How to avoid filters in off-line power supplies

In AC-DC switched mode power supplies high frequency filter components are necessary to bring the conducted emissions within the limits of the specifications VDE 0871/5. As these power line filters considerably contribute to total size, weight and cost of the power supply, any improvement is highly desirable. In this paper a new operation mode is presented, which offers the possibility to reduce the filter in the power range below 100 W or even to completely avoid the additional filter components as demonstrated with a 10 W supply for a rechargeable shaver. This new method is based on the effect, that the conducted emissions are transferred to the mains impedance only during the recharging time of the storage capacitor, this means when the rectifier bridge is conducting. During the nonconducting phase, the diodes are nearly perfect filter components. Utilizing this different behaviour gives a tremendous benefit for the size and cost of the power supplies. Possible applications are in audio/video, household equipment or in the telecommunication area