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Use buck-boost LED drivers for automotive headlamp

Posted: 11 Jun 2013     Print Version  Bookmark and Share

Keywords:automobile  high brightness  HB LED  ECU  dimming 

The benefits of using LEDs in automotive headlights have several positive implications. First, they never need to be replaced, since their solid state longevity 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 optimise 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 regulation 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
In order to maximise fuel mileage while minimising 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.

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