By: Jim Patterson, Senior Field Applications Engineer; Ted Stokes, Director of Applications Engineering;
Lj Ristic, VP and GM, Sensor Solutions Business Unit
As electronic, microcontroller based, electricity meters have replaced electromechanical meters, methods for measuring electrical current have evolved to allow accurate calculation of power and energy consumption. Measuring current requires a sensor or transducer to generate a voltage which is proportional to the current flowing from the AC mains into the load.
Traditional current sensors include shunt resistors, current transformers, and Rogowski coils.
Shunt resistors are inexpensive and highly linear, but because the current to be measured must flow through the shunt, their use is limited to single-phase applications, unless additional hardware is provided to isolate the metering chip from the electrical mains. Since the output is defined by Ohm’s Law,
Vout= I x Rshunt, the signal is limited by the resistance selected. Increasing the shunt resistance will increase the signal level presented to the metering chip, but that will increase power dissipation in the shunt, which may lead to heating problems at high currents.
Current transformers (CTs) provide isolation from the current carrying conductor, and their output voltage for a given current can readily be scaled by changing the burden resistor, but CTs tend to be bulky, relatively expensive, and may be subject to core saturation due to large AC currents, DC currents, or externally applied magnetic fields.
Rogowski coils also provide isolation from the current carrying conductor. Because they have no magnetic core material, they are immune from core saturation, but may be influenced by external AC magnetic fields. The Rogowski coil output is proportional to the differential of current (Vout=k di/dt) thus high frequency components in the signal, such as impulse noise and high harmonics, are amplified and may provide output levels that are outside of the range of the analog to digital converter (ADC) in an electric meter analog front-end (AFE) or system on chip (SOC). Crocus tunneling magnetoresistance (TMR) sensors can be used to implement contactless current sensing with scalable output levels.
TMR Sensor for Smart Metering
The Crocus magnetic sensor comprises a TMR junction whose resistance (ROUT) changes in response to an applied magnetic field, and a field line which can be driven to control the resistance with a small electrical current. The field line,(RIN), represents the input of the sensor and has a nominal resistance of 30Ω.
An applied magnetic field makes ROUT increase or decrease depending upon the magnetic field orientation.
Current in the input, or RIN, is used to bias ROUT near the middle of the transition region, where the response is most nearly linear. With static bias, the linear range is not large enough to provide linear current measurement over the 1000:1 or 2000:1 required for electricity metering applications. To overcome this limitation Crocus has developed a closed loop circuit which controls the field line current and maintains the sensor bias near the midpoint of the transition region.
Closed Loop Solution
Figure 1 illustrates the closed loop sensor circuit. Two sensors are connected in a half-bridge configuration between the positive and negative supply rails. A constant voltage bias provides field line current to which sets ROUT. The resistors put the bridge output at the midpoint of the supply rails. The bridge output is buffered by a voltage follower so that the bridge resistance does not affect the feedback circuit.
The sensors are oriented with opposite polarities, so that an applied magnetic field will increase the resistance of one sensor and will decrease the resistance of the other, moving the bridge output from the midpoint toward one of the supply rails. The buffered bridge output voltage drives two feedback amplifiers, which have equal but opposite gains. The feedback amplifiers drive the end of the field lines that are not connected to the bias supply, increasing or decreasing the field line current until the magnetic field due to the field line current cancels the effect of externally applied magnetic field. Thus the current in the field line is proportional to the current in the external conductor. Sense resistors in series with the field lines are used to sense the current in the field lines, eliminating thermal effects on the field line resistance.
The output voltage VOUT is the differential voltage across the sense resistors amplified by the output differential amplifier, and can be scaled to match the metering chip’s ADC input voltage range. The output voltage reference can be the meter chip’s positive supply voltage, ground or supply midpoint. The board does not make electrical contact with the current carrying conductor, but is attached so that the conductor is centered above both magnetic sensors in parallel with their axis of alignment.
The gain of the sensor board in volts/amp is determined by two factors:
1. The distance from the center of the current carrying conductor to the sensors
2. The gain (VOUT/VIN) of the differential output amplifier
Decreasing the sensor to conductor distance will increase the low-current sensitivity and increase the output voltage for the same current. The gain of the differential output amplifier may be modified to provide either amplification or attenuation so that the appropriate voltage is presented to the metering chip being used.