Thermal impedance - what is the purpose?

Question

If I want to compute the junction temperature of a component, I can by using its thermal resistance (case to junction) and if I know the power dissipated by the component and the temperature of the case. It allows to know the difference of temperature between the junction and the case of the component in steady state only. In other words, it means that the different heat capacitors of the component are charged to their steady value.

If I took a diode, the average power dissipated over a period is given by the following formula:

$$P_d = U_{D0}*I_{average} + R_{d}*I_{rms}^2$$

If the current through the diode is DC, which is not my case, the previous formula can be simplified:

$$P_d = U_{D0}*I_{DC} + R_{d}*I_{DC}^2$$

where Ud0 and Rd are extracted from the below curve:

Now what s the purpose of thermal impedance?

Here is a thermal impedance extracted from a datasheet:

It gives the thermal impedance of the junction to case of the component in function of the duty cycle of the power. In acSMPS, the component dissipates power only (almost) when it conducts.

Nevertheless in the previous calculation, we have also considered the fact that the power was not DC by taking the average current and the rms current.

I do not understand the difference. What should I use?

Answer 1

It takes time for the heat to conduct away from the hotspot, and the thermal resistance does not take that into account as that's DC usage.

So if on average your component dissipates 1W into heat, there is a big difference how much the internal silicon junction heats up momentarily, if you have a constant 1W dissipation at DC, or if there is 10W dissipation for 10% of the time where it heats up rapidly and 90% of time it is let to cool down to let the heat conduct away.