Closed-loop current transducer with switched mode amplifier

1. A closed-loop current transducer system for measuring a current (Ip) flowing in a primary conductor, the system powered by series connected voltage supplies and comprising a magnetic field detector, a signal processing circuit including an amplifier circuit connected to the magnetic field detector, and a compensation coil connected to the amplifier circuit and configured to generate an opposing magnetic field seeking to cancel a magnetic field generated by the current flowing in the primary conductor, characterised in that the amplifier circuit comprises a first switched mode power stage fed by the voltage supplies and configured to drive the compensation coil and supply a compensation current to a measurement resistor connected directly to a first reference voltage (Vref), where the voltage across the measurement resistor provides non-differential measurement of the current flowing in the primary conductor, and at least a second independently regulated switched mode power stage fed by the voltage supplies with an output connected to a second reference voltage through an Inductor and configured to output a balancing current, whereby the first and second reference voltages are the same or different.

2. Current transducer system according to claim 1 , wherein the second switched mode power stage is configured to output the balancing current of an amplitude between 0.1 and 1.5 times the compensation current output, but of opposite sign.

3. Current transducer system according to claim 2 , wherein the second switched mode power stage is configured to output the balancing current (Ibal) of an amplitude between 0.7 and 1.2 times the compensation current output, but of opposite sign.

4. Current transducer system according to claim 1 , wherein the second switched mode power stage is configured to output the balancing current (Ibal) which flows into the reference voltage (Vref, Vref 2 ).

5. Current transducer system according to claim 1 , wherein the first switched mode power stage comprises a modulator, one or more drivers, and a single half bridge circuit.

6. Current transducer system according to claim 1 , wherein the second switched mode power stage comprises a modulator, one or more drivers, and a single half bridge circuit, where the modulator and drivers are identical or essentially identical for both first and second power stages.

7. Current transducer system according to claim 1 , wherein the magnetic field detector includes a fluxgate inductor with an output processed and connected to a modulator of the first switched mode power stage.

8. Current transducer system according to claim 1 , wherein the magnetic field detector comprises a first fluxgate inductor and a second fluxgate inductor, the first fluxgate output being connected to the first switched mode power stage via the signal processing circuit.

9. Current transducer system according to claim 8 , wherein the first fluxgate inductor is driven by a self-oscillating circuit and configured to generate a signal used to determine a compensation current driving the compensation coil.

10. Current transducer system according to claim 1 , wherein the second switched mode power stage is connected to the first switched mode power stage via a summing circuit comprising resistors, differential amplifiers and a regulation circuit to regulate the current in the second switched mode power stage.

11. Current transducer system according to claim 1 , wherein the amplifier circuit comprises a differential current transducer configured to sense a difference between supply currents (I_L, I_H) of the voltage supplies and connected to a regulation circuit that regulates the balancing current Ibal of the second switched power stage.

12. Current transducer system according to claim 10 or 11 wherein the regulation circuit comprises a PI regulator.

13. Current transducer system according to claim 1 , wherein the amplifier circuit comprises a voltage sensing circuit configured to measure a reverse voltage across diodes on the voltage supplies, and a regulator configured to regulate the balancing current (Ibal) to decrease a current loading of capacitors on the voltage supplies.

14. Current transducer system according to claim 13 wherein the regulation circuit comprises a regulator and differential amplifiers connected to the regulator configured to measure the reverse voltages on the diodes, the regulation circuit being configured to regulate said reverse within a defined band.

15. Current transducer system according to claim 14 wherein the defined band is between 0 and 5 volts.