Automotive headlight design for automotive lighting systems

One of the biggest challenges facing automotive lighting system designers is how to maximize all the advantages of the latest generation of high-brightness LEDs. Because high-brightness LEDs generally require accurate and efficient DC current sources and dimming, LED driver ICs must be designed to meet these requirements under a variety of conditions. Therefore, the power solution must be highly efficient, rugged, and very reliable, while also being very compact and cost-effective. It can be said that in terms of driving high-brightness LEDs, one of the most demanding applications is automotive headlight applications, because such applications must withstand the test of the harsh automotive electrical environment and must provide high power, generally between 50W and 75W It must also fit into an enclosure with very limited space, while maintaining an attractive cost structure while meeting all these requirements.

Automotive LED headlights

High-brightness LED headlights have many advantages, such as small size, extremely long life, low power consumption, and enhanced dimming function. These advantages have become a catalyst for the widespread adoption of high-brightness LED headlights. Some car manufacturers, including Audi, Mercedes-Benz, and recently joined Lexus, have designed distinctive driving lights with LEDs. These lights surround the headlights. If you compare the headlights to your eyes, then these driving lights Just like eyebrows, manufacturers do this to highlight the characteristics of the brand and let people know which brand of car they are driving before they can see what the car looks like. From a design perspective, these applications are unique, and the design challenges they face are different compared to low-beam and high-beam headlights.

We all know that the main function of the headlights is to provide forward lighting at night or when the weather conditions are not ideal (for example, in rain, snow, and fog). The need for higher illuminance has been the main driving force for the development of headlamps. In the 1980s, halogen lamps were the industry standard. With their 50W electric power, these lamps can provide a light output of approximately 1,500 lm, which is a 50% increase in light output compared to previous generations. The success of this light output conversion (light output per watt) is 30 lm / W. In the mid-1990s, high-intensity discharge (HID) xenon lamps became mainstream because these lamps can provide light output up to 80 lm / W, allowing manufacturers to provide greater overall light output. However, xenon lamps also have shortcomings. For example, in order not to cause oncoming vehicles to see the road clearly, they need to be accurately adjusted; the working life is relatively short, only 2,000 hours; the use of toxic mercury vapor; the manufacturing cost is very high. As the efficacy of high-brightness LEDs continues to increase, such LEDs have become more desirable products for headlights. Five years ago, the high-brightness LEDs that were used in automobiles provided 50 lm / W efficiency, which was not enough for headlight applications. However, the current LED design provides 100 lm / W efficiency, and it is estimated that it will not take a few years. The efficacy will exceed 150 lm / W, thus even exceeding the best high-intensity discharge lamps. LEDs can provide about the same amount of light output per watt, but also have other benefits, such as long life, ruggedness, and environmentally friendly design, which make the use of LEDs to form a new generation of headlights very attractive.

There are several positive implications for using LEDs in car headlights. First of all, these LED lights never need to be replaced, because their reliable life span is more than 100,000 hours (equivalent to 11 and a half years of service life), and even exceeds the life of the car. Therefore, car manufacturers can permanently embed LEDs in the headlight design without leaving room for replacement. This also allows the car style to be greatly changed, because the LED lighting system does not require the depth or area of ​​high-intensity discharge lamps or halogen lamps. In terms of providing light output (measured in lumens) by input electric power, high-brightness LED lamps are also more efficient than halogen lamps (and will soon exceed high-intensity discharge lamps). This has two positive effects. First of all, LED lights draw less electric power from the car bus, which is especially important in electric cars and hybrid cars. Equally important, LED lights reduce the amount of heat that needs to be dissipated in the lighting system, eliminating the need for bulky and expensive heat sinks. Finally, by using high-brightness LED arrays and electronically controlling or dimming the LED arrays, LED headlights can be easily designed to optimize lighting for many different driving conditions.

Design Parameters

In order to ensure the best performance and a long working life, LED needs an effective driving circuit. No matter how wide the input voltage source is, these driver ICs must provide accurate and efficient DC current sources and accurate LED voltage regulation. Second, the driver IC must provide dimming methods and multiple protection functions to prevent LED open or short circuit failures. In addition to being able to work reliably with the automotive power bus that is very harsh in the electrical environment, the driver IC must also be cost-effective and use space efficiently.

Challenges caused by automotive electronic transients: stop / start, cold start and load dump

In order to maximize fuel mileage while reducing carbon emissions as much as possible, alternative power drive technologies are constantly evolving. Regardless of whether these new technologies incorporate hybrid electric, clean diesel or more traditional internal combustion engine designs, they are likely to adopt a stop-start motor design. This design is very common in almost all hybrid vehicles worldwide. Stop-start motor designs are already common. Many European and Asian car manufacturers have been incorporating this design into traditional gasoline and diesel vehicles. Ford Motor Company of America has announced that it will use a stop-start system in many 2012 models for the US market.

As far as the engine is concerned, the concept of a stop-start system is easy to understand. When the vehicle is stopped, the engine is turned off, and then when the vehicle is required to start again, the engine is restarted immediately. In this way, when the car is suspended due to traffic conditions or red lights while driving, it will not consume fuel and emit carbon. This stop-start design can reduce fuel consumption and carbon emissions by 5% to 10%. However, the biggest challenge faced by this type of design is how to make the entire start-stop situation unnoticeable to the driver. In order to avoid the driver's awareness of the start-stop capability of the car, there are two major design obstacles: the first is the rapid restart time. Some manufacturers use the enhanced starter design to reduce the restart time to less than 0.5 s, making the restart truly imperceptible; the second design challenge is to keep all electronic systems directly powered by the battery when the engine is turned off, including Air conditioning system and lighting system, and at the same time still maintain sufficient battery power reserve to quickly restart the engine when accelerating.

In order to incorporate the stop-start function, it is indeed necessary to make some modifications to the design of the power transmission system. The alternator in the past may also be used as an enhanced motor starter to ensure a quick restart. In addition, a stop-start electronic control unit (ECU) must be added to control when and how the engine is started and stopped. When the engine / alternator is turned off, the battery must be able to power the vehicle's various lights, environmental control systems, and other electronic systems. In addition, when the engine is needed again, the battery must be able to power the starter. This extreme battery loading situation introduces another design challenge, this time an electrical challenge, because restarting the engine requires a large amount of current draw, which in turn may temporarily pull the battery voltage down to 5V. The challenge for the LED driver is that when the battery bus voltage briefly drops to 5V and then returns to the nominal 13.8V (the charger returns to a stable state at this time), it continues to provide a good and stable output voltage and LED current.

When the car engine is in a cold or freezing temperature for a period of time, a "cold car start" situation occurs. In this case, the oil becomes extremely viscous, requiring the engine starter to provide more torque, which in turn causes it to be drawn from the battery. More current. Such a large load current may pull the battery / main bus voltage down to less than 5V during ignition, after which the voltage generally returns to the nominal 13.8V. For applications such as engine control, driving safety, and navigation systems, it is important to maintain a good and stable output voltage (usually 5V) when a cold vehicle starts, so that the power supply system can continue to work when the vehicle starts .

If the battery cable is accidentally disconnected while the alternator is charging the battery, a "load dump" situation will occur. This may happen when the battery cable becomes loose while the car is working or when the battery cable breaks while the car is running. This sudden disconnection of the battery cable may cause transient voltage spikes up to 60V, because the alternator attempts to fully charge the non-existent battery. The transient voltage suppressor on the alternator usually clamps the bus voltage between 30V and 34V and absorbs most of the inrush current. However, the DC / DC converter and LED driver downstream of the alternator have to withstand transient voltage spikes up to 36V. When such transient events occur, it is required that these LED drivers are not only not damaged, but also must continuously adjust the output voltage and LED current.