144 PRZEGLĄD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 91 NR 10/2015 Abdolreza RAHMATI 1 , Adib ABRISHAMIFAR 1 , Atila SKANDARNEZHAD 1 Faculty of Electrical Engineering, Iran University of Science and Technology (IUST), Tehran, Iran (1) doi:10.15199/48.2015.10.29 A New Technique to Suppress the Common Mode Conduction Noise of the Boost Converter Abstract. EMI filter is an essential part of the modern switching power supplies contributing to significant price and size. This paper presents a new method to reduce the common mode (CM) electromagnetic interference (EMI) of the boost converter. Conventional methods rely on impedance mismatch or active noise cancellation, but the proposed method uses a passive branch with matched impedance to suppress the CM current. An external inductor is connected between the transistor drain and middle of input dc bus before EMI filter. A Wheatstone bridge is then identified in the CM equivalent circuit of the converter with these additional components. By selecting the inductor value properly the Wheatstone bridge will be balanced and results in suppression of the input CM current. The analysis shows that the presented method can guarantee the required attenuation for CM conduction noise. Finally, a comparison between the balanced and unbalanced structure is presented using computer simulation by Pspice. Streszczenie. W artykule zaprezentowano nową metode redukcji składowej wspólnej elektromagnetycznych interferencji w przekształtniku typu boost. W metodzie zastosowano pasywny układ do dopasowania impedancji w celu zmniejszenia składowej wspólnej prądu. Wstawiony w układ mostka dławik umożliwia zrównoważenie tego mostka. Nowa technika zmniejszania składowej wspólnej szumu w przekształtniku typu boost. Keywords: Electromagnetic interference (EMI), common mode (CM), parasitic capacitor, Impedance balancing. Słowa kluczowe: interferencje elektromagnetyczne, składowa wspólna, kompresja szumu, przekształtnik boost Introduction The size of energy storage elements in switching power supplies decreases along with the increase of switching frequency. Therefore, high density power supplies generally need to high switching frequency and fast semiconductor devices. Increased switching frequency with high voltage and current change rates (dv/dt, di/dt) has injurious effects on the electromagnetic compatibility (EMC) issue of the switching converters. Thus, it is necessary to use the EMI filter to limit the high frequency components of converter input current below a permitted level which is defined by international standards [1], [2]. Since the system engineers do not know the details and exact nature of the switching converter then the design of EMI filters is not a trivial work. Also, interconnection among the EMI source impedance and EMI filters impedance may cause to poor noise attenuation. Existing EMI filter design methods usually ignore the noise impedance of the switching power supplies (SPS), then trail and error attempt is often used in filter matching design [3], [4]. Passive EMI filtering method decreases the conduction noise level by employing inductors and capacitors to create impedance mismatch in the current pass. On the other hand active filtering method needs to active devices or changes the switching algorithm to reduce the noise power over the defined frequency domain. Conducted EMI noise has two parts: differential mode (DM) and common mode (CM) part. CM component behaviour is usually more complex to be modelled rather than DM type because it involves chassis ground path which is not completely obvious [5]. On the contrary, the DM part has not any path to chassis ground and almost its amplitude is little, then, this paper focuses only on the CM noise filtering subject. A measurement circuit named line impedance stabilization network (LISN) is used and inserted between the converter and input source to provide specific measuring impedance for conducted noise amplitude and to isolate them at radio frequency from each other. The LISN setup used here is a 50uH/ 50Ω type. This paper introduces a passive noise suppression way to establish the balanced Wheatstone bridge using only a low size inductor. The LISN gauge is placed in middle of the bridge branches, then, noise current doesn’t flow through it if be balanced. Finally, simulation is done using Pspice. Boost converter and its parasitic capacitor Boost converter is a well-known and high-aptitude circuit especially for power factor correction (PFC) applications. Figure 1, shows a simple non-isolated boost converter. High dv/dt Rout 50-ohm Va Vin 12V Vout L 1mH Vp C 47uF Vb D BAL74 Q IRF130 . Fig.1. Boost converter circuit As seen, transistor drain node has greater voltage pulsation (high dv/dt) rather than other nodes. Figure 2, displays nodes voltage of V d , (V a -V d ) and (V b -V d ). The last two mentioned voltages are in-phase and have the same pulsation amplitude. Both voltages define the CM noise current contents which flows via a parasitic capacitor rests between the transistor case and heatsink body, then this current returns back through ‘a’ and ‘b’ nodes. In order to reduce the junction temperature, the transistor is usually mounted on a heatsink which is firmly connected to chassis. Fig.2. Nodes voltage of the boost voltage shown in Fig.1