Tom Hendrick, Jose Duenas TI Designs Precision: Verified Design ±15A Current Sensor using Closed-Loop Compensated Fluxgate Sensor Reference Design

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1 Tom Hendrick, Jose Duenas TI Designs Precision: Verified Design ±15A Current Sensor using Closed-Loop Compensated Fluxgate Sensor Reference Design TI Designs Precision TI Designs Precision are analog solutions created by TI s analog experts. Verified Designs offer the theory, component selection, simulation, complete PCB schematic & layout, bill of materials, and measured performance of useful circuits. Circuit modifications that help to meet alternate design goals are also discussed. Circuit Description This single-supply closed loop current transducer solution is designed to accurately measure dc, ac and pulsed currents. Featuring an ideal sensitivity of mv/a, this design allows galvanically isolated current measurements up to 55 A before reaching saturation. The nominal input range of the transducer is ±15 A. Design Resources Design Archive TINA-TI DRV421 All Design files SPICE Simulator Product Folder Ask The Analog Experts WEBENCH Design Center TI Designs Precision Library An IMPORTANT NOTICE at the end of this TI reference design addresses authorized use, intellectual property matters and other important disclaimers and information. TINA-TI is a trademark of Texas Instruments TIDUA92A-August 2015-Revised January 2016 TIPD196 ±15A Current Sensor Using Closed-Loop Compensated Fluxgate 1

2 1 Design Summary The design requirements are as follows: Supply Voltage: 5 V Nominal Input: ±15 A Measuring Range: ±55 A Ideal Sensitivity: mv/a Reference Voltage: 2.5 V Output Range: 0.22 V 4.78 V Maximum Shunt Voltage: 570 mv Bandwidth of interest: 200 khz The design goals and performance are summarized in Table 1. Figure 1 depicts the measured transfer function of the design. Table 1. Comparison of Design Goals, Calculations, and Measured Performance Goal Calculated Measured Sensitivity (mv/a) Total Unadjusted Error (%FSR) Calibrated Error (%FSR) ± 0.2% ±0.102% % % Figure 1: Measured Transfer Function 2 TIPD196 ±15A Current Sensor Using Closed-Loop Compensated Fluxgate TIDUA92A-August 2015-Revised January 2016

3 2 Theory of Operation Closed loop current transducers use a ferro-magnetic core with a magnetic sensor inserted into a gap in the core. The core picks up the magnetic field created by the current flowing through the primary winding, I PRIM. The magnetic field is measured by the magnetic sensor and passed on to a signal conditioning stage for filtering and amplification. The coil driver stage provides current to the compensation coil, I COMP, which creates an opposing magnetic field that cancels the effect of the primary current. The compensation current passes through a shunt resistor creating the differential voltage input for a precision sense amplifier. The amplifier gains the shunt voltage and drives the output stage of the transducer. The resulting voltage output, V OUT, is proportional to the current flowing through the primary winding as shown in the transfer function defined in Equation 1. Although Figure 2 shows the primary current passing only once through the magnetic core, the most general case is that of a core with a primary winding consisting of N P turns and a secondary winding consisting of N S turns. The N P and N S terms affect the module output as shown in Equation 1. Primary Winding I Comp R Sense Magnetic Core Compensation Coil Windings Signal Conditioning Coil Driver Sense Amplifier VOUT Field Probe I PRIM Figure 2: Closed Loop Sensor Block Diagram VOUT I N P N s Where the terms are: - VOUT: transducer output, - I PRIM : primary current to be measured, - N P : number of turns in the primary winding, - N S : number of turns in the compensation coil winding, - R SENSE : shunt resistor, and - G SA : gain of the sense amplifier. PRIM R SENSEGSA ( 1 ) 2.1 Sources of Output Error The physical construction of the circuit and the component tolerances will introduce error in the transfer function of the transducer. Several sources of errors are discussed in the following sections. TIDUA92A-August 2015-Revised January 2016 TIPD196 ±15A Current Sensor Using Closed-Loop Compensated Fluxgate 3

4 2.1.1 Fluxgate Sensor Offset Voltage The closed loop nature of these transducers compensates for any nonlinearities of the fluxgate sensor; however, the offset error of the fluxgate remains. Section 3.1 shows how the remaining fluxgate offset can be addressed with the DRV421 demagnetization feature Sense Amplifier and Internal Reference The amplifier offset voltage (V OS ) is a contributing source of offset error in closed loop sensors like the one depicted in Figure 2. The magnitude of offset error at the output equals the product of the amplifier gain and input-referred offset voltage along with the offset voltage from the fluxgate sensor. Many implementations use a difference amplifier to sense the voltage across R SENSE, in those cases, the exact voltage reference level does not play a role in the system error because the reference voltage is used for the reference input of the difference amplifier and the transducer output is measured as V out with respect to V REF Shunt Resistor The shunt or sense resistor will have a direct impact on the overall gain of the transducer. Using a sense resistor with tight tolerances and properly sized to reduce the effects of self-heating will directly impact the circuit performance. Moreover, resistors with low temperature coefficients are the best choice for this application in order to minimize the temperature drift caused by the shunt Magnetic Core The magnetic core introduces an error due primarily to: a) different coupling and magnetic gains between the primary and compensation currents, and b) cross coupling from the primary conductor directly to the sensor. 4 TIPD196 ±15A Current Sensor Using Closed-Loop Compensated Fluxgate TIDUA92A-August 2015-Revised January 2016

5 3 Component Selection 3.1 Integrated Fluxgate Sensor, Signal Conditioning, Coil Driver and Sense amplifier The DRV421 was chosen to minimize the component count and error sources. This IC has an integrated fluxgate sensor, a compensation coil driver and a precision differential amplifier for the output stage. The H-Bridge driver stage can provide up to 250 ma to the compensation coil and the differential amplifier stage provides a gain of 4 V/V to the shunt resistor. As mentioned in section 2.1.1, although the fluxgate sensor offset voltage will not be cancelled by the normal feedback loop operation, such error can be removed (along with the core magnetization) by using the demagnetization feature of the DRV421 (pin DEMAG). The demagnetization feature can be used either at startup or during operation, but care must be taken that the primary current is zero whenever the core is demagnetized. Please consult the DRV421 datasheet for further details on the demagnetization feature. Since the DRV421 contains the fluxgate sensor, signal conditioning, coil driver and sense amplifier all integrated in a single package, the entire system shown in Figure 2 is simplified to that shown in Figure 3. Figure 3: Closed Loop System Using DRV421 with Integrated Fluxgate Sensor 3.2 Magnetic Core Selection The sensor module has a compensation coil and a primary coil. For this design, the SC2912 module from Sumida was chosen. This module offers a magnetic core with magnetic gain of 500 μt/a, galvanic isolation tested up to 4.3 kvrms (50 Hz/60 Hz for 1 minute), and consists of a 966-turn compensation coil plus a set of four primary conductors. In this design, all four primary conductors are connected in parallel to yield a single turn for the primary winding (N p ) capable of carrying the maximum current from the design objectives. The resulting secondary current (I s ) is 57 ma as shown in Equation 2 for a primary current of 55 A (I p ). TIDUA92A-August 2015-Revised January 2016 TIPD196 ±15A Current Sensor Using Closed-Loop Compensated Fluxgate 5

6 I p N P (55A)1 I s 57mA N s 966 ( 2 ) 3.3 Shunt Resistor Selection A shunt resistor placed in series with the compensation winding current path is needed to provide a voltage to the output differential sense amplifier stage. Selection of an appropriate shunt resistor is dependent on the amount of current flowing through the secondary coil as well as the gain of the difference amplifier. The gain of the difference amplifier in the DRV421 is 4 V/V. With a design target of ±570 mv voltage across the shunt (V shunt ) for ±55 A of primary current, the value of R shunt is calculated in Equation 3. V shunt(max) 570mV shunt(max) 10Ω I s(max) 57mA R ( 3 ) Output Voltage The output voltage of the difference amplifier will be directly proportional to the primary current and it will be biased at the reference voltage (V REF ). With ±55 A of primary current, the output voltage (V out ) with respect to the reference voltage (V REF ) can be calculated using Equation 4. V out(max) V V shunt G Where G DA is the voltage gain of the difference amplifier. (570mV)4 2.28V REF DA ( 4 ) This design uses a 5V source and a fixed 2.5V reference so the total output voltage swing will be from 0.22 V to 4.78 V Module Sensitivity The sensitivity of the module is calculated using Equation 5. Sensitivity G G' R shunt G N S DA N P (10Ω)(4V / V)(1) ( 1) mV / A 966 ( 5 ) Where the variables are: - (G/G ): ratio of the magnetic gain of the primary winding to the magnetic gain of the secondary winding, - R shunt : value of shunt resistor, - G DA : voltage gain of the difference amplifier, - N P : number of turns in the primary winding, and - N S : number of turns in the compensation coil winding. The sensitivity calculated above corresponds to a core with ideal ratio of 1 between the magnetic gain of the primary winding to magnetic gain of the secondary winding. More information about the G/G ratio can be found in the application note SLOA224 - Design with the DRV421: Control Loop Stability. 6 TIPD196 ±15A Current Sensor Using Closed-Loop Compensated Fluxgate TIDUA92A-August 2015-Revised January 2016

7 3.3.3 Dummy Shunt Resistor As explained in section of the DRV421 datasheet, it may be necessary to add a dummy shunt resistor in series with the REFIN terminal of the DRV421. The value of this resistor needs to be four times that of the selected shunt resistor value. For this design, that yields a value of 40 ohm. The dummy shunt resistor in this design is designated by R4 in the circuit schematic. The PCB schematic can be found in the Appendix and more information about the function of the dummy shunt resistor can be found in the DRV421 datasheet. TIDUA92A-August 2015-Revised January 2016 TIPD196 ±15A Current Sensor Using Closed-Loop Compensated Fluxgate 7

8 4 Calculated Performance 4.1 Uncalibrated Error The major sources of error associated with the selected components are listed in Table 2. Table 2. Typical Error Contributions Specification DRV421 Fluxgate Sensor Offset (µt) DRV421 Differential Amplifier Offset 1 (µv) DRV421 Differential Amplifier Gain Error (%FS) DRV421 Differential Amplifier Linearity (ppm) Maximum Shunt Tolerance (%) Typical 1 ± These errors can be translated to equivalent primary current values as shown in Equations FluxgateError(T) 1m T FluxgateOf fset(a) 2mA ( 6 ) CoreMagneticGain(T/A) 500mT/A DiffAmpOffset(V) 10mV DiffAmpOff set(a) 0.24mA ( 7 ) ModuleSensitivity(V/A) mV / A GainError(%FS) 0.02 GainError( A) I (55A) 11mA ( 8 ) MAX Linearity(ppm) LinearityE rror(a) IMAX 0.66mA ( 9 ) 6 10 ShuntTolerance(%) ShuntToler anceerror(a) IMAX 55mA ( 10 ) Referred to the output. 2 The shunt resistor tolerance is actually a maximum (not typical) specification. 8 TIPD196 ±15A Current Sensor Using Closed-Loop Compensated Fluxgate TIDUA92A-August 2015-Revised January 2016

9 Table 3 lists the errors referred to the full-scale primary current of 55 A. Table 3. Typical Error Values Error Cause DRV421 Fluxgate Sensor Offset DRV421 Differential Amplifier Offset DRV421 Differential Amplifier Gain Error DRV421 Differential Amplifier Linearity Typical Error (ma) Shunt Tolerance 55 Once expressed in amperes of equivalent primary current, one can take the root of the sum of the squared terms (RSS) to obtain a probable estimate for the total output error of the system Error (ma) mA ( 11 ) The full-scale error is then calculated using Equation 12. Error(A) Error(%FS) % ( 12 ) I (A) 55 MAX Note that the expected error is dominated by the shunt tolerance which is a maximum specification provided by the shunt manufacturer. An additional source of error (not considered in these calculations) is due to the ratio of the magnetic gain of the primary winding to the magnetic gain of the secondary winding (referred to as G/G in Equation 5). Further characterization data on the variability of G/G for the core used in this design is needed to calculate the error component. Unfortunately such data was not available at the time of writing this document. However, this error as well as the offset errors from the fluxgate sensor and differential amplifier plus the shunt tolerance can be calibrated out. More information about the G/G ratio and its effect on the total system error can be found in the application note SLOA224 - Design with the DRV421: Control Loop Stability. 4.2 Noise Contributions Below are calculations for the noise contributions of the fluxgate sensor and the differential amplifier. NoiseDensity BW FluxgateNoise CoreMagneticGain FILTER 1.5nT/ Hz 1900Hz 500mT/A 0.13mA RMS ( 13 ) Where the NoiseDensity term corresponds to the fluxgate sensor only and the integration-to-flatband corner frequency is 1.9 khz. The DRV421 datasheet provides more details on the noise specifications and the integration-to-flatband corner frequency of the IC. TIDUA92A-August 2015-Revised January 2016 TIPD196 ±15A Current Sensor Using Closed-Loop Compensated Fluxgate 9

10 DA_ND BW 170nT/ Hz 200kHz DiffAmpNoi se 1.84mA ( 14 ) RMS Sensitivity mV/A DA_ND is the differential amplifier noise density. The sensor output bandwidth of interest for this design is 200 khz (denoted by BW). 4.3 Temperature Drift Measurements across temperature were not performed for this design. However, the DRV421 datasheet provides measurement results for the DRV421 used with other magnetic cores in closed-loop current transducers. Below are calculations pertaining to this design for the shunt drift, differential amplifier offset drift and differential amplifier gain drift when temperature increases from 27ºC to 85ºC. iffa o o ( iffampoffset rift)( T) 0.4mV/ C(85 27) C 0.56mA Sensitivity mV/A ( 15 ) Offset rif t o o o iffampgain rift(ppm/ C) ( T) 1ppm/ C(58 C) iffa Gain rift IMAX 55A 3.19mA ( 16 ) o o o TempCoeff(ppm/ C) ( T) 25ppm/ C(58 C) hunt IMAX 55A 79.8mA S rift ( 17 ) The DRV421 datasheet provides more details on the temperature drift specifications of the IC. 10 TIPD196 ±15A Current Sensor Using Closed-Loop Compensated Fluxgate TIDUA92A-August 2015-Revised January 2016

11 5 PCB Design The PCB schematic and bill of materials can be found in the Appendix. 5.1 PCB Layout The two-layer printed circuit board (PCB) used in this design measures 2.36 x 1.38 as shown in Figures 4A and 4B. The DRV421 and supporting circuitry occupies the top-copper layer. The bottom-copper layer contains a solid ground plane which provides a low-impedance path for return currents. There are two light-emitting diodes (LED) in the PCB to indicate an over-range condition (D3) and an error condition. The DRV421 datasheet provides more information on the over-range and error conditions. Figure 4A: PCB Top Layer TIDUA92A-August 2015-Revised January 2016 TIPD196 ±15A Current Sensor Using Closed-Loop Compensated Fluxgate 11

12 Figure 4B: PCB Bottom Layer 12 TIPD196 ±15A Current Sensor Using Closed-Loop Compensated Fluxgate TIDUA92A-August 2015-Revised January 2016

13 6 Verification & Measured Performance A 5V power supply provided power to the transducer circuit. The primary current was provided by a 100 A dc source whose output was swept in the range of 0 A to 55 A. Figure 5 shows the printed circuit board with the sensor and primary winding. Figure 5: Primary Winding, Magnetic Core and PCB 6.1 Transfer Function The measured dc transfer function is shown in Figure Measured Uncalibrated Full-Scale Error The full-scale error (%FSR) of the output is calculated using Equation 18. The uncalibrated full-scale range error is plotted over the 0 A to 55A range in Figure 6. Note that the worse error observed is %. The expected performance for negative currents is the same as that observed for positive currents. Full Scale Error (%FSR) I 100 PRIMARY VOUT VREF mv / A 55A ( 18 ) TIDUA92A-August 2015-Revised January 2016 TIPD196 ±15A Current Sensor Using Closed-Loop Compensated Fluxgate 13

14 Figure 6: Full-Scale Error vs. Input Current 6.3 Measured Sensitivity and Calibrated Full-Scale Error The sensitivity of the specific module used can be obtained using an end-point straight line fit to the measured data. The slope of such line is calculated in Equation 19. ( VOUT VREF ) 55A (VOUT VREF ) 0.1mA ( 25.45mV) Sensit 41.47mv / A (19) MEAS The calibrated full-scale range error is plotted over the 0 A to 55A range in Figure 7. Note that the worse calibrated error observed is %. 14 TIPD196 ±15A Current Sensor Using Closed-Loop Compensated Fluxgate TIDUA92A-August 2015-Revised January 2016

15 Figure 7: Calibrated Error vs. Input Current TIDUA92A-August 2015-Revised January 2016 TIPD196 ±15A Current Sensor Using Closed-Loop Compensated Fluxgate 15

16 7 Modifications The reference board includes two LEDs as indicators for over-range and error conditions. These components are not a critical need for the design and can be left off for cost savings. Larger currents can be measured with the DRV421 by selecting a magnetic core with different turn ratio on the compensation coil and primary coil. The current measurement range can also be modified by changing the value of the shunt resistor according to Equations 2, 3 and 4. For systems that require a tighter uncalibrated error, choosing a shunt resistor with 0.05% tolerance can lower the corresponding error as shown in Equation TIPD196 ±15A Current Sensor Using Closed-Loop Compensated Fluxgate TIDUA92A-August 2015-Revised January 2016

17 A.1 Electrical Schematic Figure A-1: Electrical Schematic TIDUA92A-August 2015-Revised January 2016 TIPD196 ±15A Current Sensor Using Closed-Loop Compensated Fluxgate 17

18 A.2 Bill of Materials Figure A-2: Bill of Materials 18 TIPD196 ±15A Current Sensor Using Closed-Loop Compensated Fluxgate TIDUA92A-August 2015-Revised January 2016

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