Automotive headlamps demand buck-boost LED drivers
Background
The very definition of an automobile is in more flux today than ever before. For the last century, it has been dominated by internal combustion drive trains, mainly gasoline powered with a smattering of diesel drive trains. However, we now have automotive drive trains ranging from purely electric (EV) to high efficiency internal combustion, to a myriad of combinations (commonly referred to as hybrids). All of these designs share a common goal of increasing fuel efficiency while simultaneously reducing carbon emissions. New power train designs include direct fuel injection, turbo charging, engine stop/start systems, regenerative braking, higher ethanol content fuels and cleaner diesel combustion. As more hybrids are developed, they are becoming much more dependent on cleaner electric power sources. Despite this level of progress, one aspect of their designs has remained relatively constant, and that is the need to provide forward illumination for driving at night and in less than perfect weather conditions. Furthermore, the means to generate the required light illumination has also evolved from Halogen lamps to High Intensity Discharge (HID) Lamps to most recently high brightness (HB) LED based designs, fueling yet another avenue for HB LED growth.
The market size for high brightness (HB) LEDs is expected to reach $12 billion this year and grow to $20.2 billion by 2015 with a 30.6% CAGR ramp (source: Strategies Unlimited). One of the key application areas driving this significant growth is the LEDs used in automobile designs. Applications range from headlights, daytime running lights and brake lights; to instrument cluster display backlighting, as well as all kinds of in-cabin vanity lighting. However, in order to maintain this impressive growth rate, LEDs must not only offer enhanced reliability, reduced power consumption and more compact form factors, but they must also enable innovative designs such as steerable headlights and antiglare dimming. Furthermore, in an automotive environment, all of these improvements must be optimized while also withstanding the rigors of the relatively caustic automotive electrical and physical environment. It goes without saying that these solutions must offer very low profile, compact footprints while simultaneously enhancing overall cost-effectiveness.
Although LEDs have been used in daytime running lights, brake lights, turn signals and interior lighting for several years; headlamp specific applications are relatively few. Currently a handful of production vehicles are offered with LED headlamps, including the Audi A8 and R8, Lexus’s LS600h and RX450h, the Toyota Prius and Cadillac’s Escalade. Some estimate that the current LED headlamp market was around $1B in 2011 and is expected to surpass $2B in 2014.
One of the biggest challenges for automotive lighting systems designers is how to optimize all the benefits of the latest generation of HB LEDs. As HB LEDs generally require an accurate and efficient DC current source and a means for dimming, the LED driver IC must be designed to address these requirements under a wide variety of conditions. As a result, power solutions must be highly efficient, robust in features and reliability while being very compact and cost effective. Arguably, one of the most demanding applications for driving HB LEDs is found in automotive headlamp applications as they are subjected to the rigors of the automotive electrical environment, must deliver high power, typically between 50W to 75W, and must fit into very space constrained enclosures, all while maintaining an attractive cost structure.
Automotive LED Headlamps
Benefits, such as small size, extremely long life, low power consumption and enhanced dimming capability are the catalyst for the wide spread adoption of HB LED headlights. Several manufacturers, such as Audi, Mercedes and most recently, Lexus have used LEDs to design very distinctive driving lights or “eyebrows” around the headlights to make clear that it is in fact their brand, long before the car can be seen. Although these applications are very distinctive from a design perspective, they do not have the same level of design challenges as do both the low beam and high beam of the headlights.
We all know that the primary function of headlamps is to provide forward illumination at night or in less than ideal weather conditions such as rain, snow and fog. The need for a higher level of illumination has been the primary driver for the evolution of the headlamp. In the 1980’s Halogen based lights became the industry standard, with 50W of electrical power they could deliver approximately 1,500 lumens of light which was a 50% improvement over their predecessors. This translates into an efficacy (known as light output per watt) or light delivered per watt of 30 lumens/watt (lm/W). In the mid 1990’s xenon based high intensity discharge (HIDs) lamps became popular as they could deliver up to 80lm/W, enabling manufacturers to deliver even greater total light output. However, they also have shortcomings such as the need to be accurately adjusted so not to blind oncoming traffic, relatively short operational lives of 2,000 hours, the use of toxic mercury gas and are expensive to manufacture. As the efficacy of HB LEDs continues to improve they have become more desirable for headlight applications. Five years ago, production HB LEDs offered efficacies of 50lm/W which were not sufficient for headlight applications, however current LED designs offer 100lm/W with estimates that this will exceed 150lm/W in the next few years surpassing even the best HID lamps. The ability of LEDs to offer roughly the same amount of light output per watt and their other benefits such as long life, ruggedness and their environmentally friendly design makes them particularly attractive to power the next generation of head lights.
Figure 1. Production LED Headlamp
The benefits of using LEDs in automotive headlights have several positive implications. First, they never need to be replaced, since their solid state longetivity of up to 100K+ hours (11.5 service years) surpasses the life of the vehicle. This allows automobile manufactures to permanently embed them into headlight designs without requiring accessibility for replacement. This also enables styling to be dramatically changed as LED lighting systems do not require the depth or area as HID or Halogen do. HB LEDs are also more efficient than Halogen bulbs (and are soon to surpass HIDs) at delivering light output (in lumens) from the input electrical power. This has two positive effects. First, it drains less electrical power from the automotive bus, which is especially important in EVs and hybrids, and equally important, it reduces the amount of heat that needs to be dissipated in the display eliminating any requirement for bulky and expensive heat sinking. Finally, by using arrays of HB LEDs and electronically steering or dimming them, they can easily be designed to optimize lighting for many different driving conditions.
Design Parameters
In order to ensure optimal performance and long operating life, LEDs require an effective drive circuit. These driver ICs must deliver an accurate and efficient DC current source and accurate LED voltage regualtion regardless of wide variations in the input voltage source. Secondly, they must offer a means of dimming and offer a wide array of protection features just in case a LED open or short circuit is encountered. In addition to operating reliably from the electrically caustic automotive power bus, they must also be both cost and space effective.
Automotive Electronic Transient Challenges: Stop/Start Cold Crank and Load Dump Conditions
In order to maximize fuel mileage while minimizing carbon emissions, alternative drive technologies are continuing to evolve. Whether these new technologies incorporate electric hybrids, clean diesel or a more conventional combustion engine designs, the chances are that they will also incorporate a stop-start motor design. Already prevalent in virtually all hybrid designs throughout the world, many European and Asian and car manufacturers have been incorporating this design into conventional gas and diesel vehicles as well. In the USA, Ford recently announced that it will incorporate stop-start systems into many of its 2012 domestic models.
The concept of a stop-start system for the engine is straight forward, the engine is shut off when the vehicle comes to a stop and then restarted immediately before the vehicle is required to move again. This eliminates the fuel used and emissions generated whilst the car is stopped in traffic or at a stop light. This stop-start design can reduce fuel consumption and emissions from 5% up to 10%. However, the biggest challenge to these designs is making the entire stop-start scenario imperceptible to the driver. There are two major design hurdles to make the stop-start capability invisible to the driver. The first is a quick restart time. By using an enhanced starter design some manufacturers have reduced the restart time to under 0.5 seconds, making it truly invisible. The second design challenge is to keep all of the electronics, including air conditioning system and lighting powered directly from the battery while the engine is turned off and still maintaining enough reserve to quickly restart the engine when it’s time to accelerate.
In order to incorporate a stop-start feature, the drive train does require some design modifications. Namely, what was once the alternator may also double as an enhanced motor starter to ensure a quick restart, additionally a stop-start electronic control unit (ECU) must be added to control when and how the engine starts and stops. The battery must be capable of powering the vehicles lights, environmental control and other electronics, while the engine/alternator is turned off. Additionally, it must be capable of powering the starter when the engine is once again needed. This extreme loading of the battery introduces yet another design challenge, this time electrical as the large draw of current required to restart the engine can temporarily pull the battery voltage as low as 5V. The challenge for the LED driver is to continually deliver a well regulated output voltage and LED current when the battery bus voltage briefly drops to 5V, then returns to a nominal 13.8V when the charger returns to steady state conditions.
A “cold crank” condition occurs when a car’s engine is subjected to cold or freezing temperatures for a period of time. The engine oil becomes extremely viscous and requires the starter motor to deliver more torque, which in turn, draws more current from the battery. This large load current can pull the battery/primary bus voltage below 5V upon ignition, after which it typically returns to a nominal 13.8V. It is imperative for some applications such as engine control, safety and navigation systems to require a well regulated output voltage (usually 5V) through a cold crank scenario so as to continually operate power systems while the vehicle starts.
A “load-dump” condition occurs when the battery cables are accidentally disconnected while the alternator is still charging the battery. This can occur when a battery cable is loose while the car is operating, or when a battery cable breaks while the car is running. Such an abrupt disconnection of the battery cable can produce transient voltage spikes up to 60V as the alternator is attempting to fully charge an absent battery. Transorbs on the alternator usually clamp the bus voltage somewhere between 30V and 34V and absorb the majority of the surge; however DC/DC converters and LED drivers downstream of the alternator are subjected to transient voltage spikes as high as 36V. These LED drivers are not only expected to survive, but must also continually regulate output voltage and LED current through this transient event.
A New Automotive Synchronous Buck-Boost HB LED Driver
Fortunately, there is a new solution to these dilemmas, Linear Technology’s LT3791 LED driver. The LT3791 is a synchronous buck-boost DC/DC LED driver and voltage controller with can deliver over 100W of LED power. Its 4.7V to 60V input voltage range makes it ideal for a wide variety of applications including automotive, truck and even avionics HB LED headlights. Similarly, its output voltage can be set from 0V to 60V enabling to drive a wide range of LEDs in a single string.
A typical 50W headlight application is shown in figure 2 below. This application uses a single inductor to accurately regulate a 25V string of LEDs at 2A to deliver 50W of LED power. This circuit offers a 50:1 PWM dimming ratio which is ideal for anti-glare auto-dimming requirements. Both input and output (LED) current is monitored while fault protection is provided to survive and report and open or shorted LED condition.
Figure 2. 98% Efficient 50W (25V, 2A) Buck-Boost LED Driver with 50:1 Dimming Ratio (for full resolution click here).
Its internal 4 switch buck-boost controller operates from input voltages above, below or equal to the output voltage making it ideal for applications, such as automotive, where the input voltage can vary dramatically during stop/start, cold crank and load dump scenarios. Transitions between buck, pass-through and boost operating modes are seamless, offering a well regulated output in spite of wide variations of supply voltage. The LT3791’s unique design utilizes three control loops monitor input current, LED current and output voltage to deliver optimal performance and reliability.
The LT3791 uses four external switching MOSFETs and can deliver from 5W to over 100W of continuous LED power with efficiencies up to 98% as can be seen in figure 3. In conventionally powered vehicles high efficiency is important as it minimizes the need for heat sinking enabling a very compact low profile footprint. However, in EVs this power savings adds valuable miles of vehicle range between charges.
LED current accuracy of +6% ensures constant lighting in an LED string while + 2% output voltage accuracy offers several LED protection features and also enables the converter to operate as a constant voltage source. The LT3791 can utilize either analog or PWM dimming as required by the application. Furthermore, its switching frequency can be programmed between 200kHz and 700kHz or synchronized to an external clock. Additional features include output disconnect, input and output current monitors, and integrated fault protection.
Figure 3. LED Efficiency in Figure 2
Conclusion
The continual acceleration of HB LED applications, especially those found in automotive head lights, is being driven by an insatiable demand for higher performance and cost effectiveness. These demands must be enabled by new HB LED driver ICs. Therefore, these LED drivers must provide constant current in order to maintain uniform brightness, regardless of input voltage or LED forward voltage variations, operate with high efficiency, offer very wide dimming ratios and have a variety of protection features to enhance system reliability. Of course, these LED driver circuits must also offer a very compact, low-profile and thermally efficient solution footprint. Fortunately, Linear Technology is continually redefining its family of LED drivers to meet these challenges “head-on” with HB LED driver ICs like the LT3791. In addition, we have developed an entire family of high current LED driver ICs aimed specifically at automotive application, ranging from advanced forward lighting headlamps to LCD backlighting. As automotive lighting systems continue to demand higher performance LED drivers, designers will have innovative IC solutions to satisfy them.
About the author: Jeff Gruetter is Sr. Product Marketing Engineer, Power Products, at Linear Technology Corporation
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