# Experimental Study of Radiated Emission Due to Secondary Common-Mode Current in Buck Converters
## 4 Noise Measurements in the Experimental System
### Common-Mode Currents  
To validate the generation mechanism of the SCM noise, measurements of the CM currents were performed in the laboratory. The configuration of the experimental system for CM current measurements has already been shown in Figure 2. Figure 6 is a picture of the constructed experimental system. Every measurement in this article was performed when the input DC voltage was 200 V; the switching frequency was 100 kHz; the duty was 0.5; and the output power was 100 W. Dry cell batteries were used to supply the control circuit for preventing the radiated emission from the power supply of the control circuit. A spectrum analyzer (FPL1003, ROHDE&SCHWARZ, Munich, Germany) and current-monitor probe (F-52B, Fischer Custom Communications, Torrance, CA, USA) were used to measure the CM currents. The measured CM current is displayed on the spectrum analyzer as the voltage value. Thus, the measured value is transferred to the current value based on the frequency characteristics of the transfer impedance obtained from the datasheet of the probe. As shown in Figure 7, the PCM current was measured by clamping the power lines P and N together near the LISN with the probe, and the SCM current was measured by clamping the power lines and the ground line E together. Since the measured results were obtained as voltage values in the spectrum analyzer, the measured results were converted to current based on the transformer impedance of the current-monitor probes provided by the developer. Here, the two-winding CMI connected to the PCM is called the primary common-mode inductor (P-CMI), and the three-winding CMI connected to the SCM is called the secondary common-mode inductor (S-CMI). The measurements of CM currents were performed in the frequency range from 30 to 300 MHz  
Figure 5: Process flow to validate the generation mechanism of radiated emission. when no CMI was connected; the P-CMI was connected; and the S-CMI was connected near the power converter. Each CMI was fabricated by applying five turns of multiple windings of the same polarity to a nickel-zinc ferrite core (43 material, model number: 5943002701, Fair-Rite, Singapore) for each noise propagation mode. Figure 8 shows each CMI and a configuration of the entire experimental system when the CMIs are connected. Figure 9 shows the measured frequency characteristics of the PCM and SCM impedances (\(Z_{\text{PCM}}\) and \(Z_{\text{SCM}}\)) of each fabricated CMI. The measurements were performed using an impedance analyzer (E4990A, Keysight, Santa Rosa, CA, USA) in the frequency range from 1 kHz to 100 MHz. For the P-CMI, \(Z_{\text{PCM}}\) was measured by connecting the terminals of the two windings shorted together to the measurement terminals of the impedance analyzer. \(Z_{\text{PCM}}\) and \(Z_{\text{SCM}}\) of the fabricated S-CMI were measured according to the connections, as shown in Figure 9a. Figure 9b confirms that the PCM impedance of the P-CMI and the SCM impedance of the S-CMI show similar frequency characteristics due to the equal turn numbers of the two CMIs. Up to about 2 MHz, the impedance increases with a +20 dB\(\Omega\)/dec  
Figure 6: Experimental setup for measurement of the CM currents in the laboratory. slope. Above 2 MHz, the slopes of impedance increase are about +10 dB/\(\Omega\) due to the complex permeability of the NiZn ferrite core. Note that the PCM impedance of the S-CMI corresponds to the leakage inductance of the S-CMI. The PCM impedance of S-CMI is very small compared to the SCM impedance because of the winding arrangement's high coupling between the windings. Moreover, it can be confirmed that the winding resistance is dominant from 1 kHz to 10 kHz in the measured PCM impedance of S-CMI. Figure 8: Configuration of the experimental system when the CMIs are connected. Figure 7: Measurement point of the CM currents. Figures 10 and 11 show the measured PCM and SCM currents. The background noise shown in Figures 10 and 11 was measured when only the cooling fan of the buck converter was operating.