Shielding-Cancelation Technique for Suppressing Common-Mode EMI in Isolated Power Converters

Abstract

In an isolated power converter, the interwinding capacitance of the transformer is the critical coupling path for the generating of common-mode (CM) noise. The shielding technique, for the purpose of blocking the electric coupling between the adjacent windings, has been widely used to deal with the CM noise. For the transformer with a single shielding layer, the displacement current that flows between the shielding layer and its adjacent windings still causes CM noise. In this paper, the shielding-cancelation (SC) technique is proposed by combining the shielding technique and the winding cancelation method together: The shielding layer is adjusted to have the same turns number and winding direction with its adjacent windings. In this way, the dv/dt on the interwinding capacitance is always zero; thus, the CM noise can be effectively suppressed. A flyback converter prototype is built in the laboratory, and the performance of the SC technique on suppressing the CM noise is verified by experiments.Index Terms-Common mode (CM) noise, isolated power converter, noise cancelation, shielding, transformer. 0278-0046

A General Transformer Evaluation Method for Common-Mode Noise Behavior

“…For CM noise, it will flow through L and N lines in the same direction to ground. The CM noise is harder to be suppressed compared to the DM noise because of its complex conduction paths influenced by stray inductive parameters and capacitive A series of CM noise eliminating techniques have been proposed in [4][5][6][7][8][9][10], and these techniques can be mainly categorized into two types: The first is to eliminate the CM noise flowing through the parasitic capacitance between the drain of the primary MOSFET and the ground. For this part of CM noise, if the heatsink is attached to the primary MOSFET, it will, via the parasitic capacitance between the heatsink and the ground to line impedance stabilization network (LISN) [11].…”

“…Adding double shielding copper foil between the adjacent primary and secondary windings can totally eliminate CM noise in the transformer path, but this technique is impractical since it will increase the manufacturing cost, winding loss, and complicate the manufacturing process [5]. The basic CM noise balance principle of [6][7] is to achieve zero voltage difference between the adjacent primary and secondary winding layers, which can make sure that the induced electric charge in the secondary side is equal to zero. Then there is no CM current flowing through transformer coupling path.…”

“…However, this model is too complex to achieve circuit analysis. The equivalent lumped capacitance is used to represent the CM noise behavior of the transformer [7,[13][14][15], which can keep the displacement current rule. Then, the CM noise behaviors of the isolated power converters such as flyback converter, forward converter, and LLC resonant converter are analyzed in [7,8] based on displacement current rule.…”

“…The equivalent lumped capacitance is used to represent the CM noise behavior of the transformer [7,[13][14][15], which can keep the displacement current rule. Then, the CM noise behaviors of the isolated power converters such as flyback converter, forward converter, and LLC resonant converter are analyzed in [7,8] based on displacement current rule. With the help of the previous efforts [7,8,[12][13][14][15][16], this paper will propose a general transformer evaluation method for CM noise behavior.…”

“…Then, the CM noise behaviors of the isolated power converters such as flyback converter, forward converter, and LLC resonant converter are analyzed in [7,8] based on displacement current rule. With the help of the previous efforts [7,8,[12][13][14][15][16], this paper will propose a general transformer evaluation method for CM noise behavior. Only a signal generator and an oscilloscope can effectively evaluate the CM noise behavior of the transformer in the low-frequency range, which is easy to be implemented without the expensive cost of the professional CE EMI testing instruments.…”