The total headlight system is usually composed of multiple converters, each regulating a different part of the system. Sometimes the high-beam and low-beam functions are combined into one string. In this case, the high beam is implemented when all LEDs in that string are fully on. The low beam is implemented by pulse-width modulation (PWM), dimming the LEDs in the string to provide a lower-lumen output. More frequently, however, the low beams are implemented by one or two strings at full brightness, and the high-beam function combines the low-beam strings with another string or two at full intensity. That approach allows the manufacturer to use the additional strings to increase the beam spread of the high beam by mechanically positioning the lights facing an adjacent angle.
The LED-based headlight mainly includes two basic electronic components: the LED array and the lighting control unit (LCU). The LCU usually houses all the current regulators and other power converters, as well as the microprocessor, which communicates with other electronic control units (ECUs) throughout the automobile. Upstream, the body control unit (BCU) controls the LCU and manages all the body functions in the car. The LED array is located on a heavily heat-sinked assembly that contains the LEDs, thermistors for measuring temperature, and coding resistors, which are a simple, cost-effective way to program the LCU power output.
Future headlight systems
This system most likely will become even more complex as additional functionality is added to the headlight, including dynamic fading for cornering lights, dynamic anti-glare systems, and increased safety functionality. Current headlight systems already use camera input to run auto-leveling motors, which respond to changes in the position of the car relative to the terrain. These cameras can be used to control dynamic light output, as well.
From a power electronics point of view, it makes sense to use a two-stage topology for a dynamic front lighting system. In a DC link topology, for example, a boost converter takes the 12V battery input and provides a stable high-voltage DC rail. Then, independent buck converters can be used to drive each series LED string separately in the system. This boost plus buck system (see photo below) is inherently more efficient and provides better mitigation of electromagnetic interference than the single-stage buck-boost systems, since the input and output current are both continuous. Additionally, the buck converter is an excellent choice for regulating output current, due to its high output impedance, which makes it look like a true current source.
Future headlight systems will use two-stage power architectures.
The cascaded DC link approach has better dynamics, as well, since the first stage can ballast the input transients of the car battery to provide a well-regulated DC rail with significant energy storage capability, while the second stage can ensure consistent regulation at all times. It is also much easier to perform high-speed PWM dimming on a buck stage with little or no output capacitance, allowing for much more resolution and a higher contrast ratio when PWM dimming. This amounts to a higher level of controllability in the system, providing the possibility for dynamic fading of the different parts of the system.
Since the automotive market is not as cost-sensitive as consumer electronics, this move to two-stage LED headlight drivers could have staying power in the market due to performance improvement and additional safety functionality.
The idea of being able to control the output of the light to such a degree is going to prove very useful to the automotive industry. I wonder what other industries might be able to take advantage of this flexibility.
I do marvel at contemporary automobiles. As one who works with microcontroller devices in many types of applications, it is no supprise that the lights mentioned here use a microcontroller to control functionality. Whatius really interesting is that the lighting microcontroller talks to the body control microcontroller.
This is just one step to an optimal lighting architecture that is automated. I can remember the days when I put driving lights (with a 1 mile range) on my Austin Healey. I had to be careful of where I used them, but no longer. Just program the LED lights and they will sense the on-coming traffic.
naperlou, Based on the amount of heat being dissipated by the LCU/LEDs, it makes sense to remove the headlamp driver function from the Body Control Module (BCM). The LCU will be control by the BCM through CAN (Control Area Network) communications.
James, now a day's most of the incandescent and traditional illuminant lamps are getting replacing either by CFL or LED bulbs. When we look in energy saving angle LED lamps are the preferable one, but its little bit expensive when compare with CFL. I think in coming days, when mass production happens, the cost may come down further. In future, it may replace all the existing lighting sytem across all sections like automobile domain, street lights etc
I recently replaced the landing light in my Cessna with an LED lamp. The original landing light pulled 20A and the lifetime was maybe 50 hours. Between the vibration in cowl mounted lights and the engine heat, incandescents just don't last long. In fact, every rental I ever used the landing light in burned out or would trip the breaker after 5 minutes. The LED replacement pulls about 2A and is rated for thousands of hours. With the lowered current draw and increased reliability I have the landing light on anytime the aircraft is moving. The FAA has approved LED replacements for most lighting systems. Even though there are no single bulb replacements for some navigation lights that have reflective surfaces inside the bulb, you can replace the entire light as a system.
I have to disagree with this part of this article:
"Unfortunately, this increase in complexity yielded little benefit in size or performance over the incandescent light."
Have they ever driven a vehicle equipped with HID lighting?
I converted my motorcycle from a dual halogen setup (H4 / H7) to dual "bi-xenon" HID projectors (4500k). Holy cow! It is almost a life changing experience to ride at night with the lights on high beam! :)
The performance difference in light output is incredible and I don't see how that can be denied.
That said, I'd love to install LED headlights on a vehicle...
@Jim- Just FYI the Ducati Panagale uses LEDs instead of HID or halogen headlamps. That will undoubtedly migrate downward into "everyday" bikes. There will be a significant safety benefit when tail lights go LED...as you know, incadescent brake lights fail often due to vibration. Often enough that most bikes have two bulbs in the tail light assembly.
I'm also surprised that car styling doesn't take advantage of the flexibility that LED's allow. Most LED-equipped cars just put them behind the classic round or rectangular shape.
The author writes "Sometimes the high-beam and low-beam functions are combined into one string. In this case, the high beam is implemented when all LEDs in that string are fully on. The low beam is implemented by pulse-width modulation (PWM), dimming the LEDs in the string to provide a lower-lumen output."
What a crock! Any lighting engineer worth his salt knows that the difference between high and low beam in a car is not simply more or less light, there is a legal requirement to ensure beam shape. For low beam the beam is supposed to reach virtually zero at ~100m by throwing the beam down. This is achieved (with incandescents) with a structure of baffles, the reflector and the position of the filament relative to the reflector. High beam on the other hand is a more open beam that throws much further. In the earlier days the power distribution was 50W for low beam and 55W for high beam, but the bulk of the lighting distance came from the beam difference. Basically according to this authors understanding of vehicle lighting low beam is allowed to shine in through the rear window of the car in front. I tell you it is not.
Kenish, the other more important aspect is LED requires a small amount of power to produce high intensity light beam, when compare with the HID or Halogen lamps. This will help to avoid battery drainage and hence save a considerable mount of power. If the rear and parking lamps are also replacing by LED, then power usage will be more and complete battery power drainage on overnight parking.
It will be a real benefit for drivers if the new LED headlights are mandated to not be as blinding as the HID lights are. IT appears that most of them are used primarily to blind oncoming drivers in city driving.
Presently the LED portion of vehicle headlamp assemblies appears to be an attempt at "styling" more than providing a useful amount of illumination. At least that is the way it looks to me when I see one oncoming. OF course, for those who drive an expensive car, I am certain that it is important to them that everybody else know that they are driving an expensive vehicle.
Probably it will be possible to produce some very useful arrangements of LED lighting on the vehicle, and certainly the best implementations will have the electronics for each light assembly in a separate module, both for heat dissipation and to reduce the cost of replacements, since automotive electronic assemblies do fail on occasion. REplacing a $750 dollar vehicle control module because one light's driver has failed would turn me against a brand of autos for a very long time. It would be good for the auto makers to realize that, and for some brand to take advantage of the lower price of single function modules.
Absolutely, William K. One would think that with the new computer controlled technologies and the ability to hyper-focus these lights, we should be able to address the issue of the oncoming driver (or walker, or biker,....).
Now, do these lights have some integral heater in them? As a Wisconsinite, I've driven in my share of snowstorms in which the headlights were the only thing in front not caked in snow because they produced enough heat to keep it melted. The issue isn't so much that I can't see (if that were the case, I would know enough to pull over and clean them off), but that other drivers can't see me at a reasonable distance.
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