Understanding semiconductor thermal resistance

Question

Most datasheets I read through have a thermal resistance rating attached to the particular package the semiconductor comes in (i.e. TO-220, D2PAK, TO-3P, etc), usually in the form of θJA (junction-to-ambient) °C/W.

Some questions:

1. What is the difference between θJA and θJC, and which should be used to calculate temperature rise above ambient (assume 25°C room temperature) with the formula P = I·V, where V is voltage dissipated (i.e. linear regulator dropoff), I is current, and P is power in watts?

2. How do you calculate the needed heatsink rating in °C/W using the prementioned ratings (without use of a fan for forced airflow, static heatsink only)?

Answer 1

θJC is the temperature difference of the junction to the case of the device per watt. θJA is the temp. diff. of junction to ambient per watt.

Think of these values θJA and θJC as resistances and think of power P as current and temperature rise as voltage and apply the well known U=R*I

I know this is a bit strange but it works perfectly for calculating heat transfers.

Lets assume ambient is 25°C, θJC=150°C/W, θJA=200°C/W and P=0.1W. If you think like I said above, you get a temp. diff. of 20°C for the junction which means the junction is at 45°C. The case will then be at 30°C.

The heat sink acts like a parallel resistor in our alternative model, which reduces the case-to-ambient „resistance“.