Figure 10-1 shows the schematic of a single-phase AEF circuit with CM noise attenuation achieved using the TPSF12C1-Q1 active filter IC. The design includes both regulator-side and grid-side Y-capacitors. A LISN provides a suitable interface to an EMI receiver for EMI measurement from 150 kHz to 30 MHz.

As shown in Figure 10-1, a square-wave signal derived from a function generator represents a convenient source for CM noise excitation, and a 1-nF capacitor mimics a realistic CM noise-source impedance of a switching regulator. Adjusting the amplitude and transition times of the source voltage sets a suitable noise amplitude and spectral envelope measured at the LISN.

This simple low-voltage test with signal injection facilitates a safe and expedient performance characterization of the filter in an EMI chamber prior to connection to a switching regulator in a high-voltage operating environment.

Figure 10-2 shows the filter board implementation. Figure 10-3 presents EMI results with AEF disabled and enabled, using both quasi-peak (QP) and average (AV) noise detectors. As evident in Figure 10-3, AEF provides up to 30 dB of CM noise attenuation in the low-frequency range (100 kHz to 3 MHz), which allows a filter using two 2-mH nanocrystalline chokes to achieve an equivalent CM attenuation performance as a passive filter design with two 12-mH chokes.