Low-energy LEDs make automotive lighting more efficient

introduction

LEDs continue to gain popularity in automotive applications, thanks to their long lifespan and the flexibility they offer for body styling and interior design. Some may be surprised that this momentum has not been stymied by the fact that LEDs are more expensive to implement than traditional incandescent bulbs. So what is the reason for the current surge in popularity of LEDs? The answer is surprisingly simple; the front of any vehicle is like a person's face, and its aesthetics have a profound impact on potential buyers. LED lighting for headlights, daytime running lights, and turn signals offers far greater design flexibility than either xenon or incandescent bulbs. This allows body designers at the front of the vehicle to achieve design options that were previously impossible. Audi's A-series cars exemplify this design flexibility with LED lighting: their daytime running lights are placed in a linear array below the headlights and turn signals. If you've ever seen one of these cars in your rearview mirror, you know exactly what I'm talking about.

However, headlights themselves have been an elusive application for LEDs in automotive environments. The primary reason for this is the thermal design of the LEDs and their associated driver circuitry. Unlike conventional light bulbs, which are essentially “heaters” and operate at high temperatures, LEDs and their driver circuitry require a heat sink to be integrated as part of their housing to move heat away from the LED. Obviously, the conversion efficiency of the LED driver and its associated power losses have a significant impact on the thermal design of the LED housing used in automotive headlight configurations. All of the above precautions are necessary, as high temperature operating environments have a negative impact on the light output and operating life of the LED.

1 LED and performance

The light output of high-power or high-brightness LEDs has surpassed the important milestone of 100 lm/W. In fact, some manufacturers have claimed to have achieved 200 lm/W of LED light output in the lab. Clearly, therefore, LEDs have surpassed incandescent bulbs in terms of luminous efficacy (a standard 60W bulb has a light output of 15 lm/W). Or, to put it another way: light output is expressed as the ratio of the amount of light output from a source (in lumens) to the amount of power (in watts) used to produce it. Even so, it is predicted that by 2012, LEDs with a light output of 150 lm/W will be widely available on the market. Another added benefit is the lifespan of LEDs. Depending on how you calculate it, white LED bulbs can last at least 10,000 hours, with some claiming as much as 50,000 hours, compared to around 1,200 to 1,500 hours for incandescent bulbs. In addition, LEDs are "green" because they do not contain any hazardous materials.

The cost of LED lighting is falling very fast. In the past 12 months, the price of specialized white LEDs (some of which are used in LED bulbs and account for most of their cost) has dropped from about $5 a few years ago to less than $1.00. Many analysts in the LED industry predict that it will be affordable for consumers to replace incandescent bulbs with LED bulbs in the next 12 months. Some LED manufacturers claim that they have designed light-emitting chips that can power LED bulbs and produce light as bright as the 75W incandescent bulbs widely used in most private homes. To be able to output this amount of light, such LED chips usually require 12W to 15W of power.

Specifically for cars, headlights must have a light output of at least 800 lumens. Cree, an LED manufacturer, has launched its XLampXP-G series of LEDs, which it claims have the highest lumen density among lighting LEDs currently on the market. These LEDs are packaged in the company's XP series and can provide up to 400 lumens of light output (at 1A) and more than 130lm/W of light output (at 350mA). So, given the energy savings that LED headlights will bring to car owners (they are typically 75% more efficient than standard versions of headlights), their use will reduce the annual fuel consumption of cars.

How much fuel cost can LED actually save? Michigan's Transportation Research Institute (UMTRI) at the University of Michigan evaluated this in a 2008 study. In that study, they investigated the power consumption of exterior lighting on buses by comparing a traditional system that used 100% incandescent and halogen lighting sources with a new system that used 100% LED lighting sources with comparable lighting effects. The results showed that the full LED system saved more than 50% of the light energy. This not only saves fuel costs for gasoline-powered vehicles, but also reduces the overall carbon emissions of automobiles by 1% to 2% per year.

2 Drive a 25W headlight

Today, a 25W white LED headlight can be configured with an array of 18 series-connected LEDs, with 350mA flowing through them to produce the desired light output. However, how to drive such a configuration efficiently and simply is a major obstacle facing designers. The LT3956 monolithic LED driver recently introduced by Linear Technology is a viable solution. The LT3956 is a DC/DC converter designed to act as a constant current and constant voltage regulator. It is well suited to driving high current, high brightness LEDs (see Figure 1).

Figure 1: 25W white LED headlight driver with 94% efficiency

The device features an internal low-side N-channel power MOSFET rated for 84V/3.3A and driven from an internal regulated 7.15V supply. The fixed frequency, current mode architecture enables stable operation over a wide range of supply and output voltages. A ground referenced voltage feedback (FB) pin serves as the input for several LED protection features and also enables the converter to function as a constant voltage source. A frequency adjust pin allows the user to set 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 current sensing is the most flexible approach for driving LEDs, allowing boost, buck mode, or buck-boost mode configurations. The PWM input provides up to 3000:1 LED

D dimming ratio, while the CTRL input provides additional analog dimming capability.

The LT3956 can achieve a conversion efficiency of approximately 94% (depending on the input voltage and operating frequency). This is shown in the efficiency curve given in Figure 2.

Figure 2 shows the relationship between the efficiency and VIN of the LT3956 circuit.

This high conversion efficiency enables a simpler and more straightforward thermal design for the LED headlight housing, as the LED driver does not significantly contribute to the heat generated by the LED itself. 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 reducing space and weight requirements.

3 Conclusion

For high-power LED drivers to be used in automotive environments, certain features are a must. Obviously, it must be able to use a conversion topology that meets both the input voltage range and the required output voltage and current requirements to provide sufficient current and voltage for different types of LED configurations. However, they should also have the following features: wide input voltage range, wide output voltage range, high efficiency conversion, tightly regulated LED current matching, low noise, constant frequency operation, independent current and dimming control, wide dimming ratio range, small and compact footprint, and very few external components.

Even with the above performance characteristics, the LED driven by the LED driver must be able to provide the required light output (lumens) from the lowest possible power level without causing significant thermal design limitations. Fortunately, for headlight designers, they already have both high-efficiency LEDs and high-performance LED drivers, which can achieve what has been considered mutually exclusive in the past (i.e., providing high light output from low input power levels).

Figure 3 Product Picture