How to reduce EMI and reduce power supply size through integrated active EMI filters

Design engineers engaged in low electromagnetic interference (EMI) applications usually face two major challenges when designing: how to reduce the electromagnetic interference in the design while reducing the size of the solution. Front-end passive filtering can reduce the conductive EMI generated by the switching power supply, thereby ensuring compliance with the conductive EMI standard, but this method may contradict the requirement of increasing the power density of low EMI design, especially considering the higher switching speed. The adverse effect of the overall EMI signal. These passive filters are often bulky, accounting for 30% of the total volume of the power solution.Therefore, while increasing the power density, effectively reducing the size of the EMI filter is still the system

Design engineers engaged in low electromagnetic interference (EMI) applications usually face two major challenges when designing: how to reduce the electromagnetic interference in the design while reducing the size of the solution. Front-end passive filtering can reduce the conductive EMI generated by the switching power supply, thereby ensuring compliance with the conductive EMI standard, but this method may contradict the requirement of increasing the power density of low EMI design, especially considering the higher switching speed. The adverse effect of the overall EMI signal. These passive filters are often bulky, accounting for 30% of the total volume of the power solution. Therefore, while increasing the power density, effectively reducing the size of the EMI filter is still the primary task of the system designer.

Active EMI filtering technology is a newer EMI filtering method that can reduce electromagnetic interference, allowing engineers to significantly reduce the size of passive filters, reduce costs, and improve EMI performance. In order to illustrate the main advantages of active EMI filters in terms of EMI performance improvement and space saving, in this article, I will review the results of the design of an automotive synchronous buck controller that integrates active EMI filter functions.

EMI filtering

Passive filtering uses inductors and capacitors to create impedance mismatches in the EMI current path, thereby reducing the conducted emissions of the power circuit. In contrast, active filtering can sense the voltage on the input bus and generate an inverted current that can directly offset the EMI current generated by the switching stage.

In this context, take a look at the simplified passive and active filter circuit in Figure 1, where iN And ZN Respectively represent the current source and impedance of the Norton equivalent circuit of differential mode noise for DC/DC regulators.

How to reduce EMI and reduce power supply size through integrated active EMI filters
Figure 1: Conventional passive filter (a) and active filter (b) circuit installation and activation

In Figure 1b, an active EMI filter configured with voltage sensing and current cancellation (VSCC) uses an operational amplifier circuit as a capacitance multiplier to replace the filter capacitor in a passive design (CF). As shown in the figure, the sensing, injection, and compensation impedance of the active filter uses a relatively low capacitance value and a small component size to design the gain term, using GOP Express. The effective active capacitance is determined by the operational amplifier circuit gain and the injection capacitor (CINJ) set up.

Figure 1 contains the expression for the effective filter cutoff frequency. Efficient GOP The inductance and capacitance values ​​of active design can be reduced, and the cut-off frequency is equivalent to that of passive design.

Filter performance optimization

Figure 2 compares passive and active EMI filter designs based on conducted EMI testing. This type of design uses peak and average detectors to meet the International Special Committee on Radio Interference (CISPR) 25 Category 5 standards. Each design uses a power stage based on the LM25149-Q1 synchronous step-down DC/DC controller to provide 5V and 6A output through a 13.5V car battery input. The switching frequency is 440kHz.

How to reduce EMI and reduce power supply size through integrated active EMI filters
Figure 2: Comparison of passive filter schemes (a) and active filter design under equivalent power level working conditions (b)

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

How to reduce EMI and reduce power supply size through integrated active EMI filters
Figure 3: Comparison of the filtering performance of active EMI filters in disabled (a) and enabled (b) states

Save PCB space

Figure 4 provides a comparison of the printed circuit board (PCB) layout of the passive and active filter stages, and the results are shown in Figure 2. The space occupied by the inductor is reduced from 5mm x 5mm to 4mm x 4mm. In addition, the two 1210 capacitors that drastically decrease with the applied voltage are replaced by several small and stable 0402 devices suitable for active EMI filter sensing, injection and compensation. The area occupied by the filter solution has been reduced by nearly 50%, while the volume has been reduced by more than 75%.

How to reduce EMI and reduce power supply size through integrated active EMI filters
Figure 4: Comparison of the PCB layout size of passive (a) and active (b) filter designs

Advantages of passive components

As mentioned earlier, compared with inductors in passive filter designs, active EMI filters have lower filter inductance values, which can reduce the space occupied and costs. In addition, inductors with smaller physical dimensions usually have a winding geometry with a lower parasitic winding capacitance and a higher self-resonant frequency, thereby improving the filtering performance in the higher conduction frequency range of CISPR 25: 30MHz to 108MHz.

Some automotive designs require two input capacitors to be connected in series to ensure fail-safe robustness when connected directly through the battery-powered rail. Therefore, the active circuit can save additional space, because the series connection of small 0402/0603 inductors and injection capacitors can replace multiple 1210 capacitors. Smaller capacitors can simplify the procurement process of the device, because such devices can be bought at any time and are not subject to supplier restrictions.

Concluding remarks

We will continue to pay attention to EMI, especially in automotive applications that use voltage sensing and current injection active filters to achieve low EMI signals, and ultimately reduce the space and volume occupied, while reducing the cost of the solution. The integration of active EMI filter circuits and synchronous buck controllers helps to solve the trade-off between low EMI and high power density in DC/DC regulator applications.

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