Current transformer with improved performance
The present disclosure describes techniques and designs that may be implemented for improving current transformer accuracy (e.g., while reducing additional complexity and costs). In some aspects, described embodiments may implement usage of a current mirror to reduce current transformer error. Additionally or alternatively, some described embodiments may implement Hall-effect sensors (e.g., in place of conventional first stages in a two-stage current transformer design). Such embodiments, as well as other described embodiments, may improve accuracy of current transformers in a more efficient manner (e.g., current transformer accuracy may be improved while reducing or minimizing additional complexity, costs, etc.).
1 . A current transformer comprising:
a transformer comprising a primary winding, a sense winding and a drive winding;
a first series combination comprising the sense winding coupled to a circuit ground;
a second series combination comprising the circuit ground, a current source coupled to the circuit ground, and an output of the current source coupled to said drive winding;
a Hall-effect sensor coupled to the current source and comprising a sensor input, a sensor output, and the circuit ground, wherein said sensor input is configured to sense current in said primary winding, whereby the current source is driven by the sensor output, thus generating the current source output; and
wherein the first series combination is coupled in parallel with the second series combination.
2 . The current transformer of claim 1 further comprising:
an operational amplifier comprising an inverting amplifier input, a non-inverting amplifier input, and an amplifier output, wherein one of the amplifier inputs is coupled to said circuit ground and the other amplifier input is coupled to the drive winding and to the sense winding such that the amplifier inputs are coupled in parallel with said first series combination and said second series combination; and
a burden resistor coupled between the other amplifier input and the amplifier output.
3 . The current transformer of claim 1 further comprising:
said transformer comprising another drive winding;
the first series combination further comprising a current mirror coupled at a current mirror input to the sense winding, wherein the current mirror is coupled to the circuit ground such that the current mirror input is interposed between the sense winding and the circuit ground; and
a current mirror output of the current mirror coupled in to the other drive winding.
4 . The current transformer of claim 3 further comprising:
an operational amplifier comprising an inverting amplifier input, a non-inverting amplifier input, and an amplifier output, wherein one of the amplifier inputs is coupled to said circuit ground and the other amplifier input is coupled to the drive winding and the sense winding such that the amplifier inputs are coupled in parallel with said first series combination, and said second series combination; and
a burden resistor coupled between the other amplifier input and the amplifier output.
5 . The current transformer of claim 1 further comprising:
the first series combination further comprising a current mirror coupled at a current mirror input to the sense winding wherein the current mirror is interposed between the sense winding and the circuit ground; and
a current mirror output of the current mirror coupled in parallel with said current source such that an output of the current mirror flows through the drive winding.
6 . The current transformer of claim 5 further comprising:
an operational amplifier comprising an inverting amplifier input, a non-inverting amplifier input, and an amplifier output, wherein one of the amplifier inputs is coupled to said circuit ground and the other amplifier input is coupled to the drive winding and the sense winding such that the amplifier inputs are coupled in parallel with said first series combination, and said second series combination; and
a burden resistor coupled between the other amplifier input and the amplifier output.
7 . The current transformer of claim 1 further comprising:
a transresistance amplifier comprising an inverting amplifier input, a non-inverting amplifier input, and an amplifier output, wherein one of the amplifier inputs is coupled to the circuit ground and the other amplifier input is coupled to the sense winding at the transresistance amplifier input; and
a summing node coupled to the transresistance amplifier output and interposed between the hall sensor and the current source and configured to add an output voltage of the Hall-effect sensor and an output voltage of the transresistance amplifier, wherein the current resulting from the adding of the voltages flows through the drive winding.
8 . The current transformer of claim 7 further comprising:
an operational amplifier comprising a second inverting amplifier input, a second non-inverting amplifier input, and a second amplifier output, wherein one of the second amplifier inputs is coupled to said circuit ground, and the other second amplifier input is coupled to the drive winding and the sense winding such that the second amplifier inputs are coupled in parallel with said first series combination, and said second series combination; and
a burden resistor coupled between said other second amplifier input and the second amplifier output.
9 . The current transformer of claim 7 further comprising:
a programmable gain device coupled to and interposed between the Hall-effect sensor and the current source wherein the programmable gain device is configured to control an output voltage gain of the Hall-effect sensor; and
a control system including a processor, wherein the transresistance amplifier further comprises a voltage output and wherein the control system is configured to receive the transresistance amplifier voltage output and adjust the programmable gain device to minimize the voltage at the output of the transresistance amplifier.
10 . The current transformer of claim 9 further, wherein said control system is a computing device selected from a group consisting of an analog computer, a digital signal processor, a microcontroller, and a similar computing device.
11 . The current transformer of claim 9 wherein said control system is equipped with an analog-to-digital converter to convert the output voltage of the transresistance amplifier to a digital signal, a control output coupled to said Hall-effect sensor and said programmable gain device, and a program memory.
12 . The current transformer of claim 9 wherein said programmable gain device is selected from a group consisting of an electronic potentiometer, a multiplying digital-to-analog converter, an analog multiplier, and a device providing a similar function.
13 . The current transformer of claim 1 , wherein the output voltage of the Hall-effect sensor is applied to the current source and the gain of the current source is adjusted to result in a current approximately equal to I IN /K, where I IN is the current in the K, primary winding and K=N S /N P , where N S is a number of sense winding turns and N P is a number of primary winding turns wound on a core of the current transformer, whereby a required current in the sense winding is reduced.
14 . The current transformer of claim 1 , wherein the current source is voltage-controlled.