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Sidban
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Life of LED Lamp

07/01/2011 4:01 AM

We have a 12 LED lamp fixture with a substantial heat sink. Each LED is 1 Watt and has a warm white colour. They are connected in series. At an operating current of 300 mA, the heat sink temperature stabilises at around 47 degrees C. My query is, if I maintain the heat sink temperature at 47 degrees C, can I double the operating current ? If so, what will be be effect on the life of the LEDs ?
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JohnDG
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#1

Re: Life of LED Lamp

07/01/2011 4:15 AM

NO. The chip temperature is going to be higher than the heatsink temperature. If you run at higher current, you'll have to pull more heat out of the heatsink to maintain the same (heatsink) temperature to compensate for more heat in the chip. The chip temperature will rise dangerously - the power dissipated goes up as the square of the current (so you'd have 4 times the 'amount of heat' to get rid of.

The LEDs will fail much sooner (if not immediately).

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JohnDG
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#2
In reply to #1

Re: Life of LED Lamp

07/01/2011 4:47 AM

" ... the power dissipated goes up as the square of the current ... " duh, - this isn't true for LEDs (sorry - not enough coffee yet). The voltage woun't double if the current doubles - but the voltage drop will increase, so twice the current will give more that twice the power dissipation. The LED life will definitely be shortened.

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Sidban
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#5
In reply to #1

Re: Life of LED Lamp

07/04/2011 4:54 AM

Thanks for the enlightening answers. I am trying to increase the brightness of the lamp. At 600 mA operating current I am pumping out more heat out of the system thereby even lowering the temperature of the heat sink. The junction temperature will be related to the heat sink temperature in some way. In this case will the life of the LEDs still be shortened ?
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JohnDG
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#6
In reply to #5

Re: Life of LED Lamp

07/04/2011 5:16 AM

"The junction temperature will be related to the heat sink temperature in some way" - true, but there will be a thermal gradient between the junction and the radiating surface of the heatsink. The more energy you put into the LED (given a constant surface temperature of the heatsink, maintained by more forced-air cooling or whatever), the higher the junction temperature will be. Without knowing the thermal impedance of the chip-to-carrier and carrier-to-heatsink interfaces, and full details of the heatsink, it is impossible to say what the actual junction temperature rise will be.

Exceeding the maximum continuous current rating of the LED will definitely shorten its life, no matter how hard you cool it.

As redfred suggested, your best bet to get more light out would be to go to pulsed drive (but it would involve a rather more complicated power supply).

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redfred
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#3

Re: Life of LED Lamp

07/01/2011 11:40 AM

What JohnDG eventually said is absolutely correct. If you plug your numbers of 300 mA and 3+1/3V into the Shockley diode equation, you can predict how much more heat will be dissipated by your diode at the higher 600 mA current. [To do this properly, one should do an iterative approximation for the temperature rise instead of just using the nominal VT value of 26mV for room temperature. Also being an LED, more photon-assisted recombination-generation will be happening so this simplified equation won't be accurate. But this is just a useful approximation anyway.] Now I don't know which LEDS you are trying to use nor what your drive circuitry topology is but I'm not comfortable with LEDS operating at 300 mA that produce so much heat that a heat sink stabilizes at 47°C.

However, I believe it is rare that the actual peak current in the LED array will be the direct/frequent cause of failure of the fixture. The added heat from this higher current in the LEDS and other components will be raising the temperature of the capacitors of your circuitry that drives these LEDS. The lifetime of these capacitors is orders of magnitude shorter than the LED at room temperature. By raising the capacitor's environmental temperature they will fail sooner than the LED.

Now there is a peak current limit for any semiconductor junction where one single pulse at that current will damage the crystal lattice structure itself. If you stay below this pulsed current peak with a pulsed current then you be getting the maximum safe operation and less total heat dissipation. You will have to now consider the parasitic capacitance across the band gap in your calculations. The current crossing this capacitance will not be producing light. Likely this will give you a much cooler operation.

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Haajee
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#4

Re: Life of LED Lamp

07/02/2011 4:55 PM

Not clear.

Do you want to operate lamps on double the current?

Cooling cannot increase the max operating currents or voltage so wattage.
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