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Research and Design of a Common Mode Hybrid EMI Filter For Switch-mode Power Supply Wang Ping Tao Chenbin Zhang Jinghai School of Electrical Engineering and Automation, Tianjin University, China E-mail: pingw@tju.edu.cn Abstract–After the study of electromagnetic interference (EMI) mechanisms in switching mode power system (SMPS) Z1 and the soft switch, this paper proposes a novel hybrid filter (HEF) to suppress commend mode EMI in SMPS. This paper reviews the limits of pure passive and active EMI filter and discusses the advantages of the hybrid EMI filter (HEF). By Z2 A studying the insertion loss, impedance compatibilities, and reliability of the hybrid EMI filter, a new HFF topology is proposed in this paper. And an example of HEF for flyback Fig.1: the HEF topology model converter is designed. And the topology has been simulated in Saber. The simulation result verifies its advantages in better 1.Passive Filter Design attenuation, lower cost, and higher efficiency. The equivalent circuit with a LC filter is shown in Fig. 2. Keywords–CM interference, hybrid active/passive filter, Zg L matching network, switch mode power system. I. INTRODUCTION Ug C ZL U2 Today, as nonlinear devices were widely used, the SMPS becomes a large interference source [1]. These fast switching devices generate high-voltage dv/dt and high commend-mode voltages, causing some serious problems Fig. 2: Equivalent circuit with a LC filter such as premature winding failures, ground leakage currents, shaft voltages and bearing currents, and In Fig.2, ZL is the equivalent impedance of load resistance conducted or radiated EMI. RL. Ug is noise source, and Zg is noise impedance. The natural frequency of the LC filter is To suppress EMI, conventionally passive filters have been widely used. However, passive filters have some 1 0 (1) disadvantages which limit SMPS to be smaller, such as LC large size, fixed compensation, and subtle impedance calculation [2]. Recently, some active filter topologies are then the insertion loss of the LC filter is developed, most of which are not used in large current converters yet[3]. An alternative to above two filters is to P 2 ( L)2 . use a hybrid EMI filter. ILR 10lg( 1 ) (1 2 ) 2 (2) P2 0 RL2 In this paper, mating network is used to LC passive filter to ignoring the effects of impedance mismatching. And in In different SMPS, the circuit structure, high impedance of active part, a feedforward filter is adopted. Its the elements, semiconductors used and working frequency characteristics are analyzed and the current sensor is are different. Therefore, the source impedance Zg is discussed. Simulation and experiment result are presented, different and hard detected. And the load impedance ZL is demonstrating that good noise attenuation of this HEF. also different when SMPS connected different part of the system, and it is also changing in a large scale. So Zg and II. HEF DESIGN ZL are not certain, and impossible to meet impedance mating condition ZL=Zg, which means the EMI filter The HEF topology in this paper is shown in Fig.1 [4]. The cannot works at its best status. So it is difficult to design noise current is first attenuated by the passive filter. Then an appropriate EMI filter suitable for most SMPS. the noise current volume must be controlled in a range that the active filter can accept. In this paper, a LC filter is In this paper, mating network was adopted to design chosen as passive filter, and a feedforword filer is chosen passive filter part. And the matching network is formed by as an active filter. The Z1 is high impedance to noise, Z2 is some matching devices such as inductors, capacitors and low impedance to noise, A is active filter. resistors in input and output of the passive filter. The matching network could cancel the bad effects because of ZL≠Zg. The matching network used in this paper is shown in Fig.3. the insertion loss is (1 p )(1 s ) ILs K K 0 (9) p s Fig.3: mating network To realize that IL is nearly 20dB in passive part, after a In Fig.3, Cp and Rp formed ‘P’ network, and Cs and Rs series of calculation, the topology is shown in Fig.4. formed ‘S’ network. The capacitive reactance of Cp is much larger than Rp at working frequency, while its capacitive reactance is much smaller than Rp above cutoff frequency fc. And at work frequency, impedance loss of Rs is decreased by low impedance choke Ls, for the impedance Ls is much smaller, shorting Rs. And above cutoff fc, the impedance is much higher than Rs, which prevents resonance and decrease impedance mismatching. Fig.4: Passive filter with matching network To simplified calculation, voltage attenuation factor K is used to calculate insertion loss IL. And K0 is defined as voltage attenuation factor when no matching network 2. Active Filter Design exists. Then, the voltage attenuation factor K is 2.1.Current Sensor The measurement of EMI noise currents makes necessary Ug Ug U2 K K0 (3) the use of current transformer (CT) with a wide frequency Um U1 Um bandwidth and without distortion. An elementary structure based on a toroidal current transformer, a winding, and a Then the matching effects of P network and S network are resistor load is used in this paper. The equivalent circuit is defined as shown in Fig. 5. 1:n K min | U g p U1 min (4) Lμ R K c | f fc K min | U 2 s Um Fig. 5: Equivalent circuit of CT min (5) K c | f fc Since the frequency of the EMI noise is as high as Then constrains of Rp and Rs are 30MHz,it is necessary to discuss its high frequency performance. Fig.6 shows the high frequency equivalent circuit of current transformer [5]. R p X 1o (L) p 1 2 (6) X 2 s (L) Rs s2 1 X10(ωL) presents the open-circuit impedance at f=f1 viewed from input, and X2S(ωL)presents short-circuit impedance at f=f1 viewed from output. P network is connected at input port of LC filter, and then Fig.6: High frequency equivalent circuit of CT the equivalent source impedance Zpg is formed by P network impedance Zp in parallel with Zg. Ll1 and Ll2 are the primary and secondary leakage Simultaneously, the equivalent load impedance Zsm equals inductance, C1 and C2 are the magnetizing transformer the impedance that S network impedance in series with Zs. inductance, and Cps1 and Cps2 are the primary and Furthermore, Zpg and Zsm changes with Zg, Zm and secondary inter winding capacitance, Lu is excitation frequency. Here, the effect on input impedance by inductance of CT. To analysis the circuit, the simplified impedance matching is presented by ε, which is the ratio model of CT is shown in Fig.7. of input impedance viewed to source and load by filter with and without the matching network. Then Z pg Z g // Z p 2 p Z1i min (7) Z 2i m in Z sm Z m Z s (8) 2 s Consider the worst condition, ε is chosen to be 0.1. Then Fig.7: Simplified model of CT Then the transfer function is III .SIMULATION EXPERIMENTAL RESULTS I2 1 (10) The output filter was used in a full bridge DC/DC I1 s 3C1C2' R ' ( L1 L'2 ) s 2C1 ( L1 L'2 ) sR ' (C1 C2' ) 1 converter. And the filter is used to attenuate common-mode noise. The simulating circuit is shown in To guarantee small phase shift, it is necessary to make L u Fig.10. Fig.11 shows the noise currents with and without large. Under certain n and R condition, to prevent active filter. i-g is noise current without active filter, i-c is magnetic core saturating, the volume of core is the active filter injection current, and i-q is the noise current after compensation. Obviously, the active part c L I p 2 produces compensate current that is same size of the noise Vc 2 (11) current, but contrary phase. Bsat μc is the permeability of the core, Ip is the peak value of induced current, Bsat is the largest allowable flux density. The current sensor and amplifier circuit is shown in Fig.8. Fig.11: noise currents with and without active filter The effectiveness of the HEF is shown in Fig. 12. From Fig.8: The current sensor and amplifier circuit the results, the HEF performs good attenuation. The enlarging result is shown in Fig 13. After HEF the noise 2.2. current injection circuit current is attenuated 40dB, and its voltage peaked is The current injection circuit is shown in Fig.9. whittled. Fig.12: Effectiveness of HEF Fig.9: The current injection circuit The Op amp used in this paper is LM6361, and C3 prevents direct current injecting to main circuit, and R4 is used to reduce the dissipation of BJT. 3. Novel HEF topology The novel topology of HEF is shown in Fig.10. The Fig.13: Enlarged Fig.12 nulling method we choose here is feedforward, of which the input ig is noisy and the output iq is quiet. Fig.10: The topology of HEF Fig.14: high frequency performance comparison between HEF and PEF Another passive EMI filter (PEF) with the same filtering result was made in this paper. It also designed with matching network. The devices shown in Fig.4 are Rp=50Ω ， Cp=25nF ， L=1mH ， C=250nF ， Rs=91Ω ， Ls=100μH. Fig.14 shows the frequency spectrum comparison between hybrid filter and passive filter at high frequency. By introducing passive devices to active filter, the high frequency performance of HEF is much improved, which is only a little worse than passive filter. However the size of the HEF is only 20% of PEF. So the HEF has much advantage in realization. IV. CONCLUSION To cancel the common-mode voltage generated by the PWM inverter in the drive system, a novel structure of active output EMI filter is proposed by combining it with a passive filter to become a hybrid filter. The matching network was added to traditional LC passive filter, which is to ignoring input and output impedance of EMI filter. the results show good attenuation to common-mode noise and relatively smaller in size. The HEF has a good future in realization. REFERENCES [1] Pairodamonchai, P., Suwankawin, S., Sangwongwanich, S., “Design and Implementation of A Hybrid Output EMI Filter for High Frequency Common-Mode Voltage Compensation in PWM Inverters”, Power Conversion Conference - Nagoya, 2007. PCC '07 2-5 April 2007, pp.1484 – 1491. [2] Wenjie Chen, Xu Yang, Zhaoan Wang, “An Active EMI Filtering Technique for Improving Passive Filter Low-Frequency Performance”, IEEE Transactions on Electromagnetic Compatibility, 2006,Vol.48,No.1,pp.172-177. [3] Mingjuan Zhu, Perreault, D.J., Caliskan, V.,Neugebauer, T.C., Guttowski, S., Kassakian, J.G., “Design and evaluation of Feedforward Active ripple filters,” Power Electronics, IEEE Transactions on Vol. 20, No.2, Mar 2005 ,pp.276 – 285. [4] Wenjie Chen, Xu Yang, Zhaoan Wang, “Systematic evaluation of hybrid active EMI filter based on equivalent circuit model”, Power Electronics Specialists Conference, 2006,18-22 June 2006, pp.1 – 7. [5] Chen Wenjie, Yang Xu, Wang Zhaoan, “A study on design of an active emi filter for integrated power electronics modules”, Proceedings of the CSEE, Vol. 25, No. 24, 2005,pp.51-55.

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