In practical applications it is typical that the component is driven by a periodic power waveform (switching power supplies, LED modules etc.) It is often useful to analyze responses on periodic excitations in the frequency domain instead of the time domain. Zth curves can be converted to frequency domain through the Fourier transformation.
The periodic waveform can always be decomposed as a constant power and harmonics. The constant part is always positive; the harmonics show changes of positive and “negative” power value. For the constant part the measured system shows the full thermal impedance. However, when the same power is applied at higher frequencies, we experience smaller temperature growth because the heat does not reach the outer structures; it is stored “locally” in the capacitance of the material nearer to the source of heat. The temperature changes are delayed compared to the change in power; due the capacitances, there is a phase shift between power and temperature.
Figure 3-13: Complex loci of the multi-chip module
In case of a driving point the real part is always positive and the imaginary part is always negative (distributed thermal resistance and capacitance elements). Transfer type impedances run into other quarters of the plane (i.e. the phase of thermal changes may be opposite at the measurement point as that of the excitation point).
