Active current transformer circuits for low distortion sensing in switched mode power converters

The current transformer (CT) is frequently used for sensing applications in switched mode power converters. Advantages are that galvanic isolation is inherently incorporated, bandwidth is high, losses are low and that a high-amplitude output signal may be derived. Because of this combination of advantages it may be preferred to other current sensing technologies such as the Hall-effect sensor or the sense resistance. However, it exhibits some limitations. A compromise exists between the amplitude of the output signal and the distortion present in it due to droop. Droop results as some of the input current under measurement diverts away from the "ideal" transformer within the CT's equivalent circuit and into its magnetizing branch. In addition, where the CT is used for sensing unidirectional current pulses, the duty cycle of the pulses has to be restricted if saturation of its core material is to be avoided. This paper describes techniques based on the incorporation of an active load and synchronous rectification for reducing the distortion due to droop and allowing operation at extended duty cycles. Experimental results are given for a dual transformer arrangement used to sense the choke current drawn by a boost converter circuit.