The power density of modern LEDs makes it possible to use this type of lighting in vehicle headlights. The actual light source is insensitive to vibration and shocks. But like any other electronic component, it also needs to be operated within certain temperature limits. Small fans specially adapted to the needs of headlight operation offer a solution. To adapt the headlight to the car’s needs, all components need to fulfil a few basic requirements. This also applies to the fans that are used.
Constantly higher light output from comparably small chip surface enables light developers to produce headlight units that are small and compact. This opens new possibilities for vehicle design and saving energy, as LED headlights potentially require less energy than other illuminants, with better light values. In ten cycles of the New European Driving Cycle (NEDC), one vehicle model displayed a CO2 reduction of more than one gram per kilometre. The European Commission therefore officially classifies the LED headlight as innovative technology for lowering CO2 emissions.
The semi-conductor elements and their control, which are largely resistant to mechanical influences, present however fundamental requirements for intelligent thermal management in the headlight housing. The developers at ebm-papst have taken on this challenge and have developed compact and robust fans, which are specially adapted to the demands of the new headlight technology (Fig 1). The most important fan characteristics for functioning thermal management in LED headlights are described in detail below.
LED technology requires new ways of thinking
When you compare the old lamps with modern LED lights, there are many differences. Often the focus is solely on efficiency, i.e. the much greater energy efficiency of LEDs and the potentially better colour temperature of the light. Modern semiconductors now achieve degrees of efficiency that are four times greater than those of halogen headlamps. In numbers, that is currently around 90 lumens per watt, equivalent to a 75 W bulb or a car headlight with a 55 W halogen light. The high luminance generated on a very small chip surface means that the chip is subjected to a high local heat stress.
Substantial waste heat is generated even in an LED with a current physical efficiency of around 30 %, plus the losses in the optical sealing compound and in the ballast module. Even in an energy-saving LED headlight housing, with several LED chips the amount of heat to be dissipated adds up to a few watts quickly. As semiconductors generally become less efficient at higher temperatures, also known as “derating”, the LED substrates also suffer efficiency loss as chip temperature rises. This leads to shorter service life. Different limits apply, depending on the chip type. A possible service life of 100,000 hours (11.4 years) for a chip in a well cooled environment quickly becomes “only” 15,000 to 30,000 hours.
The same is true of components in the ballast unit. A fall in temperature of just 10 °C, for example, can double the service life of electrolyte capacitors, i.e. from 5,000 hours at 105 °C to around 40,000 hours at a moderate 75 °C. Another reason for the need for reliable cooling processes is that modern headlamps are legally required to be delivered only as compact units, which do not allow for component replacement. According to the internationally valid ECE requirements, LEDs for automotive technology may only be installed in cars as modules. This means that the diodes must be encapsulated in a component in a way that protects them from manipulation.
LED cooling is more than just moving air
Ambient air is the coolant of choice in operation around the world. A fan built into a headlamp can apply this air precisely to the areas where heat must be dissipated or where air is otherwise required (Fig. 2). The latter is also important for LED lights. Depending on the ambient conditions, air moisture can gather in non-hermetically sealed headlight housings. This steams up the reflector and the lens. Due to the low levels of waste heat compared to traditional halogen lamps, this water only evaporates if the (waste) air is conveyed precisely in the housing. The fan used therefore assumes a further task vital for the reliable operation of the headlight, alongside the purely cooling function. This requires the operating noise to be kept to a minimum. An aerodynamic impeller with winglets or sickle-wing profile aids this further.
But there are other things to consider. In a vehicle, the required operating temperature range is between around -40 °C to +120 °C. There are also the various climactic factors such as air moisture and salt content, as well as any dust that may be in the air. When driving, attention also has to be paid to the additional vibration, shock and impact stresses, as well as electromagnetic influences. A fan needs to be able to resist these, whilst having the most compact construction possible. Furthermore, the materials used also need to be resistant. The plastics used, for example, may not release plasticizers (so-called fogging) as these can lead to the headlight becoming permanently blinded. Plastics that prevent fogging require other processing parameters, for example oil cooling instead of water cooling in the injection moulding tool and adjusted cycle times. Even seemingly trivial things must be taken into account, such as laser marking for bar codes and type designations instead of the usual (adhesive) labels or ink marking. Our ability as a manufacturer to fall back on decades of experience and expertise in development and production is something that end users quickly recognise in the reliability and long service life of our products.
If the fan is mechanically stable, with maintenance-free bearings and made from fogging-resistant materials, the next question is that of operating parameters. Integrated motor electronics with outward data interface allow the fan to be used universally in a range of applications, as well as for fine tuning the thermal management in individual headlights via the vehicle’s on-board electronics, for example by changing the speed to adjust air flow.
The design principles of modern LED headlights call for reliable thermal management. Small, rugged fans exactly tuned to the specific characteristics of the vehicle and of LED technology ensure reliable dissipation of excess heat, allowing trouble-free headlight operation for more than a decade. The very tight integration of optics, electronics and cooling in an LED headlight calls for early design consultations among all specialists to ensure optimum results in terms of economic and lighting efficiency.