# Thermal power dissipation for a diode with heatsink

I have a circuit that has a diode that needs to support 3A through it. The supply is at 7.2V and is therefore ~22W.

I have currently selected a schottky diode show here. Key specs are it can support up to 5A, has thermal resistance junction to case of 12K/W, and a max junction temp of 150C. The heatsink I plan on using has a thermal resistance of about 3C/W.

I am trying to do some thermal calculations to make sure the device can support my usecase. I am using an online calculator such as this one and input the specs from above. Two things I have questions on:

1. The units are in K/W. Looking online, it seems people treat C/W and K/W the same, and I am not sure why as they are different units; however if you convert the K to C values, you will get all negative values. Once again more confusion.

2. What do I use for power here? Using 22W (system power) gives ridiculous results and states that the heat sink will not work, so I am assuming that I use the max forward voltage at max current (~.36V) times the current I plan on running it at (3A) and take .36 * 3 = 1.8W. This then was input to the calculator showing a junction temp lower than what the diode is spec'd to (i.e. I can use it). Am I correct using this power?