Effect of duty cycle on common mode conducted noise of DC-DC converters

Along with the increasing price of copper material, there is more demanding on cost price of switching power supplies due to those weighty inductors such as common-mode (CM) chokes. The total capacitance of CM filter is usually limited by safety standard. As a result, large bulky CM inductor is usually required. In some applications, such as silver-box power supply, wherein the CM noise is not mainly from PFC part, the dc-dc will dominant the CM noise level. For the purpose of reducing CM filter, bare CM noise of typical dc-dc converters was investigated and the effect of duty cycle on EMI noise is discussed in detail. Presuming the corner frequency of EMI filter and its size as well is determined by the low-frequency bare noise, by optimizing the noise spectrum in the most sensitive frequency range and creating a valley in the vicinity of 150 kHz for the noise spectrum, the frequency of critical noise spectrum to be attenuated can be increased and so does the corner frequency of CM filter. Experiments are performed to verify the benefits in a typical power supply design.