Design comparison of active and passive EMI filters

The design engineers who use low interference signals (EMI) in the work will generally face two tests when designing: how to reduce the volume of the scheme while reducing the design interference signals. The front passive filter can reduce the conductive EMI generated by the switching power supply, thus ensuring compliance with the conductive EMI standard, but this method may contradict the need to improve the customized power of low EMI, especially considering the adverse impact of high switching speed on the entire EMI signal. This type of passive filter is usually huge, accounting for 30% of the total volume of the power supply scheme. Therefore, it is still the first requirement of the system designer to reasonably reduce the volume of EMI filter while increasing the power.

Active EMI filtering theory is a relatively new EMI filtering method, which can weaken the interference signal, enable engineers to greatly reduce the size of passive filters, control costs, and improve EMI characteristics. This paper will review the design results of vehicle synchronous step-down controller equipped with active EMI filter function to show the key advantages of active EMI filter in terms of EMI performance increase and space saving.

EMI filtering

Passive filtering uses inductance and capacitor to obtain impedance mismatch through EMI current, thus reducing the transmission and transmission of the circuit. In contrast, the active filter can sense the voltage of the input bus, generate the opposite current, and directly resist the EMI current generated at the switch level.

In this context, please look at the simplified passive and active filter circuits in Figure 1, where iN and ZN show the current source and impedance of the Norton equivalent circuit to distinguish the difference between DC/DC regulators.

Figure 1. Installation and activation of conventional passive filter (a) and active filter (b) circuits

Figure 1b is equipped with voltage sensing and current clearing devices. (VSCC) Active EMI filter uses the computational amplifier circuit as the capacitor multiplier, rather than the filter capacitor in passive design. (CF)。 As shown in the figure, GOP is used to display the induction, introduction and compensation impedance of the active filter. The gain term is designed with lower capacitance value and smaller component specification. The efficient active capacitor is set by calculating the amplifier circuit gain and introducing the capacitor (CINJ).

Figure 1 includes the relationship of reasonable filter cut-off frequency. High-efficiency GOP can reduce the inductance and capacitance of active customization, and the cut-off frequency is equal to the passive design.

Optimize filtration performance

Figure 2 compares the design of passive and active EMI filters based on transmission EMI test. This design and application of peak and mean detectors can meet the requirements of the International Special Committee on Radio Interference. (CISPR) Class 255 standard. According to the power level of LM25149-Q1 synchronous step-down DC/DC controller, the conclusion after the active EMI filter circuit is turned on and off is shown in Figure 3 according to the 13.5V vehicle battery input. The weakening of the middle and low frequency of the active EMI filter is not significant compared with the unfiltered or initial noise signal. The peak EMI level of 440 kHz fundamental frequency weight has been reduced by nearly 50 dB, which enables designers to meet the strict management of EMI more smoothly. Output 5V and 6A. The switching frequency is 440kHz.

Figure 2. Comparison of passive filter scheme (a) and active filter design (b) under equivalent power level operating conditions

Figure 3 shows the results when the active EMI filter circuit is enabled and disabled. Compared with unfiltered or original noise signals, the attenuation of middle and low frequency of active EMI filter is less obvious. The peak EMI level of the fundamental frequency component of 440 kHz has been reduced by nearly 50 dB, which makes it easier for designers to meet the strict requirements of EMI.

Figure 2. Comparing the filtering performance of the active EMI filter in the disabled (a) and enabled (b) states

Save PCB space

Figure 4 shows that the layout of passive and active filter level printed circuit boards (PCBs) is relatively good, as shown in Figure 2. The space occupied by inductance is reduced from 5mmx5mm to 4mmx4mm. In addition, two 1210 capacitors are replaced by several small 0402 devices with stable values, which are suitable for the sensing, introduction and compensation of active EMI filters. The area of this filtering solution has been reduced by nearly 50%, while the volume has been reduced by more than 75%.

Figure 4. Comparison of PCB layout dimensions of passive (a) and active (b) filter designs

Advantages of passive devices

As mentioned earlier, compared with the inductance in the passive filter design, the active EMI filter has a lower filter inductance value, which can reduce the occupied space and reduce the cost. In addition, inductors with smaller physical specifications generally have a winding resistance geometry with lower survival winding capacitance and higher self-resonance frequency, and then improve the filtering characteristics in the higher transmission operating frequency band of CISPR25: 30MHz to 108MHz.

In some car designs, two input capacitors must be connected in series to ensure the invalid protection stability after being connected through the battery-powered track. Therefore, the active circuit can save additional space, because the series connection of small 0402/0603 inductance and lead-in capacitor can replace multiple 1210 capacitors. Small capacitors can simplify the procurement process of equipment, because these equipment can be purchased at any time and are not limited by dealers.

Epilogue

We pay close attention to EMI, especially the use of voltage induction and current introduction active filters in automotive applications to complete low EMI signals, thus reducing the space and volume occupied and reducing the cost of solutions. The integration of the active EMI filter circuit and the synchronous step-down controller is conducive to dealing with the DC/DC voltage regulator measurement problem between low EMI and high power density.