I'm planning to make a power supply with the LM2596 (adjustable voltage variant). In the datasheet I read that efficiency for VIN=12V, VOUT=3V, ILOAD=3A is 73%. That means, if I'm not mistaken, that 3 × 3 / 73 × 27 ≈ 3.33W will be wasted as heat.
When a heatsink is rated as 20K/W, does that mean that for every watt that is dissipated as heat, the temperature of the heatsink rises with 20K? When the heatsink is connected properly, is this the value that I should calculate with, or is the temperature of the component always higher than that of the heatsink, s.t. I should actually calculate with for example 22K/W?
In this example then, the temperature rise with a 20K/W would be 3.33 * 20 = 66.7K, which is still OK because the chip can handle up to 125°C according to the datasheet.
Is this correct? I'm a little bit worried about being OK with my component getting around 90°C, even though the datasheet says it's fine.