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I'm not sure to correctly understand the datasheet of Traco THN24 family. I read this:

Thermal impedance – Natural convection 17.6°C/W

but the DCDC is rated for 20W without de-rating (100% output power) at ambient temperature of 60 °C with natural convection and without heatsink.

This makes no sense for me, because the temperature should rise up to 20W * 17.6 °C/W = 352 °C!

Of course I'm wrong, otherwise the product cannot be used at the specified power. Where is my mistake?

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    \$\begingroup\$ You have to multiple the 20W by the inefficiency, if the converter is 85% efficient, 0.15 * 20 = 3W of waste heat * 17.6 degC/W = 53 degree temp rise. \$\endgroup\$ – Dean Franks Apr 11 '18 at 8:09
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The efficiency is given as a typical figure, so reduce it a little for a safety margin.

Output power: 20 W
Efficiency: 0.85
Input power = 20 W/0.85 = 23.53 W
Waste power = 23.53 W - 20 W = 3.53 W
Temperature rise = 3.53 W * 17.6 K/W = 62.1 K

Resulting case temperature at 20 °C ambient = 82 °C.

The data sheet says the maximum case temperature is 105 °C so it will work.

However I would not like a component to be that hot (e.g. it might severely reduce the life of other components near it) so I would be looking at adding a more substantial heat sink than the optional one and some forced air flow if it was going to be run at that heat dissipation.

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