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DESIGN & PRODUCTS AUTOMOTIVE ELECTRONICS ment. This also enables styling to be dramatically changed as restart time. By using an enhanced starter design some manu- LED lighting systems do not require the depth or area as HID facturers have reduced the restart time to under 0.5 seconds, or Halogen do. HB LEDs are also more efficient than Halogen making it truly invisible. The second design challenge is to keep bulbs (and are soon to surpass HIDs) at delivering light output all of the electronics, including air conditioning system and from the input electrical power. This has two positive effects. lighting powered directly from the battery while the engine is First, it drains less electrical power from the automotive bus, turned off and still maintaining enough reserve to quickly restart which is especially important in EVs and hybrids, and equally the engine when it’s time to accelerate. important, it reduces the amount of heat that needs to be dis- In order to incorporate a stop-start feature, the drive train sipated in the display eliminating any requirement for bulky and does require some design modifications. Namely, what was expensive heat sinking. Finally, by using arrays of HB LEDs and once the alternator may also double as an enhanced motor electronically steering or dimming them, they can easily be de- starter to ensure a quick restart, additionally a stop-start elec- signed to optimize lighting for many different driving conditions. tronic control unit (ECU) must be added to control when and how the engine starts and stops. The battery must be capable Design parameters of powering the vehicles lights, environmental control and other In order to ensure optimal performance and long operating life, electronics, while the engine/alternator is turned off. Addition- LEDs require an effective drive circuit. These driver ICs must ally, it must be capable of powering the starter when the engine deliver an accurate and efficient DC current source and ac- is once again needed. This extreme loading of the battery curate LED voltage regulation regardless of wide variations in introduces yet another design challenge, this time electrical as the input voltage source. Secondly, they must offer a means the large draw of current required to restart the engine can tem- of dimming and offer a wide array of protection features just in porarily pull the battery voltage as low as 5V. The challenge for case a LED open or short circuit is encountered. In addition to the LED driver is to continually deliver a well regulated output operating reliably from the electrically caustic automotive power voltage and LED current when the battery bus voltage briefly bus, they must also be both cost and space effective. drops to 5V, then returns to a nominal 13.8V when the charger returns to steady state conditions. Transient challenges: stop/start cold crank A “cold crank” condition occurs when a car’s engine is sub- and load dump conditions jected to cold or freezing temperatures for a period of time. The In order to maximize fuel mileage while minimizing carbon emis- engine oil becomes extremely viscous and requires the starter sions, alternative drive technologies are continuing to evolve. motor to deliver more torque, which in turn, draws more current Already prevalent in virtually all hybrid designs throughout the from the battery. This large load current can pull the battery/pri- world, many European and Asian and car manufacturers have mary bus voltage below 5V upon ignition, after which it typically been incorporating stop-start motor design into conventional returns to a nominal 13.8V. It is imperative for some applications gas and diesel vehicles as well. In the USA, Ford recently an- such as engine control, safety and navigation systems to require nounced that it will incorporate stop-start systems into many of a well regulated output voltage (usually 5V) through a cold crank its 2012 domestic models. scenario so as to continually operate power systems while the The concept of a stop-start system for the engine is straight vehicle starts. forward, the engine is shut off when the vehicle comes to a stop A “load-dump” condition occurs when the battery cables are and then restarted immediately before the vehicle is required to accidentally disconnected while the alternator is still charg- move again. This eliminates the fuel used and emissions gener- ing the battery. This can occur when a battery cable is loose ated whilst the car is stopped in traffic or at a stop light. This while the car is operating, or when a battery cable breaks stop-start design can reduce fuel consumption and emissions while the car is running. Such an abrupt disconnection of the from 5% up to 10%. However, the biggest challenge to these battery cable can produce transient voltage spikes up to 60V designs is making the entire stop-start scenario imperceptible as the alternator is attempting to fully charge an absent bat- to the driver. There are two major design hurdles to make the tery. Transorbs on the alternator usually clamp the bus voltage stop-start capability invisible to the driver. The first is a quick 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 continu- ally regulate output voltage and LED current through this transient event. Synchronous buck-boost HB LED driver Fortunately, there is a new solution to these dilemmas, Linear Technology’s LT3791 LED driver is a synchronous buck-boost DC/DC LED driver and voltage controller which can deliver over 100W of LED power. Its 4.7V to 60V input voltage range makes it well suited for a wide variety of applications including automotive, truck and even avionics HB LED Fig. 2: 98% efficient 50W (25V, 2A) buck-boost LED driver with 50:1 dimming ratio. headlights. Similarly, its output voltage can be 22 Electronic Engineering Times Europe November 2012 www.electronics-eetimes.com


EETE NOVEMBER 2012
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