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What is K/W from heat sinks?


Capricious
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Capricious,
That's right. A heatsink that is rated at 2.4 K/W increases its temp 2.4 degrees Kelvin for each Watt that it must dissipate, in standing calm air. This is called its thermal resistance. You use this value to determine if the heatsink is large enough or whether it will need forced air cooling with a fan. I guess they use the term "Kelvin" because they don't know what your ambient temp will be. Many parts are spec'd with degrees Celcius/W.
There are additional thermal resistances that must be accounted for:
1) Between a transistor's junction and its case.
2) Between the transistor's case and the heatsink. Because the surfaces are not completely flat, heatsink compound grease conducts heat and helps to fill the small voids. But it must be a very thin layer.
3) An insulator also has a thermal resistance rating.
4) The amount of mechanical pressure between the transistor's case and the heatsink. Many "clips" fasteners do a poor job compared to bolts and nuts.

If your heatsink requirement caculates as being too large, or you don't want to use a fan, then add parallel output transistors (with emitter resistors to equalize their gain differences) so that they can share the current and heating. The driver transistor will also run cooler since a power transistor's gain decreases when its current increases. So 2 paralleled transistors have more gain than 1.

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Capricious, in case you are interested:
The Kelvin temperature scale is the base unit of thermodynamic temperature measurement in the International System of measurement. It is defined as 1/ 273.16 of the triple point (equilibrium among the solid, liquid, and gaseous phases) of pure water. The kelvin (symbol K) without the degree sign is also the fundamental unit of the "Kelvin scale", which is an absolute temperature scale named for the British physicist William Thomson Baron Kelvin. This scale has as its zero point absolute zero (the theoretical temperature at which the molecules of a substance have the lowest energy). Many physical laws and formulas can be expressed more simply when an absolute temperature scale is used, so the Kelvin scale has been adopted as the international standard for scientific temperature measurement. The Kelvin scale is related to the Celsius scale. The difference between the freezing and boiling points of water is 100 degrees in each, so that the kelvin has the same magnitude as the degree Celsius.
Below is the conversion.

MP

post-555-1427914165208_thumb.jpg

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