October 18, 2018

By: Jim Harrison Guest Blogger, Lincoln Technology Communications

The use of LED lighting technology in automotive headlamps has made for some very handsome styling treatments recently—and some not so pretty. The lamps give the styling departments an edge—and they can provide performance/safety improvements as well. Plus, LEDs are much more energy efficient—which in this case is the same as improving gas mileage or EV range.

While parking and tail lights are nearly all LED-based, headlamps take a sizeable amount of power and accompanying driver design effort and have been slower on the take-up.

Way back in the 1890s, as soon as there was an automobile, people started driving at night. They used oil-burning lanterns (kerosene) as lighting devices. In 1908, the first automotive headlamp bulbs in the U.S. had a carbon filament that contained a vacuum and were not gas filled. The very first standard electric headlamps were introduced in 1899 on the Columbia Electric car. The famous Peerless Motor Company of Cleveland made electric headlamps standard on their fancy cars in 1908.

Figure 1. The 1911 Peerless 45-HP Model 32 had stylish headlights. (Photo by Alf van Beem)

On October 1, 1908, a fella named Henry watched the first Model T go out the door. It wasn’t until 1915 that Ford replaced the T’s acetylene and oil lamps with electric headlamps powered by a magneto. In 1940, the modern sealed beam headlight became the standard. For the next 17 years the government mandated the round 7-inch lamp. In 1957 the law changed to allow lights of different sizes and shapes, as long as they illuminated the road properly.

The first full LED headlamp was introduced in 2007. Now LED headlamps are becoming common. Standard full LED headlights draw 15W to 18W, whereas halogen bulb versions draw 55W to 65W and HID (high-intensity discharge) bulbs draw around 42W. Note there are some LED headlamp retrofit kits that produce an extreme amount of light and take a lot of power—25,000 lumens and 60W. A look at available LED headlight replacement bulbs online shows prices from $9.50 to $239.00 for a two-bulb set. That’s quite a range! It shows the headlight market is in a bit of a flux period, to say the least.

A Look at Headlamp Performance

A standard halogen headlight bulb will produce around 1300 lumen. Xenon HID bulbs are roughly two-and-a-third times brighter, at about 3000 lumen. HID lamps require more power to start up, but once they are on they operate at considerably lower power than halogen. On most car models, HID lighting is only used for the low beams while the high-beam light is done by separate halogen lights because high beams need to be turned on and off instantly, which HID is not good at. For some vehicles, the HIDs provide both the low  and high beam via a shutter that moves up and down when prompted, so there is no delay.

Automotive electronics designers look to various suppliers for SMT LED and LED matrix chips for headlamps. Recent single SMT LED chips have achieved 175lm/W efficiencies at 2W and 350lm. But much higher power single LEDs are available.

As an example, we can look at the CREE XHP70B-00-0000-0D0HN240H chip (part of their X-Lamp Series). The 7.0mm x 7.0mm footprint part puts out 1590lm at 85°C and 1751lm at 25°C. Efficiency is around 133lm/W. The lamp is configurable to 6V or 12V by PCB layout and maximum drive current is 2.4A at 12V. Test current is 1.05A. The lamp has a 4000k color temperature and a CRI of 80. These premium high-power lamps cost around $8 each at 1,000 qty. Many versions (bins) are available, at lower and higher cost. You may need only two of these 1590lm chips for a headlight assembly—perhaps three with high beam.

Maybe the foremost LED claim-to-fame for this application is longevity. The CREE part is expected to retain 85% of its light output after 23k hours of operation. This is 2.6 years continuous, so a 15-year operational lifetime is easy to see.

Other Lighting Design Considerations

For the designer, there are also other things to consider. In a vehicle, the required operating temperature range is between -40 °C to +120 °C. There are also the various climactic factors such as air moisture and salt content, as well as dust. There are strong vibration and shock stresses, as well as electromagnetic influences. One area of the design that needs prime attention is radio-frequency interference (RFI). Interference from LED driver high-speed PWM switching can cause problems for any of the myriad of electronic systems on the automobile, including everything from engine control and the data buses to the radio and the USB connections. RFI testing is essential.

High beams should provide night visibility out to 480 feet, while low beams yield at least 330 feet. More than half of midsize SUV headlights tested by the IIHS rated marginal or poor. The only two vehicles with a good rating used HID lamps. The beam shape is very important and many cars with LED lamps have not figured out how to optimize this. More than half of the 79 headlight variants evaluated in the IIHS tests have too much glare. The insurance institute says headlights often come off the factory line poorly aimed, which can cause glare and render the move to brighter LEDs wasted effort. Complaints about glare from oncoming headlights are very common, research by the National Highway Traffic Safety Administration indicates.

Another potential advantage for LED lamps is matrix lights. Individual lamps in a matrix are able to illuminate specific areas in a very targeted manner. The area where an oncoming car is can be darkened while either side of it is illuminated. Matrix headlights are not yet legal in the U.S. Automakers are pushing for government approval to open up even more options.

Controlling the Lamp

Next, we need to drive the LED lamp(s) as efficiently as possible and control intensity. There are many ICs available for this task. One great example of a LED controller suitable for headlights is the MAX20096/MAX20097 dual-channel, synchronous, n-channel, high-current buck LED drivers. The ICs use a proprietary PWM current control method that does not require any loop compensation and maintains a nearly constant switching frequency for relatively easy switching noise/RFI mitigation. Inductor current sense is achieved by monitoring the current in the bottom switching device.

The ICs operate over a 4.5V to 65V input range and each channel drives two external n-channel power FETs. The converter switching frequency can be as high as 1MHz, keeping components small. The LED PWM frequency is normally 200Hz.

The MAX20096 has an SPI interface that permits the output voltages and currents on both channels and the junction temperature to be read out and output current level to be set. Protection features include current-limit, overvoltage, and thermal shutdown. This device comes in a thermally enhanced 5mm x 5mm, 32-pin side-wettable TQFN package and is specified to operate over the -40°C to +125°C automotive temperature range.

The MAX20097 is available in a 28-pin thermally enhanced TSSOP package. It does not have the SPI interface, relying instead on an open-drain fault flag (FLTB) that goes low in case of an open string, shorted string, thermal shutdown, or overvoltage activation in either channel.

Figure 2. A recent BMW headlight assembly (Photo from AutoPhography).

The chips are suitable for automotive high-beam/low-beam headlamps and/or signal/daytime running lights. Supply current for both devices is 10mA maximum, and they support analog dimming control. The chip regulates the average current in the switch-mode inductor. LED current can be set from zero to 3A using an analog input or digital control via PWM. Dimming via PWM can also be accomplished via the SPI interface on the MAX20096. Each channel is independently adjustable. PWM dimming maintains the same LED color regardless of brightness, whereas LEDs may change color with analog dimming.

Driver circuity efficiency is about 93% driving two to four series LEDs. LED headlamps are usually specified at 35W to 50W, which these ICs can handle using a 14V to 48V supply. An eval kit is available for both versions.

Two Alternative Chips

Maxim also has the MAX20090 single-channel device that offers a flexible driving scheme allowing boost, high-side buck, SEPIC, or buck-boost mode configurations. The chip’s PWM control input provides LED dimming ratios of up to 1000:1. It comes in a 4mm × 4mm, 20-pin TQFN package.

The MAX20078 single-channel controller operates over a 4.5V to 65V input range and uses  Maxim’s proprietary PWM current control method. This device can run at switching frequencies as high as 1MHz, and it features ultra-fast response and pseudo fixed-frequency synchronous buck regulation. The chip’s high- and low-side gate drivers have peak source and sink current capability of 2A. It comes in a 3mm × 3mm, 16-pin TQFN or a 16-pin TSSOP package.

Driver circuity efficiency is about 93% driving two to four series LEDs. LED headlamps are usually specified at 18W to 40W, which all these ICs can handle using a 14V to 48V supply.

It’s no doubt LED headlamps will be standard equipment on all cars and trucks in a few years. The LED driver designs will migrate from cars to busses and trains and tractors and...