## 6 Experimental Results
### CM Conducted Emission  
In order to measure the CM EMI delivered by the Buck converter, the setup recommended in the CISPR-25 standard [31] was adopted. Such a test-bench is sketched in Figure 13a (not in scale), and the common-mode EMI was measured by means of the CM sensing network described in [32]. Two LISNs were exploited since the power supply cables are 20 cm long. A picture of the experimental setup is shown in Figure 13b, where the key components are labeled too. The load, which is connected to the output port of the Buck converter, is an electronic load operating as variable resistance, so different values of output current can be tested.  
In order to verify that the CM conducted EMI are related to the switching voltage (\(v_{\text{sw}}\)) through a capacitive coupling, as previously described in Section 2, the frequency spectrum of the switching voltage (\(V_{\text{sw,dBV}}\)) was subtracted from the spectrum of the common mode voltage (\(CM_{dB\
u}\)) measured using the spectrum analyzer. Then, the magnitude of the transfer function \(\left|TF\right|\) was evaluated at each frequency as  
\[\left|TF\right|_{\text{dB}}=CM_{dB\mu V}+G-(V_{\text{sw,dB}}+120), \tag{9}\]  
where \(G=5.4\) dB is the attenuation introduced by the CM sensing network. The \(\left|TF\right|\) envelope is shown in Figure 14 by solid line. By fitting such an envelope with a straight line having a slope of 20 dB/dec, the dashed curve is obtained. It is worth noticing that even though the propagation path can be complex, in actual fact the equivalent model presented in Section 2 is effective. As far as the designed Buck converter is concerned, from the straight line approximation it is possible to estimate the equivalent parasitic capacitance, resulting in \(C_{\text{p,sw}}=385\) fF.  
Figure 14: Common-mode transfer function obtained by experimental results (continuous line) and the fitted one (dashed line). The capacitive model is effective in predicting the CM EMI propagation.  
Figure 13: (**a**) Block diagram of the test setup to measure the common-mode EMI delivered by the device under test (DUT), and (**b**) a photograph of the experimental setup compliant with CISPR-25 standard.  
Once the capacitive coupling was assessed, the common-mode EMI was measured for different values of source inductance, as shown in Figure 15 for a load current equals to 3 A. More precisely, surface mounted air core inductors, with nominal values of \(3.7,6.6,12.5,25\,\mathrm{nH}\), were exploited to test the proposed technique. As it can be noticed from Figure 15, the use of the source inductance is effective in reducing the peak caused by the turn-on oscillation, as well as the spectral components in the (60 MHz, 90 MHz) frequency range. Indeed, by increasing the value of the source inductance from 3.7 nH up to 25 nH, the peak at \(f_{\mathrm{r,on}}\) moves to lower frequencies and its amplitude is reduced. More precisely, in case of 3 A output current and \(L_{\mathrm{s}}\) = 6.6 nH, the peak is approximately 20 dB lower than the oscillating case. Such an aspect assess the effectiveness of the proposed technique, since the insertion of the source inductance introduces a negative feedback during the transistor turns-on, and modifies the switching voltage, eventually.