# A Peak Absorption Filtering Method for Radiated EMI from a High-Speed PWM Fan
## 4 Filter Suppression of Radiated Interference  
According to the analysis of the interference path and the main source of radiation, the interference voltage was generated by the switching device in the inverter circuit, which entered the windings of the fan and caused radiated interference. The voltage for the port of the windings was calculated via the field-circuit simulation model shown in Figure 13. Taking the U phase as an example, the simulation results are presented in Figure 19, compared with the spectrum of the voltage at the winding port obtained by FFT. The calculation results demonstrate a consistency between the simulation and measurement. Therefore, the circuit simulation model can effectively guide the design of EMI filters. For the filter circuit topology design, if the low-pass filter includes an inductive component, the inductance value of the selected device will be at the mH level due to the lowest frequency of the interference peak of 15.5 kHz, resulting in a large filter size. As a  
Figure 19: Comparison of the simulation results of the winding voltages with the measurement results. (**a**) Measurement results; (**b**) simulation results. Figure 17: Simulation results of PCB surface current. Figure 18: Voltage at three-phase winding ports in the time-domain. miniaturized motor, axial flow fans have limited internal space, which makes it challenging to accommodate larger-sized components. Hence, this paper proposes a non-inductive RC snubber circuit connected in parallel between the drain and source of each MOSFET, absorbing the voltage peaks due to high-frequency switching. The circuit topology is shown, and the RC snubber circuit in parallel with the MOSFET is marked in red in Figure 20.  
The determination of RC parameters was achieved through circuit simulation. By adding parallel resistor and capacitor elements in the circuit simulation of Figure 13, the parameter combinations of resistors and capacitors in the filter were varied separately, and the absorption effect of the filter was measured by calculating the frequency-domain voltages of the winding. Using this simulation model, we calculated the winding voltage under two scenarios:  
1. With a fixed capacitance of \(C=10\) uF, the resistance values were varied as \(R=1\)\(\Omega\), \(10\)\(\Omega\), and \(100\)\(\Omega\). 2. With a fixed resistance of \(R=10\)\(\Omega\), the capacitance values were varied as \(C=1\) uF, \(10\) uF, and \(100\) uF. As depicted in Figures 21 and 22, the comparison of peak voltages with the voltage without filter across the winding terminals revealed that a smaller resistor and a larger capacitor yielded a more effective suppression of peak voltages. Additionally, to preclude overcurrent in the circuit, the resistance should not be chosen to be excessively small. Furthermore, considering the spatial constraints of the PCB, the capacitor value should also not be excessively large. The parameters of the filter were determined as \(R=10\)\(\Omega\), \(C=10\) uF. Figure 21: Simulation results of winding voltages with different \(R\) when \(C=10\) μF. Figure 20: RC filter circuit topology. The device met the space size requirements of the fan and can be integrated into the PCB as shown in Figure 23. The red squares mark where the RC devices were installed, in parallel with each MOSFET. The control PCB was designed and directly processed by the authors for the integration of the complete fan, which is a four-layer board including key components such as MCUs, inverter circuits, and RC filter circuits. The comparative results before and after filtering for the fan radiated interference measurement are shown in Figure 24. The measurement results show that the designed RC filter had a significant absorption effect on the radiated interference in the frequency band. Radiated interference attenuation reached a maximum of 41.9 dB. Peak radiated interference in the frequency band was reduced below the standard limits.