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Datasheet of heatsink.

The heatsink says it's thermal resistance is 24K/W.

Let's say my device dissipates 5 Watts. Assume a perfect conduction from the case to the heatsink, and junction to case is 0K/W.

If I use the specified value, the heatsink should rise 120K above ambient. However, from the graph it says it will be about 96K above ambient (unsure which curve is correct).

Further, how do I interpret the effect of forced air cooling according to the graph?

enter image description here

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  • \$\begingroup\$ The temperature rise of a chosen heatsink must be equal or less than the maximum allowable surface to ambient temperature difference at the same dissipation. The lower right to the upper left curve is for natural convection. This heatsink it is not proper selected, except if you want to operate your device at the fright edge. \$\endgroup\$ – GR Tech Dec 17 '14 at 12:32
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The specified value of 24K/W assumes that the performance is linear. That chart is obviously anything but linear.

Forced air changes this "base" 24K/W to the amount given on the chart. So if there was a flow of 400fpm the thermal resistance would drop to about 6.3K/W.

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The spec sheet tells us that the 24K/W figure is using natural convection at 75 degrees C above ambient.

Looking next at the falling line on the graph, the arrows indicate that it graphs thermal resistance against air flow. We see that the line never reaches 24: the values of thermal resistance are much smaller for all the plotted air flow values.

Looking now at the rising line on the graph, the arrows indicate that it graphs temperature against power. Drawing a vertical line at 1 Watt, we read a temperature rise of 28C, indicating a thermal resistance of 28. This is much larger than any value on the Air Velocity curve: it is close to the quoted value for natural convection. It is not 24, because the line is a curve: 24 is only specified for one point on a curve, the 75 degree above ambient point.

Drawing a horizontal line at 75C, we read approximately 3.25W, indicating a thermal resistance around 21.

This indicates that (a) The graph is wrong: it should go through (75C, 3.125W), or (b) 24K/W was wrong, and the graph is correct, or (c) The power/temperature line is drawn for different conditions than the quoted 24K/W figure: different ambient, different mounting, different air flow etc.

In any case, as the graph shows, the value 24K/W is not exact for any other point, or any other conditions, but it is approximately true for condtions approximately the same as specified.

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  • \$\begingroup\$ From using my judgement I can tell that the rising graph is for natural convection and the other is for forced cooling, but how do those arrows specify which line corresponds to which axis? \$\endgroup\$ – tgun926 Dec 17 '14 at 11:57
  • \$\begingroup\$ The arrows attached to each line point to the axis used for that line. Obvious only after you know... \$\endgroup\$ – david Dec 26 '14 at 6:13

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