Is my heat sink properly sized for my LED strip?

Question

I am looking for some help in sizing a heat sink for an LED horticulture light I am building. I will be attaching two Bridgelux BXEB-L1190U-30G3000-C-C3 LED strips side by side, link below to specs, to a 1244mm 840AS sink from heatsinkonline.com. The LED strips will be operating at a maximum drive current of 1.52 amps and a forward voltage of 41.5 for a total wattage of 1.52Amps * 41.5Volts * 2strips = 126 watts. The thermal resistance rating of the sink is 4.903 C/W/3”(75mm).

Specs: https://www.bridgelux.com/sites/default/files/resource_media/DS171%20Bridgelux%20EB%20Series%20Slim%20Gen3%20Data%20Sheet%2020190620%20Rev%20A.PDF

First step is to take the total length of the heatsink and divide by the thermal resistance length.

1244mm / 75mm = 16.58 thermal resistance sections in the total sink length

Now divide the total wattage as heat to be dissipated which would be a worst case scenario. I think the norm is to assume 2/3 of total watts is the amount of heat required to be dissipated.

126 watts / 16.58 = 7.59 Watts to be dissipated over the 3”(75mm) section.

So now take the number of watts times the thermal resistance to get the increase in heat sink temperature from ambient temp.

7.59 watts * 4.903 C/W/3” = 37.24 degrees Celsius increase

Now calculate the temperature of the case as temperature ambient plus heat sink thermal increase.

Tc = 37.24C + 27C = 64.24 degrees Celsius

TcMax for the Bridgelux strip is Tc=85 degrees Celsius.

Since the calculated Tc < TcMax this suggests to me that the heat sink is sized properly to support the two Bridgelux LED strips at the given wattage. Using total watts and assuming that it is all converted into heat should provide an additional buffer of safety. Please comment if there is something I have missed or not accounted for in the above calculation as I have never done this before. Thanks in advance for the help.

Answer 1

I am working with your given values: (4.903k/W : (1244mm:75mm)) * 126W = 37.24K

If your room is 25°C warm, then the heatsink is 62°C hot. The chip of the LED inside has a higher temperature! I would say you can add 30 Kelvin to it.

The 50,000 hours endurance of an LED is taken by temperatures of 20°C. If you increase the temperature of the LED-chip by 10k, then the endurance is only 25,000 hours.

20°C -> 50,000h
30°C -> 25,000h
40°C -> 12,500h
50°C -> 6,250h
60°C -> 3,125h
70°C -> 1,560h
80°C -> 0,780h

This is the time till the luminosity of the LED drops to 80%, but I have seen very dark LEDs. So do not use to much current (72% would be okay) and try to cool it down as good as possible.

You can simulate the heat spreading and temperature with ANSYS workbench. Important are here the airflow and heat sink orientation. With passive cooling, you only have laminar air flow.

You should use for every LED stripe one of these heat sinks, they are not so expensive. (I can only select 23 inch long heat sinks.)

An other calculation: In the data sheet the "BXEB-L1190U-30G3000-C-C3" have a nominal current of 700mA at voltage of 38.2V what is 26.8 Watt of power.

If the thermal resistance of 4,903k/W is correct, then we would have an other picture.

(4,903k/W : (1244mm:75mm)) * 26.8W = 7.7k

In this case you could use the one heat sink with two stripes, but I would still recommend two heat sink for a better illumination and more flexibility.

Answer 2

I agree with your calculations, but your assumptions may be optimistic.

You haven't included the thermal resistance from the strip to the heatsink. You can make this negligible with a good attachment. What is your plan?

They didn't specify the temperature for the 50,000 hr lifetime rating of the LEDs. It may be considerably less than 64 deg C.

The efficiency of a heatsink is highly dependent on its orientation. I assume that your setup will have the fins pointing up. They didn't say how the heatsink was oriented for their rated thermal resistance. Your orientation is good, but may not be the most efficient.

Be sure that air can flow freely around the heatsink.

I would buy a 6 inch piece of heatsink, bolt a chassis mount resistor to it, and run some tests to gain confidence.