Thermal impedance - what is the purpose?

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?

• For DC, you only need thermal resistance. For pulsed applications, you need to take into account thermal impedance. Aug 20, 2021 at 7:53
• I do not get why the first "themal resistance" does not take into account the effect of pulsed application as the average power dissipated is calculated via RMS and average current.
– Jess
Aug 20, 2021 at 8:00
• At the start of the pulse the junction is surrounded by a cold chip. At steady state it is surrounded by a hot chip. Aug 20, 2021 at 8:32
• Did you want a formula for estimating the junction Rjc(f,d) to add to Rjc(dc)? If you know the thermal time constant of the heatsink… Aug 20, 2021 at 13:31
• @TonyStewartEE75 I am sorry I do not get what you said ? For estimating what ? Rjc(f,d) f for frequency and d for duty cycle ? And what is Rjc ? The thermal Impedance ?
– Jess
Aug 20, 2021 at 13:38