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temperature in LED headlamp gets too hot?

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Jun 22, 2008
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how hot can led get


I am making my own LED car headlamp, as i have CAD and tooling capability + electronics knowledge.

However, its the thermal situation that's hardest.

As you know, some customers will have the headlights ON in the daytime, in Arizona at the height of summer. -Because they can.

The lamp enclosure is sealed, there is obviously no fan in it, and no water cooling system.

Even with the LEDs OFF, (Yes, "OFF") the temperature inside the lamp enclosure would be at least 85 degrees C on a hot sunny day in Arizona. -due to the sun's rays getting into the headlamp and heating it up. then just imagine how hot it will get in the headlight enclosure when the LED lamp is switched ON in this circumstance.

Please put it like this.....The MOSFET of the LED driver's SMPS should (must!) operate with its junction temperature below 110 degrees C.

-No mosfet should be operated with junction temperature above 100 degrees C.

Now, -the ambient temperature inside the lamp enclosure with the lamp OFF would be 85 degrees C. (= worst case ->but we must design for worst case).

So with the LEDs ON, we must assume that the temperature inside the enclosure would be AT LEAST 100 degrees C in this case. (i.e. at least 15 degrees C higher than the temperature in the enclosure with the LEDs OFF)

Now the LED drivers MOSFET is dissipating 1 Watt in this LED driver.

There is no heatsink, so we rely on the PCB copper to conduct heat away from the FET's case.

But doing it this way always gives an absolute minimum achievable case to ambient thermal resistance of 25 degrees C per Watt. -its impossible to get this figure lower on any PCB without fan cooling.

The mosfet's junction to case thermal resistance is 1.3 degrees C per watt.

Putting all this together means that the Mosfets junction temperature will be 100C + 1.3C + 25C = 126.3 degrees C.

(= Ambient + RthJC + RthCA)

126.3 C is way too high for a mosfet junction temperature, -but its the minimum i can get without a heatsink......

But none of the major manufacturers use heatsinks or fan cooling on their mosfets in their LED headlamp PSU's. (i definetely know this -i have insider knowledge -take your own LED headlamp apart and see for yourself)

So my question are they all getting away with this?

are they "living on a knife-edge" ?

are their MOSFETs on the verge of frying?

headlamp with a fan inside


You must change the headlamp structure to included heatsink (active or pasive)

All the best


headliamp led

If you have a look at the LED lamps, they are designed with the LED chip mounted on a heatsink that is integerated into the lamp enclosure. Due to this design, the inside and outside temperature never differ by more than a few degrees. So even in 50degC ambient in a desert, the LED chip in off state will not go above 55-60degC.

You can not use packaged LEDs in this application. They will get too hot inside and burn out in a few weeks.

make your own led headlights

I think you will find the sealed units are sealed at high vacuum to remove conductive heating and that the lens for the headlamp has special coating designed to reflect infrared radiation to reduce radiative heating. Commercial high luminance LEDs are fitted with backside heat sinks as part of their packaging, although they are fairly small.

Chuck a brick at the competitors headlamp and see if it implodes. Remember and say sorry, you were aiming at the wife and she unsportingly ducked.

design led headlamp

When I designed an LED fog lamp for a specific application I tried to use the mounting points of the lamp to the car as my heat sinks allowing the body of the vehicle to become a giant heat sink. I'm not sure in your application if you can get the pcb and a mounting point to work with your design. Also, have you thought about PWM at a high frequency and say 50 or 75?% duty cycle?

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