OT: Marchal LED Conversion for 356, An LED Headlight Project Options

[Spoke]

A Porsche 356 owner contacted me about converting a set of Marchal fog running lights to LEDs. We talked about it and he asked if I could do it in 6 weeks. My response was I've never done a high power LED headlamp so it's more of a project than a product.

The Marchal running light is shown on this file-folder picture of a 356. That's not me nor the owner. (IMG:style_emoticons/default/smile.gif)



The idea was to design an LED-only board attached to a large heat sink and use a step-up dc-dc converter on a separate board to provide 0.275Amp to the LEDs.

The LEDs are Cree XP-G3 surface mount LEDs. These have a maximum current of 2 amps. The LEDs require a lens to focus the light pattern. The XP-G3 have a wide output light pattern and can't be used 'naked'. The lens per LED are Carclo's 10049 lens with a light pattern shown below.



The LED board uses 19 LEDs with lenses and holders. The board will be backed by a very large heat sink. The main killer of LEDs is heat. 2 amps through an LED with 3.5V is 7W in less than 1/4" square area. Massive power dissipation/heat in a very small area.

The PCB really should be an aluminum PCB but they are really expensive so the PCB is FR4 and is thin at 0.8mm to minimize thermal impedance of FR4. Vias from top to bottom are liberally applied to provide as much metal top-to-bottom for heat transfer. The clear area of the bottom of the board will allow maximum heat flow to the heat sink.

[Spoke]

The converter board was redesigned with a much larger MOSFET in a D2PAK package with 0.0056 ohm ON resistance and a small heatsink for the MOSFET. The original MOSFET was in a DPAK case and had 0.35 ohm ON resistance. Quite a lot less resistance considering the MOSFET is switching almost 2 amps.

Also went with a larger inductor with a much lower series resistance.

Since the converter board will be mounted on the back of the LED heatsink, I tried to shield the converter board from the heatsink with a blank FR4 board.

With the fixture completely assembled, the case of the MOSFET still gets above 110C and the ambient air temperature inside the fixture is getting to about 80C. Pretty hot but should still be ok. This is with no air movement. (ie., the car is stationary with ambient 23C). With airflow provided, the temps drop as much as 20C.