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 config uration 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 vol tage 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 comm on-mode rejection current transformer, without the need for a Hall-effect p robe. This is achieved by subtracting the high-frequency components, detect ed across the secondary shunt resistor, from the voltage across a primary s hunt resistor connected in series with the primary of the current transform er. The resulting signal is an accurate image of the transformer magnetizin g current, which is then transferred to the secondary side by means of a lo w-bandwidth isolation amplifier. The high-frequency components are subseque ntly added, to the amplified and filtered low-frequency components, by mean s of a third transformer winding, the number of turns of which is chosen to be equal to the gain of the low-frequency amplifier.