Single Stage EMI Filter for Server Power Supply

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Adopting Gallium Nitride (GaN) device in serverpower supply, switching frequency can be increased up to theMHz range, with dramatic improvement in power density andnot at the detriment of efficiency. Furthermore, the complicatedmagnetic structure can be broken down into elemental pieces andfurther integrated into a matrix transformer structure with PCBwindings. With PCB based winding structure, opportunities forEMI common-mode noise reduction, previously inconceivable,can be realized, leading to a much improved CM noise signaturein both PFC and DC/DC converter. Consequently, a simple singlestage filter can be employed to meet the EMI requirement, forthe first time. The single stage filter with 60dB/dec attenuation ofDM noises and 40dB/dec attenuation of CM noises are deemedsufficient to meet the requirement. However, the presence of selfparasiticssuch as ESL of capacitors and EPC of inductors as wellas near- field mutual couplings between inductor and capacitorsare detrimental and renders the filter ineffective in frequencybeyond a few hundred kHz. In this paper, balance technique forCM noise reduction of PFC and shielding technique for DC/DCconverter are implemented in PCB-winding based magnetics. Tofurther enhance the filter performance at high frequency, anexhaustive finite element analysis and near field measurementsare conducted to better understand the effect of frequency to thepolarization of near field due to the displacement current. Theknowledge gain in this study enable one to minimize unwantedmutual coupling effects by means of physical placement of thesefilter components. Moreover, the unwanted ESL of DM filter capcan be reduced using the proposed X-cap structure. The mutualcoupling between capacitors is also reduced by a flux cancellationloop. With these approaches applied, a single stage filter isdemonstrated in an off-line 1 kW, 12V server power supply.

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