory. Clearly then, LEDs have surpassed an incandescent light bulb (15 lm/W for a typical 60W bulb) in terms of luminous efficacy. Or said another way, the amount of light output from a light source, measured in lumens, as a ratio of the amount of power consumed to produce it, measured in Watts. Even so, it is projected that by 2012, LEDs with 150 lm/W output will be readily available in the marketplace. Another added benefit is LED lifetime. Depending on how it is calculated, a white LED bulb has at least a 10,000-hour lifetime and some even claim up to 50,000 hours, while an incandescent bulb’s life is around 1,200 to 1,500 hours. Furthermore, LEDs are “Green,” since they do not contain any hazardous materials.
The cost of LED lighting has been coming down very quickly. The cost of individual white LEDs, several of which go into a LED bulb and make up much of the cost, have come down in price from about $5 a few years ago to under $1.00 in the last twelve months. Many LED industry analysts predict that over the course of the next twelve months LED bulb replacements for the incandescent light bulb will be priced at a level that will be acceptable for the consumer. Some LED manufacturers have already claimed that they have designed light-emitting chips that could power a LED bulb producing light comparable to the 75-Watt incandescent bulbs so commonly used in most private residences. This type of LED chip usually requires 12W to 15W of power in order to be able to output this amount of light.
Specifically in the case of cars, a headlight needs to have a light output of at least 800 lumens to be practical. Cree, a manufacturer of LEDs, has its XLamp XP-G series of LEDs, which it claims has the highest lumen density of any available lighting class of LEDs. Housed in their XP-family package, they can provide up to 400 lumens output at 1A and over 130 lm/W at 350mA. Thus, since a LED headlight will provide an energy benefit (they are typically 75% more efficient than the standard version), their use will reduce the level of fuel consumption on an annual basis. How much LEDs can actually save in fuel costs was evaluated in a 2008 study by the University of Michigan’s Transportation Research Institute (UMTRI). In it, they studied the power consumption of exterior lighting on passenger cars by comparing a traditional system using 100% incandescent and halogen lighting sources with a 100% LED equivalent. The results showed light power savings of over 50% with an all-LED system. This not only saves in fuel cost for a petrol powered vehicle but it would reduce the vehicle’s total carbon footprint by 1% to 2% per year.
Driving a 25W Headlamp
A 25W white LED headlamp can now be configured using an array of 18 LEDs in series with 350mA of current passing through them to produce the necessary light output. However, a major obstacle is how to efficiently and simply drive such a configuration. One possible solution is to use the recently introduced LT3956 monolithic LED driver from Linear Technology. The LT3956 is a DC/DC converter designed to operate as a constant-current and constant-voltage regulator. It is ideally suited for driving high current, high brightness LEDs (see Figure 1 below).
The LT3956 features an internal low side N-channel power MOSFET rated for 84V at 3.3A and is driven from an internal regulated 7.15V supply. The fixed frequency, current-mode architecture results in stable operation over a wide range of supply and output voltages. A ground based referenced voltage feedback (FB) pin serves as the input for several LED protection features and also makes it possible for the converter to operate as a constant-voltage source. A frequency adjust pin allows the user to program the frequency from 100kHz to 1MHz to optimize efficiency, performance or external component size.
The LT3956 senses the output current at the high side of the LED string. High side sensing is the most flexible scheme for driving LEDs, allowing boost, buck mode or buck-boost mode configurations. The PWM input provides LED dimming ratios of up to 3000:1 and the CTRL input provides additional analog dimming capability.
Efficiency of conversion for the LT3956 can be in the 94% range, depending on the input voltage and operating frequency. This is illustrated in the efficiency curve shown in Figure 2 right.
This high efficiency of conversion allows for a more straightforward thermal design for the LED headlight housing since this
LED driver does not contribute significantly to the heat generated by the LEDs themselves. In the example above, the LED driver has a power loss of 1.5W, which is dissipated as heat (25W * (1-0.94)). This has the added benefit of also saving on space and weight requirements.
Conclusion
For a high power LED driver to be used in an automotive environment it must have some key attributes. Clearly, they must be capable of delivering sufficient current and voltage for the different types of LED configurations in a conversion topology that satisfies both the input voltage range and required output voltage and current requirements. However, they should also posses the following features:
a) Wide input voltage range
b) Wide output voltage range
c) High efficiency conversion
d) Tightly regulated LED current matching
e) Low noise, constant frequency operation
f) Independent current and dimming control
g) Wide dimming range ratios
h) Small compact footprint with minimal external components
Even with this list of performance characteristics, the LED being driven by the LED driver has to be capable of delivering the necessary lumens of light output from the lowest possible level of power without causing significant thermal design constraint. Fortunately for the headlight designer there exists both the high efficacy LEDs and the high performance LED drivers to realize what had previously been considered mutually exclusive: high values of light output from low levels of input power.
Linear Technology • Tel: 01628 477066 • Email: uksales@linear.com • www.linear.com
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