Techniques for input ripple current cancellation: classification and implementation

Switching power supplies, due to their high-frequency switching operation, draw a substantial amount of input ripple current. Such ripple current manifest itself as electromagnetic interference (EMI). Active and passive ripple cancellation methods have been proposed in the past. This paper discusses the cancellation methods, and classifies the basic circuit configurations for ripple current cancellation. The basis of discussion is a 'nullification' process, which can be described effectively in terms of the ideal circuit element nullor. The initial focus is on differential-mode noise cancellation, but the basic technique can be applied equally to cancelling common-mode noise. The classification provides a clear guideline for synthesizing practical cancellation circuits. Both active and passive implementation examples are given and experimentally demonstrated.