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?