Keywords : switching power supply, EMI filter, operational amplifier, network parameters
1 Introduction
Traditional switching power supplies mostly use single-stage or multi-stage passive filters composed of discrete common-mode inductors, differential-mode inductors, and X and Y capacitors. These filters invariably utilize a large number of passive components, such as filter inductors and capacitors. Furthermore, due to increasingly stringent international electromagnetic compatibility (EMC) standards, single-stage filters often cannot meet EMC requirements, necessitating the use of two or more stages. This increases the size of the passive filter and consequently, increases losses. In addition, the distributed parameters of passive components, such as the distributed capacitance and lead inductance (e.g., the distributed capacitance of the differential and common-mode inductors and the series equivalent inductance of the filter capacitors), significantly impact the filtering effect, especially in the high-frequency range. These distributed parameters have a more severe and difficult-to-control effect, greatly attenuating high-frequency filtering characteristics. As the main component of common-mode and differential-mode filter inductors—the magnetic core—has frequency-varying electromagnetic parameters, and its performance is limited by manufacturing processes and materials, which imposes numerous restrictions on its application. Therefore, how to improve the power density and switching frequency of switching power supplies, and achieve modularization and integration of switching power supplies, while solving the more serious electromagnetic interference caused by these changes, is a key problem faced by power electronics technology researchers.
Overall, traditional passive EMI filters have the following shortcomings: ① They are large in size and expensive, thus failing to meet the increasingly miniaturized and high-density requirements of switching power supplies; ② Passive filters have a narrow attenuation bandwidth, requiring increased inductance and capacitance values to improve insertion loss in the low-frequency range, while in the high-frequency range, the influence of distributed parameters may cause unnecessary oscillations, affecting filtering characteristics; ③ Today's requirements for switching power supplies are to be small, lightweight, efficient, reliable, and high-power-density "green power supplies," but as filters are an important component of switching power supplies, there are limits to size reduction, thus restricting the development of switching power supplies.
The aforementioned shortcomings of passive filters mean that they cannot meet the needs of filtering technology in today's increasingly serious electromagnetic pollution. On the other hand, active EMI filters use active cancellation technology, which effectively suppresses EMI noise current and can be dynamically compensated and adjusted without adversely affecting the stability of the system. In addition, compared with passive filters, active EMI filters use semiconductor devices and electronic circuits, which can reduce their size and weight, which is conducive to integrated packaging. Therefore, it has become a new trend and development direction for research in the industry [1-4].
2. Active common-mode EMI filter for switching power supply
2.1 Basic Principles of Active Common-Mode EMI Filters
The essence of active EMI filtering technology is real-time compensation of noise signals. The active common-mode EMI filter (ACMF) proposed here works by first sampling the common-mode signal, and then dynamically outputting a compensation current (voltage) equal in magnitude but opposite in direction to the sampled noise current (voltage) through feedback. Essentially, it provides an extremely low-impedance internal loop for the common-mode current. Figure 1 shows its schematic diagram. Path1 refers to the common-mode current path from the common-mode noise source S1 to ground through the distributed capacitor CD. Without a filter, the common-mode noise inoise will be entirely injected into ground through CP. ACMF generates a compensation current, providing a low-impedance shunt branch Path2 for inoise, thus allowing it to flow along Path2 as much as possible. Ideally, icomp = -inoise, making the common-mode current flowing to ground zero, thereby attenuating the common-mode current to meet electromagnetic interference standards.
2.2 ACMF Design
The shortcomings of traditional passive electromagnetic interference (EMI) filters are analyzed, and the design concept of an active common-mode filter (ACMF) is proposed, along with its working principle. Taking a flyback switching power supply as an example, the design and application of the ACMF in this power supply are studied. Figure 2a shows the connection of the ACMF in the switching power supply system. In the figure, the common-mode filter capacitor Cy, connected by a dashed line between the primary and secondary grounds of the flyback power transformer, has a good filtering effect, but its value cannot be too large due to leakage current safety regulations. Here, the proposed ACMF is used to replace Cy to enhance the filtering effect against common-mode interference. Figure 2b shows the specific circuit of the ACMF. It consists of a broadband high-speed operational amplifier U as its core component.
Due to the unique characteristics of ACMF networks in processing common-mode interference signals, there are specific requirements for the selection of certain components in the circuit. The selection and design of key components in the ACMF circuit are as follows:
(1) Selection of Operational Amplifier U: Because the common-mode interference spectrum is wide, the operational amplifier required to handle it must have a wide bandwidth, fast response speed, high suppression of common-mode voltage in the input voltage, and the ability to output a large current. Here, the high-speed voltage feedback operational amplifier LM7171 with a unity gain of up to 200MHz and an output current of 100mA is selected. Its operating voltage is wide, ranging from 5.5 to 36V. In general, the high-frequency transformer of the switching power supply will have an auxiliary winding to provide power to the PWM/PFM control chip. In this way, the operating voltage of the operational amplifier can be directly obtained from this auxiliary winding after rectification, as shown by #1 in Figure 2a. After connecting to the winding, the operating power supply of the operational amplifier can be conveniently provided between #1 and #2.
(2) Determination of negative feedback network parameters Since the ACMF network uses voltage detection and current compensation, the gain Aiv of the ACMF can be written as:
Let the impedance between #2 and #3 be Z, then:
For ease of analysis, let's assume...
Z can then be further expressed as:
If Z=0, then UAB=0, thus achieving the theoretical short circuit between points A and B of the primary and secondary grounds in the flyback circuit in Figure 2, which is the most ideal situation. As can be seen from equation (3), to achieve the ideal situation, the gain of the feedback network should be as large as possible. The larger the gain, the smaller Z, and the better the compensation effect. However, in practical applications, from the perspective of system stability, in order to avoid oscillation, especially in high-frequency cases, the loop gain cannot be too large. Therefore, a compromise is often made between stability and gain. In the experiment, the feedback network was set to Rf=470kΩ and R4=10Ω.
(3) Selection of capacitors The output coupling capacitor C6 couples the output voltage into the circuit and also serves as an isolation between ACMF and the main circuit. Here, C6 is selected as a high-frequency capacitor with good high-frequency characteristics.
In the ACMF network, due to the "virtual short" between the inverting and non-inverting inputs of the operational amplifier during operation, and the series connection of the coupling capacitor Cps between C4 and the primary and secondary windings of the transformer, the common-mode voltage can be sampled through C4 and input to the inverting input of the operational amplifier. After passing through the operational amplifier feedback network composed of Rf, R4, and C4, as well as C6 and R5, a dynamic compensation current in the opposite direction can be output, thereby allowing the common-mode current to circulate within the ACMF network. This greatly reduces the common-mode current flowing to ground, achieving the purpose of attenuating or even eliminating the common-mode current.
Since the ACMF is connected between the primary and secondary windings of the transformer, and the Cy terminal it replaces needs to be grounded, the designed ACMF circuit must meet the isolation requirements of the transformer primary and secondary windings and the safety requirements for ground leakage current.
The above two points can be guaranteed in circuit design through the following measures.
(1) Withstand voltage test when the power supply is not working. At this time, the active filter does not work. When performing the withstand voltage test, the high-voltage high-frequency capacitor C6 is connected in series in the high-voltage pulse circuit, so it does not affect the withstand voltage requirement of the transformer. In the experimental circuit, C4 to C6 are all 560pF values and are safety capacitors that meet the safety requirements.
(2) The impedance Z expression between #2 and #3 in Figure 2a is shown in equation (3).
The frequency response curve of the AC impedance Z shown in Figure 3 can be plotted using equation (3). For ease of comparison, the impedance Z2 when only Cy=560pF is connected between #2 and #3 is given in the figure. Table 1 shows the impedance values of Z and Z2 at some frequencies. As can be seen from Figure 3 and Table 1, ACMF does not significantly reduce impedance below 400Hz and in the low-frequency range of power frequency; in the high-frequency range above 150kHz, ACMF exhibits very low impedance. Therefore, while providing an extremely low impedance path for high-frequency common-mode interference, ACMF does not increase the power frequency leakage current of the switching power supply to ground.
2.3 Experimental Results and Analysis
A flyback switching power supply with an output power of 60W (19.5V/3.5A) and a switching frequency of 58kHz was used as an example to verify the suppression effect of the designed ACMF circuit on common-mode interference. In the experiment, no passive common-mode filter was added to the prototype; only a differential-mode filter consisting of two differential-mode capacitors (0.47μF and 0.22μF) and a 12.74μH differential-mode inductor was added to filter out differential-mode noise, highlighting the filtering effect of the designed ACMF on common-mode noise. As shown in Figure 2 and the common-mode noise propagation channel, the voltage between points A and B is actually the voltage across the LISN common-mode noise sampling resistor. Therefore, the change in voltage between these two points can be measured with an oscilloscope to determine the attenuation effect of the ACMF on common-mode noise. Figure 4a shows the experimental waveform of the voltage uAB between points A and B measured with an oscilloscope when the prototype is equipped with ACMF and without ACMF, but with Cy=560pF. It can be seen that uAB is greatly reduced after using ACMF. This indicates that the designed ACMF has a significant suppression effect on common-mode noise.
Further conducted experiments were performed on the designed active EMI filter using an ER55C EMI receiver. Figure 5 shows the common-mode noise of the experimental switching power supply prototype as measured by the receiver. It is evident that the conducted experiments demonstrate that the designed ACMF effectively suppresses common-mode noise.
3. Conclusion
(1) Because active EMI filters use semiconductor devices, they have significant advantages over traditional passive filters in terms of size, weight and loss.
(2) Simulation and experimental results demonstrate that the proposed and designed active EMI filter operates stably and achieves good filtering performance within the specified conducted interference frequency range. It is highly compatible with the needs of integrated and high-density switching power supplies.
