# Thermal resistance Junction - Maximum Power dissipation

I want to understand the real meaning of the parameter 'Thermal Resistance Junction to Air (°C/W) to calculate the power dissipation. So for example if the value of this parameter is 250°C/W, should I take this value as a constant if my ambient temperature is 50°C and then calculate my power dissipation with Pd= (Tj(max)-Ta)/R(°C/W)? Or I have to determine the Thermal Resistance Junction to air as a variable parameter and calculate it considering the ambient temperature and for my case with 50°C (Ambiant temperature).

I thank you for your help.

## 1 Answer

Yes you should take that Thermal Resistance as a constant, it is a property of how a chip or device is packaged (how well the package can transfer heat) and if that heat transfer is "helped" in any way for example by using a heatsink.

If your ambient temperature is 50°C and you allow the junction in your device to reach for example 90°C then that means there's a 90°C - 50°C = 40°C temperature difference.

For a thermal resistance of 250°C/W that means 40°C / 250°C * 1 Watt = 0.16 Watt can be dissipated. Dissipate more and the junction in your device will become hotter than 90°C (assuming the ambient temperature is 50°C).

• I thank you for your answer. When I calculate the maximum power dissipation for the MB6S bridge rectifier(SMD) using the typical thermal resistance (85°C/W), I found that the P is equal to 1.47W (considering ambiant temperature to 25°C). However when I calculate the P using the current and the voltage I found P=V X I = 1 X 0.4 = 0.4 Watt. I'm really confusing about the great difference between the two results. Have you any idea about this difference ? And what is the best formula to use to calculate power dissipation in this case. I thank you for your support. – geek225 Jul 4 '19 at 11:44
• The 1.47 W is the maximum power dissipation, it means that if you dissipate more than that, the rectifier will get too hot. The 0.4 W is the actual power dissipation meaning, when 1 A flows, 0.4 W is dissipated. That's less than 1.47 W so the rectifier will not get too hot. When 0 A flows, you'd get 0 Watt and the rectifier would dissipate zero power so there would be no temperature increase at all. – Bimpelrekkie Jul 4 '19 at 11:59
• @ Bimpelrekkie: So for electronic design which formula to consider : The formula using thermal resistance or the formula considering current and voltage ? Thanks. – geek225 Jul 4 '19 at 12:57
• Both, you use the thermal resistance one for determining your maximum power dissipation then you use the current and voltage to determine the actual power dissipation. Then check that Pactual < Pmax, if that is so, no problem. If Pactual > Pmax then you have an issue and you need to change the design / use different components / add cooling – Bimpelrekkie Jul 4 '19 at 13:01