Simple mathematical transformation allows us to calculate the frequency-domain representation of the impedance. This representation can be used to calculate the junction temperature when a sinusoidal change in the heating power takes place, provided, we know the frequency of the heating waveform. This property is important for the characterization of power semiconductor devices which are driven by sinusoidal signals. In the frequency domain impedances at a given frequency are given by a complex number. Showing the real and imaginary part of the impedance values for different values results in the so called complex locus – also known as Nyquist diagrams in electrical engineering. If a power semiconductor is driven by square wave signal with a given duty cycle, the so called pulsed thermal resistance diagrams can be used to predict the effective chip temperature. The value of the pulsed thermal resistance depends on the frequency and the duty cycle of the applied square wave. This is such an important property of power semiconductor device packages that it is provided on product data sheets. In case of LEDs the pulsed thermal resistance diagrams provide essential information for the thermal effect of PWM (pulse width modulated) based dimming. Again, pulsed thermal resistance can be calculated from the Zth-functions.

In the case of LEDs measurement of the real Zth requires considering the emitted optical energy. Therefore identification of the real thermal resistance or thermal impedance of LEDs requires light output measurements (total flux measurements) performed simultaneously with the thermal measurements. LEDs driven directly from the AC mains pose other problems. Due to the highly non-linear electrical characteristics the heating power waveform contains many harmonics of the base frequency of the AC mains, therefore calculating "a" single Zth value poses some problems.

For more information and to registuer, click here.