A fluxgate current sensor circuit

Abstract

The application provides a kind of fluxgate current sensor circuit, including excitation circuit, excitation winding L1, compensation winding L2, error amplifier circuit, low-pass filter circuit and sampling circuit, excitation winding L1 is connected between two bridge arms of excitation circuit for detecting excitation current, excitation current is fed back to compensation winding L2 after error amplifier circuit, low-pass filter circuit and sampling circuit in turn, by measuring the current flowing through compensation winding L2, the value of the measured current can be known. The application adopts a more optimal excitation self-oscillation scheme for detecting small current, with higher detection accuracy.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of isolated sampling, in particular to the field of fluxgate current sensors. BACKGROUND

[0002] It is known that ordinary current transformers can only measure alternating current, and cannot measure direct current. To measure direct current, Hall current sensors and fluxgate current sensors are generally used. The magnetic ring of a Hall sensor needs to be opened to accommodate the Hall device, and the magnetic permeability of the magnetic ring is relatively low, so it can only measure currents above an ampere. The magnetic ring of a fluxgate current sensor is closed, and the magnetic permeability is relatively high, so it is relatively sensitive and can measure currents in the milliampere range.

[0003] A fluxgate current sensor uses the principle of magnetic modulation to make the magnetic ring in a symmetrical alternating saturation state. When current passes through the magnetic ring, it generates magnetic flux in the magnetic ring, making the alternating saturation state of the magnetic ring no longer symmetrical. The current signal is measured by detecting and demodulating this asymmetrical state. Saturating the magnetic core requires a certain amount of current, typically between a few milliamps and a few tens of milliamps. The key to symmetrical alternating saturation of the magnetic core is to make the positive and negative excitation currents symmetrical. If they are not symmetrical, it is equivalent to having an initial error of a milliampere. If the fluxgate is measuring a large current, this initial error can be ignored. If the measurement is in the range of a few hundred milliamps, the initial error of a milliampere will significantly affect the measurement accuracy.

[0004] A fluxgate current sensor can generally be divided into open-loop and closed-loop types. A closed-loop fluxgate current sensor has a compensation winding to compensate for the magnetic flux generated by the input current, so that the average magnetic flux of the magnetic core is always zero. The compensation current is in a turn ratio relationship with the input current, and the measurement value of the input current signal can be obtained by converting the compensation current into a voltage signal. An open-loop fluxgate current sensor uses the asymmetrical signal of the detected excitation current as the measurement value after processing by a signal conditioning circuit.

[0005] Figure 1 is a conventional excitation circuit scheme. When the excitation current increases, the voltage across resistor R1 also increases, and the gate-source voltage of the MOS transistor decreases. The turn-off of the two bridge arms is achieved by the gate-source voltage of the MOS transistor being less than the gate-on threshold voltage. The gate-on threshold voltage of the MOS transistor changes with the ambient temperature, and there are also differences in the gate-on threshold voltage parameters between different MOS transistors. Therefore, in the case of a large temperature range or a large current range, the gate-on threshold voltage of the MOS transistor may not be able to follow the change of the input signal, resulting in a large error in the measurement value. Figure 1In the prior scheme, when the left and right bridge arm gate opening threshold values are inconsistent, even if the measured current is zero, the excitation in the positive and negative directions will be asymmetric, resulting in a relatively obvious zero point deviation of the output of the entire sensor. Taking the actual test of a single power supply 5V powered fluxgate sensor as an example, it is found in the test of batch samples that there is a zero point deviation of more than 200mV, which is converted into a percentage of 0.2V / 2.5V=8%. SUMMARY

[0006] Therefore, the application provides a fluxgate current sensor circuit which adopts a more optimal excitation self-oscillation scheme for detecting small current and has higher detection accuracy.

[0007] The technical scheme provided by the application is as follows:

[0008] A fluxgate current sensor circuit, characterized in that: comprising a bridge circuit, a current limiting circuit and an excitation winding L1, the excitation winding L1 is connected between two bridge arms of the bridge circuit, the two bridge arms are alternately turned on to generate alternating excitation currents on the excitation winding L1, the current limiting circuit is connected with the two bridge arms of the bridge circuit and detects the currents flowing through the two bridge arms of the bridge circuit respectively, when the current of a bridge arm exceeds a set threshold value, the corresponding bridge arm is controlled to be turned off, and the turn-off threshold values of the two bridge arms of the bridge circuit are controlled by a same reference voltage REF1.

[0009] As a specific embodiment of the above-mentioned fluxgate current sensor circuit, the bridge circuit comprises a switch tube Q1, a switch tube Q2, a switch tube Q3, a switch tube Q4, a resistor R1, a resistor R2, a resistor R3 and a resistor R4, the source of the switch tube Q1 is connected with the source of the switch tube Q2 and then connected with a positive power supply voltage VCC, the drain of the switch tube Q1 is connected with the gate of the switch tube Q2, the drain of the switch tube Q3 and one end of the resistor R4, the drain of the switch tube Q2 is connected with the gate of the switch tube Q1, the drain of the switch tube Q4 and one end of the resistor R3, the other end of the resistor R4 is connected with the gate of the switch tube Q4, the other end of the resistor R3 is connected with the gate of the switch tube Q3, the source of the switch tube Q4 is connected with the ground through the resistor R2, and the source of the switch tube Q3 is connected with the ground through the resistor R1.

[0010] As a specific embodiment of the above-mentioned fluxgate current sensor circuit, the current limiting circuit comprises comparators COMP1 and COMP2, the same phase input end of the comparator COMP1 is connected with the same phase input end of the comparator COMP2 and the reference voltage REF1, the inverse phase input end of the comparator COMP1 is connected with the source of the switch tube Q3, the inverse phase input end of the comparator COMP2 is connected with the source of the switch tube Q4, the output end of the comparator COMP1 is connected with the gate of the switch tube Q3, and the output end of the comparator COMP2 is connected with the gate of the switch tube Q4.

[0011] Preferably, the magnetic flux gate current sensor circuit further comprises a compensation winding L2, an error amplifier circuit, a low-pass filter circuit and a sampling circuit, the excitation current detected by the excitation winding L1 is fed back to the compensation winding L2 through the error amplifier circuit, the low-pass filter circuit and the sampling circuit in sequence.

[0012] Preferably, the excitation winding L1 and the compensation winding L2 are wound on the same magnetic core.

[0013] As a specific embodiment of the magnetic flux gate current sensor circuit, the error amplifier circuit comprises a resistor R5, a capacitor C1, a capacitor C3 and an operational amplifier OP1, the resistor R5 is connected between two bridge arms of the excitation circuit in series with the excitation winding L1, the input end of the operational amplifier OP1 is connected to the two ends of the resistor R5, the capacitor C1 is connected between the inverting input end and the output end of the operational amplifier OP1, the capacitor C3 is connected between the non-inverting input end and the ground of the operational amplifier OP1, and the output end of the operational amplifier OP1 serves as the output end of the error amplifier.

[0014] As a specific embodiment of the magnetic flux gate current sensor circuit, the sampling circuit comprises a resistor R6, a resistor R7 and a capacitor C2, one end of the resistor R6 and one end of the resistor R7 serve as the input end of the sampling circuit, the other end of the resistor R6 is grounded through the compensation winding L2, the other end of the resistor R7 is grounded through the capacitor C2, and the connection point of the resistor R7 and the capacitor C2 serves as the output end of the magnetic flux gate current sensor circuit.

[0015] As a specific embodiment of the magnetic flux gate current sensor circuit, the error amplifier circuit comprises a resistor R8, a resistor R9, a capacitor C1, a capacitor C3 and an operational amplifier OP1, the resistor R8 and the resistor R9 are connected in series and then connected in parallel to the two ends of the excitation winding L1, the input end of the operational amplifier OP1 is connected to the two ends of the resistor R9, the capacitor C1 is connected between the inverting input end and the output end of the operational amplifier OP1, the capacitor C3 is connected between the non-inverting input end and the ground of the operational amplifier OP1, and the output end of the operational amplifier OP1 serves as the output end of the error amplifier.

[0016] As a specific embodiment of the magnetic flux gate current sensor circuit, the sampling circuit comprises a resistor R6 and a differential sampling circuit, one end of the resistor R6 serves as the input end of the sampling circuit, the other end of the resistor R6 is grounded through the compensation winding L2, the two input ends of the differential sampling circuit are connected to the two ends of the resistor R6, and the output end of the differential sampling circuit serves as the output end of the magnetic flux gate current sensor circuit.

[0017] The working principle of the present application will be analyzed in combination with specific embodiments, which will not be described here in detail, and the beneficial effects of the present application are as follows:

[0018] 1. By controlling the peak circuit of the excitation winding L1 through a comparator, the turn-off threshold of the two bridge arms of the excitation circuit is controlled by the same reference voltage REF1, which can eliminate the parameter difference between different individual switching transistors, and the peak current is also more stable under different temperatures. Compared with the existing solutions, the present invention has higher accuracy.

[0019] 2. By using a low-pass filter circuit to filter out the carrier frequency signal on the excitation winding L1, the cutoff frequency of the error amplifier can be set higher, thus enabling it to respond to higher measured current signals and improve the overall measurement bandwidth of the current sensor. Attached Figure Description

[0020] Figure 1 The circuit schematic of the existing excitation circuit;

[0021] Figure 2 This is a circuit schematic diagram of the first embodiment of the fluxgate current sensor circuit of the present invention;

[0022] Figure 3 This is a circuit schematic diagram of the second embodiment of the fluxgate current sensor circuit of the present invention. Detailed Implementation

[0023] To better understand the improvements made by this invention compared to the prior art, specific embodiments of this invention will be described in detail.

[0024] First Embodiment

[0025] like Figure 1 As shown, a fluxgate current sensor circuit includes an excitation circuit, an excitation winding L1, an error amplifier circuit, a low-pass filter circuit, and a sampling circuit.

[0026] The excitation circuit comprises a switch tube Q1, a switch tube Q2, a switch tube Q3, a switch tube Q4, a resistor R1, a resistor R2, a resistor R3, a resistor R4, a comparator COMP1 and a comparator COMP2, the source of the switch tube Q1 is connected with the source of the switch tube Q2, and the connection is connected with a positive power supply voltage VCC; the drain of the switch tube Q1 is connected with the gate of the switch tube Q2, the drain of the switch tube Q3 and one end of the resistor R4; the drain of the switch tube Q2 is connected with the gate of the switch tube Q1, the drain of the switch tube Q4 and one end of the resistor R3; the other end of the resistor R4 is connected with the gate of the switch tube Q4 and the output end of the comparator COMP2; the other end of the resistor R3 is connected with the gate of the switch tube Q3 and the output end of the comparator COMP1; the source of the switch tube Q4 is connected with one end of the resistor R2 and the negative input end of the comparator COMP2; the source of the switch tube Q3 is connected with one end of the resistor R1 and the negative input end of the comparator COMP1; the other end of the resistor R1 is connected with the other end of the resistor R2, and the connection is grounded; the positive input end of the comparator COMP1 is connected with the positive input end of the comparator COMP2, and the connection is connected with a reference voltage REF1.

[0027] The error amplifier circuit comprises a resistor R5, a capacitor C1, a capacitor C3 and an operational amplifier OP1; the resistor R5 is connected in series with the excitation winding L1, and the connection is connected between two bridge arms of the excitation circuit; the input end of the operational amplifier OP1 is connected with both ends of the resistor R5; the capacitor C1 is connected between the inverting input end and the output end of the operational amplifier OP1; the capacitor C3 is connected between the non-inverting input end and the ground of the operational amplifier OP1; the output end of the operational amplifier OP1 is connected with the input end of the low-pass filter circuit as the output end of the error amplifier.

[0028] The sampling circuit comprises a resistor R6, a resistor R7 and a capacitor C2; one end of the resistor R6 and one end of the resistor R7 are connected as the input end of the sampling circuit, and the connection is connected with the output end of the low-pass filter circuit; the other end of the resistor R6 is grounded through the compensation winding L2; the other end of the resistor R7 is grounded through the capacitor C2; the connection point of the resistor R7 and the capacitor C2 is the output end of the fluxgate current sensor circuit.

[0029] The switch tube Q1 and the switch tube Q2 are P-type MOS tubes, and the switch tube Q3 and the switch tube Q4 are N-type MOS tubes.

[0030] The output ends of the comparator COMP1 and the comparator COMP2 are OC gates.

[0031] The working principle of the embodiment is as follows:

[0032] The excitation circuit is a full-bridge self-oscillation circuit, the excitation winding L1 is connected between two bridge arms of the excitation circuit, is excited by alternating square wave voltage, and needs to be set with a peak current so that the excitation winding L1 enters a saturation state during each excitation. Since the turn-on threshold of the gate of the N-MOS tube Q3 and the gate of the N-MOS tube Q4 is not absolutely equal, one of the cross bridge arms will be turned on first when power is turned on. Assuming that the N-MOS tube Q3 is turned on first, the drain of the N-MOS tube Q3 will be pulled low, at this time the P-MOS tube Q2 will be turned on, and the P-MOS tube Q1 and the N-MOS tube Q4 will be turned off, at this time the excitation voltage on the excitation inductor L1 is positive on the right and negative on the left, and the voltage on the resistor R1 continuously rises, when the voltage on the resistor R1 reaches the reference voltage REF1, the output end of the comparator COMP1 will pull the gate of the N-MOS tube Q3 to the ground, at this time the N-MOS tube Q3 is turned off and the drain becomes high, at the same time the N-MOS tube Q4 and the P-MOS tube Q1 are turned on, and the P-MOS tube Q2 is turned off, the excitation voltage on the excitation inductor L1 is positive on the left and negative on the right, and the whole circuit is self-oscillated in the above-mentioned working mode.

[0033] The gate opening threshold of the MOS tube will change with the ambient temperature, and there is also a certain difference between the gate opening threshold parameters of different MOS tubes, so in the prior art Figure 1 , when the gate opening threshold of the left and right two bridge arms is inconsistent, it will lead to asymmetric excitation in the positive and negative directions, resulting in a relatively obvious error in the output of the whole sensor. In the present application, the peak value circuit of the excitation winding L1 is controlled by a comparator, the turn-off threshold of the two bridge arms of the excitation circuit is controlled by the same reference voltage REF1, the parameter difference between different MOS tubes can be eliminated, and the peak current is also relatively stable at different temperatures.

[0034] The magnetic flux gate current sensor circuit of the present application can significantly eliminate the zero point deviation of the output of the original sensor. Taking the actual test of a single power supply 5V powered magnetic flux gate sensor as an example, the batch test of the actual sample shows that the zero point deviation is about 30mV, which is converted into percentage as 0.03V / 2.5V=1.2%, and the present application has higher precision compared with the prior art.

[0035] The excitation winding L1 and the compensation winding L2 are wound in the same magnetic core, and the measured current wire passes through the middle of the magnetic core. When the measured current is 0, the excitation current on the excitation winding L1 is symmetrically positive and negative, and the average magnetic flux inside the magnetic core is 0. When the measured current is not 0, the measured current will generate a bias magnetic flux in the magnetic core, at this time the positive and negative excitations are no longer balanced, and the excitation in one direction is more likely to be saturated, and the excitation in the other direction is not easy to be saturated, at this time the excitation current on the excitation winding L1 is not symmetrically positive and negative.

[0036] The resistance R5 is used to sample the current on the excitation winding L1. When the excitation current of the excitation winding L1 is not symmetrical, the average voltage on the resistance R5 is also not zero. At this time, the error amplifier composed of the operational amplifier OP1 will amplify the voltage signal on the resistance R5, and after a low-pass filtering, the signal is fed back to the compensation winding L2. The current flowing through the compensation winding L2 will generate another magnetic flux in the magnetic core, which will offset the magnetic flux generated by the measured current, so that the average magnetic flux in the magnetic core is always zero. Therefore, by measuring the current flowing through the compensation winding L2, the value of the measured current can be known.

[0037] The excitation current on the excitation winding L1 can be called a carrier signal. The cut-off frequency of the error amplifier needs to be much lower than the carrier frequency. Otherwise, the output signal of the sensor will have obvious carrier signal, which will affect the detection quality. By adding a low-pass filter, especially a high-order filter, to filter out the signal of the carrier frequency, the cut-off frequency of the error amplifier can be set higher. In this way, the error amplifier can respond to higher measured current signals. Therefore, the output of the error amplifier is connected to a low-pass filter, which mainly functions to improve the measurement bandwidth of the entire current sensor.

[0038] Second embodiment

[0039] As shown in Figure 3 A magnetic flux gate current sensor circuit includes an excitation circuit, an excitation winding L1, a compensation winding L2, an error amplifier circuit, a low-pass filter circuit and a sampling circuit.

[0040] The excitation circuit includes a switch tube Q1, a switch tube Q2, a switch tube Q3, a switch tube Q4, a resistance R1, a resistance R2, a resistance R3, a resistance R4, a comparator COMP1 and a comparator COMP2. The source of the switch tube Q1 is connected to the source of the switch tube Q2, and then connected to a positive power supply voltage VCC. The drain of the switch tube Q1 is connected to the gate of the switch tube Q2, the drain of the switch tube Q3 and one end of the resistance R4. The drain of the switch tube Q2 is connected to the gate of the switch tube Q1, the drain of the switch tube Q4 and one end of the resistance R3. The other end of the resistance R4 is connected to the gate of the switch tube Q4 and the output of the comparator COMP2. The other end of the resistance R3 is connected to the gate of the switch tube Q3 and the output of the comparator COMP1. The source of the switch tube Q4 is connected to one end of the resistance R2 and the negative input of the comparator COMP2. The source of the switch tube Q3 is connected to one end of the resistance R1 and the negative input of the comparator COMP1. The other end of the resistance R1 is connected to the other end of the resistance R2, and then connected to ground. The positive input of the comparator COMP1 is connected to the positive input of the comparator COMP2, and then connected to a reference voltage REF1.

[0041] The error amplifier circuit comprises resistors R8 and R9, capacitors C1 and C3, and an operational amplifier OP1. Resistors R8 and R9 are connected in series and then connected in parallel to both ends of the excitation winding L1. The input terminal of the operational amplifier OP1 is connected to both ends of resistor R9. Capacitor C1 is connected between the inverting input terminal and the output terminal of the operational amplifier OP1. Capacitor C3 is connected between the non-inverting input terminal and the ground of the operational amplifier OP1. The output terminal of the operational amplifier OP1 is connected to the input terminal of the low-pass filter circuit as the output terminal of the error amplifier.

[0042] The sampling circuit comprises resistor R6 and a differential sampling circuit. One end of resistor R6 is connected to the output terminal of the low-pass filter circuit as the input terminal of the sampling circuit. The other end of resistor R6 is connected to the ground through the compensation winding L2. The two input terminals of the differential sampling circuit are connected to both ends of resistor R6. The output terminal of the differential sampling circuit is connected to the output terminal of the fluxgate current sensor circuit.

[0043] The switching tubes Q1 and Q2 are P-type MOS tubes, and the switching tubes Q3 and Q4 are N-type MOS tubes.

[0044] The output terminals of the comparators COMP1 and COMP2 are OC gates.

[0045] The working principle of the present embodiment is similar to that of the first embodiment. The main difference is that the error amplifier circuit of the present embodiment samples the voltage signal between the excitation inductors. Since the voltage between the excitation inductors is high, the voltage signal needs to be divided before sampling. When the measured current is zero, the excitation voltage between the excitation inductors L1 is symmetrical, and the average value is zero. When the measured current is not zero, the excitation voltage between the excitation inductors L1 is asymmetrical, and the error amplifier can detect the asymmetrical state and output a certain current to the compensation winding L2, so that the average value of the magnetic flux of the magnetic core is zero. Similarly, by measuring the current flowing through the compensation winding L2, the value of the measured current can be known.

[0046] The above is only a preferred embodiment of the present application. It should be noted that the above preferred embodiment should not be regarded as a limitation of the present application. The protection scope of the present application should be defined by the scope of the claims. For ordinary skilled persons in the art, several improvements and refinements can be made without departing from the spirit and scope of the present application, and these improvements and refinements should also be regarded as the protection scope of the present application.

Claims

1. A fluxgate current sensor circuit, characterized by: The bridge circuit, the current limiting circuit and the excitation winding L1 are included, the excitation winding L1 is connected between two bridge arms of the bridge circuit, the two bridge arms are alternately conducted to generate positive and negative alternating excitation current on the excitation winding L1, the current limiting circuit is connected with the two bridge arms of the bridge circuit, respectively detects the current flowing through the two bridge arms of the bridge circuit, when the current of the bridge arm exceeds the set threshold value, the corresponding bridge arm is controlled to be turned off, the turn-off threshold value of the two bridge arms of the bridge circuit is controlled by the same reference voltage REF1; The bridge circuit includes the switch tube Q1, the switch tube Q2, the switch tube Q3, the switch tube Q4, the resistance R1, the resistance R2, the resistance R3 and the resistance R4, the source of the switch tube Q1 is connected with the source of the switch tube Q2 and then connected with the positive power supply voltage VCC, the drain of the switch tube Q1 is connected with the gate of the switch tube Q2, the drain of the switch tube Q3 and one end of the resistance R4, the drain of the switch tube Q2 is connected with the gate of the switch tube Q1, the drain of the switch tube Q4 and one end of the resistance R3, the other end of the resistance R4 is connected with the gate of the switch tube Q4, the other end of the resistance R3 is connected with the gate of the switch tube Q3, the source of the switch tube Q4 is grounded through the resistance R2, and the source of the switch tube Q3 is grounded through the resistance R1; The current limiting circuit includes the comparator COMP1 and COMP2, the same phase input end of the comparator COMP1 is connected with the same phase input end of the comparator COMP2 and the reference voltage REF1, the opposite phase input end of the comparator COMP1 is connected with the source of the switch tube Q3, the opposite phase input end of the comparator COMP2 is connected with the source of the switch tube Q4, the output end of the comparator COMP1 is connected with the gate of the switch tube Q3, and the output end of the comparator COMP2 is connected with the gate of the switch tube Q4; It also includes the compensation winding L2, the error amplifier circuit, the low pass filter circuit and the sampling circuit, the excitation current detected by the excitation winding L1 is fed back to the compensation winding L2 through the error amplifier circuit, the low pass filter circuit and the sampling circuit in sequence.

2. The fluxgate current sensor circuit of claim 1, wherein: The excitation winding L1 and the compensation winding L2 are wound in the same magnetic core.

3. The fluxgate current sensor circuit of claim 1, wherein: The error amplifier circuit includes the resistance R5, the capacitor C1, the capacitor C3 and the operational amplifier OP1, the resistance R5 is connected between the two bridge arms of the excitation circuit after being connected with the excitation winding L1 in series, the input end of the operational amplifier OP1 is connected with the two ends of the resistance R5, the capacitor C1 is connected between the opposite phase input end and the output end of the operational amplifier OP1, the capacitor C3 is connected between the same phase input end and the ground of the operational amplifier OP1, and the output end of the operational amplifier OP1 serves as the output end of the error amplifier.

4. The fluxgate current sensor circuit of claim 3, wherein: The sampling circuit includes the resistance R6, the resistance R7 and the capacitor C2, the connection point of one end of the resistance R6 and one end of the resistance R7 serves as the input end of the sampling circuit, the other end of the resistance R6 is grounded through the compensation winding L2, the other end of the resistance R7 is grounded through the capacitor C2, and the connection point of the resistance R7 and the capacitor C2 serves as the output end of the magnetic flux gate current sensor circuit.

5. The fluxgate current sensor circuit of claim 1, wherein: The error amplifier circuit comprises resistors R8 and R9, capacitors C1 and C3, and an operational amplifier OP1. Resistors R8 and R9 are connected in series and then connected in parallel to both ends of the excitation winding L1. The input end of the operational amplifier OP1 is connected to both ends of the resistor R9. The capacitor C1 is connected between the inverting input end and the output end of the operational amplifier OP1. The capacitor C3 is connected between the non-inverting input end and the ground of the operational amplifier OP1. The output end of the operational amplifier OP1 serves as the output end of the error amplifier.

6. The fluxgate current sensor circuit of claim 5, wherein: The sampling circuit comprises a resistor R6 and a differential sampling circuit. One end of the resistor R6 serves as the input end of the sampling circuit. The other end of the resistor R6 is connected to the ground through the compensation winding L2. The two input ends of the differential sampling circuit are connected to both ends of the resistor R6. The output end of the differential sampling circuit serves as the output end of the fluxgate current sensor circuit.

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