Hello!

Fig. 1 (The waveforms in the scope):

Fig. 2 (The Fourier Analysis results of these waveforms):

Fig. 3 (My setting of the Fourier Analysis tool):

We can see that in Plot 1, the THD value is so high(=80936.9%). Its looks like not right... Does the PLECS calculate THD value with the DC component(=217.319)?

However, Does the THD value(=5.21%) in Plot 3 calculated with the DC component(=108.66)?

Attached are file of Fig. 1, 2, 3.

Thanks!

Jeremy

Thanks for your great answer!

I'm sorry that I didn't take care that in the first plot, the base frequency is 160kHz.

In the second plot, the base frequency is 80kHz which is specified. However, I cannot calculate the THD of "Measured voltage" displayed(≈0.48):

THD(Measured voltage in Plot 2)

≈ { [V

_{3}/sqrt(2)]^{2}+[V_{5}/sqrt(2)]^{2}+[V_{7}/sqrt(2)]^{2}+[V_{9}/sqrt(2)]^{2}+[V_{11}/sqrt(2)]^{2}+[V_{13}/sqrt(2)]^{2}+[V5/sqrt(2)]^{2}+[V_{17}/sqrt(2)]^{2}+[V_{19}/sqrt(2)]^{2}} / { [V_{1}/sqrt(2)]^{2 }}= { V

_{3}^{2}+V_{5}^{2}+V_{7}^{2}+V_{9}^{2}+V_{11}^{2}+V_{13}^{2}+V_{15}^{2}+V_{17}^{2}+V_{19}^{2}} / { V_{1}^{2 }}=(195.23

^{2}+117.138^{2}+83.67^{2}+65^{2}+53^{2}+45^{2}+39^{2}+34^{2}+30^{2}) / 585.69^{2}=0.208

which is largely different from0.48.

So I am curious that the left harmonic components will cause a so large difference? or how to calculate in the software?

edited Nov 7, 2018 by Jeremy_C