How to Increase the Lumen Output of LED Headlight Bulb?
How to increase the lumen output of led headlight bulbs? This question should be considered everyday by manufacturers. We all want to offer customers the brightest led headlight kit with best light beam. But how can we achieve that? What technical problem do we meet now? Here please allow me to explain to you.
- Select high quality LED Chip for light source if we want to make continuous high lumen LED headlight bulb, which need to be high relative luminious flux and high temperature resistance. The first choices are famous LED suppliers like Philips Lumileds, Osram, Cree, Nichia, Samsung and so on. Their LEDs have excellent performance and work well at high temperature.
- Efficient cooling technology can increase lumen output and extending LED life.
The energy conversion rate is fixed. Around 35% energy converts to light, 65% energy converts to heat. We can’t increase energy conversion rate.
And for LEDs, their Relative Luminious Flux vs. Junction Temperature is fixed too. Lumen will decrease when junction temperature goes up. (The chart of Luminious Flux vs. Junction Temperature of Cree XHP50 below)
Under a specific condition, 12V Voltage, forward current will also decrease as ambient temperature goes up. When forward current on LEDs goes down, Lumen decrease as well. (The chart of Maximum Current vs. Ambient Temperature of Cree XHP50 below)
That means if we want to increase Luminious Flux, we need to control ambient temperature at within a reasonable range. Heat produced from LEDs at work. Heat dissipation is the key point.
Except led chip, other electronics of led headlight bulb will also produce heat at work, like IC driver, capacitance and so on. These electronics themselves could go up the ambient temperature to 70 -80℃, if they are built in led bulb, the heat from LEDs and electronics will mixed and add up, temperature goes up too high. Now the reliable design is to make a driver on outside built-in these electronics. Then we only need to think about how to reduce the ambient temperature of LEDs.
There are two cooling methods for LED headlight bulb, First one is active cooling; second is passive cooling.
Active Cooling Techonology
Active cooling refers to dissipate heat by heat sink to achieve thermal balance. The heat conduction process is as follows;
Heat produced from LEDs at work → PCB board → Bulb aluminum body→ Aluminum heat sink
Each heat conduction process concerns on a production technology.
First, soldering, it has a great impact on the lifetime of LEDs. Heat produced from LED at work, if we want to transfer heat to PCB board sufficiently and fast, soldering needs to be accurate and professional. We need to select high quality solder paste, professional soldering machine and SMT equipment to make LED be soldered well. If choosing cheap solder paste, or soldering LED by hand and simple equipment, there will be bubble space between LEDs and PCB board, the connecting between two surfaces is not fully complete. I mean some areas between two surfaces aren’t connected well. The heat on these areas couldn’t transfer and will keep gathering, temperature on LED goes up, LEDs are working continuously under high temperature, as I mentioned above, LED Luminous Flux will be reduced, life will be shorted. Good soldering technology could reduce unconnected area to a minimum and makes LEDs transfer heat to PCB board sufficiently and uniformly.
Secondly, Heat transfer from LED to PCB board and will continue transferring to bulb body. On this process, PCB board plays a major role. The best PCB board is copper base PCB, because the thermal conductivity of copper is the best. Aluminum base is followed by, the worst is glass-fiber board. The surface of PCB board should be as smooth as possible for decreasing the gap between PCB board and bulb body.
In addition, we will use heat transfer silicone to fill in the gap between PCB board and bulb body, increase heat transferring. Cheap and good heat transfer silicone are widely different. Just like CPU heat transfer silicone, you guys should know that.
Next, heat will travel to heat sink and dissipate gradually. If bulb body and heat sink are integrated, heat will transfer quicker and fluently. But, it requires die-casting technique to make two metal parts integrated. Some metals are not allowed to die-casting. Copper can, it is good, but expensive and heavy. Normally we use aluminum for heat sink. Pure aluminum has high thermal conductivity, but is too soft to be processed by die-casting technology. The aluminum alloy for die-casting is low thermal conductivity, not designed to be heat sink. The best solution for auto headlight heat sink is 6063 aluminum alloy, which is tough, high thermal conductivity.
But 6063 aluminum alloy can’t be processed by die-casting technology; factories usually joint 6063 aluminum alloy heat sink and bulb body together by screw, rivet, or hydraulic machine. There is another popular method to combine them by thread, in this way, mould requires being precision.
The shape of heat sink will affect the heat dissipation performance. A larger surface area of heat sink will promote heat loss. Besides, airflow should be taken into consideration. The space between cooling fins is not allowed to be too narrow, or heat won’t dissipate fast but transfer to other cooling fins. Here we take a example, the Ebay led headlight below, there are so many cooling fins on heat sink and they are very closed, space between fins are narrow, that is not good for heat dissipation.
By comparison, you can see the Philips led headlight was designed a little fins on the heat sink, but their fins gap are spacious for airflow, heat dissipation performance will be better.
If big heat sink with large surface is better for heat dissipation, why don’t we make all led headlights with big heat sink? That is because there is limited space behind headlight housing in some vehicles. Halogen bulbs are small size while led headlight is big size, when we replace it with a led headlight bulbs, space behind headlight housing is sometimes not enough so that LED headlight can’t be installed on. So in practice, we need the heat sink as small as possible.
Yes, this is correct; heat sink can’t be made too large for avoiding installation problem. But if we make led headlight to be higher Luminous Flux, the forward current on LEDs will increase and heat will be produced more. So heat dissipation problem limits the cap of Luminous Flux of led headlights.
Passive Cooling Techonology
So except active cooling, factories also offer another solution for heat dissipation, passive cooling, that is to equip a fan on led headlight bulb, cooling the bulb by fan. The passive cooling method is simpler and more straightforward. And we are able to make led headlight bulb be small size as well as high lumen output in this method.
But it is a double-edged sword to install a fan on led headlight bulb. The heat dissipation is counting on fan. If fan has short lifespan and stop running quickly, led headlight bulb will blow in soon. So the quality is crucial. Now we generally use two types of fans, sleeve bearing fans and ball bearing fan.
Sleeve bearing fan relies on lubricating oil to work. It is low manufacturing cost, and runs fast with low noise at the beginning. After working for some time, lubricating oil will volatilize, fan rotating speed will also drop significantly. Weather and working condition decide the lifespan of a sleeve bearing fan, normally it can work 6 months to 12 months on an auto led headlight.
Ball bearing fan is much better though generally more expensive. Based on the rolling friction transmission and consisted of two ball bearings, there are a few tiny steel balls in the bearing, balls turn when their axis moves with small friction. Ball bearing fans do not suffer the same limitations of oil volatilizing as sleeve bearing fans, are more durable at higher temperatures, and has longer lifespan.
Beside, for better heat dissipation, the airflow direction of fan should be the same to the airflow direction of heat balance. Some manufacturer violates this principle to design their product for a good-looking appearance; finally heat dissipation performance is suck. You can see that Philips LED headlight don’t have any cover on air inlet. Fan will work hard while heat dissipation performance will be great.
Finally, it is the driver. When driver is put on outside, we don’t need to worry that it will transfer heat to bulb and affect the luminous efficacy of LEDs. But we need to care about what electronic inside the driver. And is output current reasonable? Is forward current on LEDs stable? Is there temperature control device in driver? They will all decide the final quality, lifespan, lumen output of a led headlight bulbs.