A DC current transformer for large bandwidth and high common-mode rejection

At first, a review of known magnetic-coupled current-sensing techniques is presented. Subsequently, a novel technique is introduced, based on a configuration discussed in a previous paper. The previous technique made use of a galvanomagnetic device (Hall-effect sensor) to sense the magnetization of a current transformer core, so that the sum of the Hall voltage and the voltage across the secondary shunt resistor would yield a faithful copy of the input current. The technique described in this paper makes use of the same principle to obtain a high bandwidth (from de to 1 MHz) and very high common-mode rejection current transformer, without the need for a Hall-effect probe. This is achieved by subtracting the high-frequency components, detected across the secondary shunt resistor, from the voltage across a primary shunt resistor connected in series with the primary of the current transformer. The resulting signal is an accurate image of the transformer magnetizing current, which is then transferred to the secondary side by means of a low-bandwidth isolation amplifier. The high-frequency components are subsequently added, to the amplified and filtered low-frequency components, by means of a third transformer winding, the number of turns of which is chosen to be equal to the gain of the low-frequency amplifier.