Heatsink suspiciously hot - Page 1
Forum section: electronics / projects

author: thexeno
author_rank: Regular Contributor
timestamp: January 05, 2016, 06:32:30 pm
content: Hi! I am lighting up 4 Luxeon Rebel LEDs. Designing them using a thermal pad certified from Lumiled to be 7°C/W on 2 layer PCB of 70um, I have the Rjc=10°C/W, Rjh = 7°C/W. I want to use them at max 130°C of Tj, 40°C of Tamb.Using one LED of 2.3W with the same conditions, I use 22°C/W heatsink using the (Tj-Ta)/P - Rjh, where Rjh=Rjc+Rch.Using 4 LEDs, I divided by 4 the heatsink resistance. Which is the same of multiplying by 4 the power and dividing by 4 the Rjh (because the resistances should be in parallel). Result: 5.5°C/W of heatsink.I bought this:http://uk.farnell.com/fischer-elektronik/ick-s-25-x-25-x-6-5/heat-sink-pin/dp/1211712But when 4 LEDs are on, touching the heatsink with a wet finger, I hear the "water frying", and the remaining flux of the solder paste near one LED (solder is a Pb alloy) start to be somewhere between viscous and hard (when is cold it is hard). I don't know why it is too hot. For sure, the heat is tranferred.Must be noted that maybe the heatsink is overloaded, that is the area of the 4 LEDs under the PCB covers the entire surface of the heatsink, meaning that it is not absorbing heat from the center, but rather from the entire square of 25mm, more or less.Any guess?

author: macboy
author_rank: Super Contributor
timestamp: January 05, 2016, 07:48:30 pm
content: Do you have airflow? As I see it, the heatsink is 5.4 deg/W with 0.5 m/s airflow, and a little better with more airflow. It is not rated for less airflow than that.

author: thexeno
author_rank: Regular Contributor
timestamp: January 06, 2016, 01:23:45 pm
content: Quote from: macboy on January 05, 2016, 07:48:30 pmReally?I may need a fan... but how do I know if the air flow must be on pushing air on the metal, rather than aspiring it? (aspiring maybe is better when air passes through a radiator opened, like a grid... pushing air is better on heatsinks where air bags on the surface..)This onehttp://it.farnell.com/fischer-elektronik/ick-s-25-x-25-x-18-5/led-heatsink-with-pins-square/dp/1850055states nothing about no forced convection, but it is sold as a natural convection heatsink. And interestingly, as the same of the previous one, is 5.2°C/W but it is far more big. the curve is different too, but states nothing on still air.Quote from: blueskull on January 05, 2016, 11:50:04 pmAbout efficiency... to find it, I need to remove from the total power of the LED (Vforward*I) a value which is lm/(lm/W) where lm is the stated lumens of the LED and lm/W the efficiency. Correct? Because I had only this regarding efficiency.Actually, the room temperature was 19°C, CONCEPTUALLY is it correct to consider the entire wattage as a worst case? It result in 9°C/W. Because considering higher wasted power keeps me on safe side.An interesting thing, you considered 100°C on surface (at least, ok) but since you didn't design the heatsink, you don't know what surface temperature corresponds to the thermal pah temperature, because the heat is not always uniform. Am I right? OR it is a good thing measuring its surface to know its thermal resistnace?

author: thexeno
author_rank: Regular Contributor
timestamp: January 06, 2016, 09:08:33 pm
content: Is it meaningful to measure the difference between the heatsink and room temperature, dividing by the power? Because I know that the heatsink did not have the same temperature on the entire surface *by definition*. What the maningful measurement mean? Probing temperature at hotter spot? At the cooler one? Fins are cold compared to central and low part of heatsink.I'm not sure that 12.5°C/W means somenthing.PS:Quote from: blueskull on January 06, 2016, 04:25:20 pmwhat?