What to look out for in vehicle LED head­lights

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.

Figure 1: Ruggedly designed fan that resists vibra­tions, shocks, heat and cold in the head­light housing

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

Figure 2: 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.