Single Stage EMI Filter for Server Power Supply

Adopting Gallium Nitride (GaN) device in server power supply, switching frequency can be increased up to the MHz range, with dramatic improvement in power density and not at the detriment of efficiency. Furthermore, the complicated magnetic structure can be broken down into elemental pieces and further integrated into a matrix transformer structure with PCB windings. With PCB based winding structure, opportunities for EMI common-mode noise reduction, previously inconceivable, can be realized, leading to a much improved CM noise signature in both PFC and DC/DC converter. Consequently, a simple single stage filter can be employed to meet the EMI requirement, for the first time. The single stage filter with 60dB/dec attenuation of DM noises and 40dB/dec attenuation of CM noises are deemed sufficient to meet the requirement. However, the presence of self-parasitics such as ESL of capacitors and EPC of inductors as well as near- field mutual couplings between inductor and capacitors are detrimental and renders the filter ineffective in frequency beyond a few hundred kHz. In this paper, balance technique for CM noise reduction of PFC and shielding technique for DC/DC converter are implemented in PCB-winding based magnetics. To further enhance the filter performance at high frequency, an exhaustive finite element analysis and near field measurements are conducted to better understand the effect of frequency to the polarization of near field due to the displacement current. The knowledge gain in this study enable one to minimize unwanted mutual coupling effects by means of physical placement of these filter components. Moreover, the unwanted ESL of DM filter cap can be reduced using the proposed X-cap structure. The mutual coupling between capacitors is also reduced by a flux cancellation loop. With these approaches applied, a single stage filter is demonstrated in an off-line 1 kW, 12V server power supply.