Fluxgate Converter for Current Sensing

Abstract

A fluxgate current converter (2) comprises a control circuit (4) and a fluxgate device (2), the fluxgate device comprising a saturable soft magnetic core (6) surrounded by an excitation coil (8), the control circuit comprising an excitation coil drive circuit connected to the excitation coil, the excitation coil drive circuit being configured to generate an alternating magnetic field to alternately saturate the soft magnetic core. The excitation coil comprises a first winding (8a) and a second winding (8b) connected together at a common point (9), the common point forming an input of the first winding and an output of the second winding, the first winding being connected in series with the first switch (S1) and the second winding being connected in series with the second switch (S2). The first switch and the second switch are controlled by the control circuit to provide an excitation coil current in the first winding, thereby generating a magnetic field in a first direction in the magnetic core, and the excitation coil current in the second winding generating a magnetic field in a second direction opposite to the first direction in the magnetic core.

Description

[0001] The present invention relates to a fluxgate converter for current measurement applications.

[0002] As is well known, fluxgate converters are used in current measurement applications, whereby the fluxgate converter measures the strength of the magnetic field generated by one or more primary conductors carrying the current to be measured. The fluxgate converter includes a saturable soft magnetic core surrounded by an excitation coil, which is connected to a control circuit configured to alternately saturate the soft magnetic core. In certain configurations, the fluxgate converter is a small magnetic field sensing component arranged in the air gap of a magnetic circuit core, which surrounds a central channel through which one or more primary conductors extend. In another known configuration, the fluxgate saturable core itself forms a central channel for the primary conductor to pass through.

[0003] In a closed loop converter, there is a compensation coil magnetically coupled to the magnetic circuit core and connected in a feedback circuit to the signal processing circuit, the compensation coil seeking to cancel the magnetic field generated by the primary conductor. Such an arrangement is well known. Although fluxgate converters can be used in an open loop manner, whereby there is no compensation coil, they are primarily used in a closed loop configuration in view of their high sensitivity.

[0004] In the configuration where the fluxgate converter is a small magnetic field sensing component arranged in the air gap of the magnetic circuit core, the compensation coil can be wound on the magnetic circuit with the air gap, while in the configuration where the fluxgate saturable magnetic core itself forms a central channel for the primary conductor to pass through, the excitation coil can be used to provide the compensation current. In the latter case, the excitation current signal is superimposed on the compensation current signal.

[0005] Fluxgate transducers provide accurate measurement of primary or differential currents flowing in one or more primary conductors. Compared to the well-known transducers using Hall effect detectors, fluxgate transducers have high sensitivity and low offset. In addition, fluxgate transducers provide good isolation between the primary and secondary sides and are able to measure low frequency AC currents (up to several kHz) and DC currents. Fluxgate sensor technology is also known for DC and AC leakage current measurements. These types of transducers are implemented in many fields, such as:

[0006] -Battery management system (measures the current flowing through the battery)

[0007] - Laboratory reference sensors (calibration equipment)

[0008] -Medical equipment (e.g., magnetic resonance imaging)

[0009] However, the electronics required to drive the excitation current for saturating the magnetic core of the fluxgate detector and for driving the compensation current render such current transducers more expensive and less compact than conventional open-loop current transducers with Hall effect detectors. In conventional fluxgate detectors, the excitation current is usually generated by an auto-oscillating circuit architecture, usually based on an H-bridge circuit. In such conventional auto-oscillating circuits, it is difficult to actively change the saturation level or to define a specific start-up sequence, for example to achieve a demagnetization process.

[0010] In certain applications, current measurement systems are associated with safety functions, in particular in the case of sensors intended to measure leakage currents or residual currents. Illustrative examples of such applications include sensors for detecting AC and / or DC leakage currents in solar panel applications or in automotive applications (e.g. for on-board charging systems).

[0011] In automotive applications, one of the applicable safety standards is the ISO26262 standard, and based on the hazard analysis and risk assessment performed, the safety level of this function is ranked as ASIL D from the quality management aspect. In automotive applications, DC leakage current may be a safety function with a level of ASIL D, the highest safety level required. This safety level requires high reliability and results in high costs.

[0012] For some applications, the current transducer should have a very low offset error. This is the case for battery monitoring devices where an offset in the measured current would cause the battery's state of charge to be too high or too low. For a leakage current sensor or residual current sensor, the sensor will monitor a current that will be zero or close to zero most of the time, and any change in the current value beyond a certain limit (threshold) should be detected. Therefore, for such applications, it is necessary to use a current sensor with a very low offset error.

[0013] However, current sensors that use a magnetic material core to measure the magnetic field have the disadvantage of generating a magnetic offset due to the influence of the residual magnetism and coercive field in the magnetic material. Although the magnetic offset in nominal operation can be minimized if the converter includes a demagnetization function to reinitialize the magnetic state of the magnetic path material, for highly sensitive fluxgate converters, conventional reinitialization may not be sufficient to accurately operate the converter.

[0014] It is an object of the present invention to provide a fluxgate converter for current measurement that is accurate and cost-effective to manufacture and implement.

[0015] It would be advantageous to provide a fluxgate transformer for current measurement that is easily adjusted or calibrated to user requirements.

[0016] It would be advantageous to provide a fluxgate transformer for current measurement that is easy to install and operate.

[0017] It would be advantageous to provide a fluxgate converter for current measurement that is reliable and meets high safety level standards.

[0018] The object of the invention is achieved by providing a fluxgate converter for current measurement and a method of operating a fluxgate converter according to the independent claims.

[0019] A fluxgate current converter is disclosed herein, the fluxgate current converter comprising a control circuit and a fluxgate device, the fluxgate device comprising a saturable soft magnetic core surrounded by an excitation coil. The control circuit comprises an excitation coil drive circuit, the excitation coil drive circuit is connected to the excitation coil, and the excitation coil is configured to generate an alternating magnetic field to alternately saturate the soft magnetic core.

[0020] The field coil includes a first winding and a second winding connected together at a common point, the common point forming an input of the first winding and an output of the second winding. The first winding is connected in series with a first switch and the second winding is connected in series with a second switch, the first switch and the second switch are controlled by the control circuit to provide a field coil current in the first winding to generate a magnetic field in the magnetic core in a first direction, and the field coil current in the second winding generates a magnetic field in the magnetic core in a second direction opposite to the first direction.

[0021] According to a first aspect of the invention, the control circuit comprises a comparator having a first input terminal connected to the excitation coil, wherein the control circuit comprises a PWM module connected to the output terminal of the comparator and configured to control the operation timing of the first switch and the second switch, including introducing a dead time during switching from one excitation coil winding to another excitation coil winding.

[0022] According to a second aspect of the present invention, the converter includes an analog measurement channel (M2) connected to the excitation coil after the first switch and the second switch, and the analog measurement channel includes an ADC module, wherein the analog measurement channel also includes a DAC, which is connected to the ADC module and also connected to an input terminal of a comparator having a first input terminal connected to the excitation coil.

[0023] According to a third aspect of the present invention, the control circuit further comprises a first resistor connected to the high side of the fluxgate device, the control circuit further comprises a third switch S3 arranged in a bypass circuit in parallel with the first resistor, the third switch being configured to short-circuit the first resistor when the switch is switched to a closed position, and wherein the analog measurement channel comprises a reference voltage source to control a power supply reference voltage for the ADC module.

[0024] According to a fourth aspect of the invention, the fluxgate converter is connected to a user system to form together a current measurement system, the current measurement system having at least three measurement channels (a first PWM channel M1, a second analog channel M2 and a third channel M3) provided by the test winding, the user system being configured to calculate a comparison of the measurement signals from the three measurement channels in order to determine correct or incorrect operation.

[0025] In an advantageous embodiment, the first input of the comparator is connected to the excitation coil after the first switch and the second switch.

[0026] In an advantageous embodiment, the second input of the comparator is connected to a DAC which supplies an adjustable reference voltage for adjusting the output level of the comparator.

[0027] In an advantageous embodiment, the fluxgate converter further comprises a combinational and sequential logic unit connected to the output of the PWM module to generate a complementary signal including the dead time provided by the PWM module.

[0028] In an advantageous embodiment, the fluxgate converter comprises a capture module connected to the output of the PWM module to capture the timing of the switching signal.

[0029] In an advantageous embodiment, the converter comprises an analog measurement channel (M2) connected to the excitation coil after the first switch and the second switch, the analog measurement channel comprising an ADC module.

[0030] In an advantageous embodiment, the analog measurement channel further comprises a DAC connected to the ADC module and also to one input of the comparator having a first input connected to the excitation coil.

[0031] In an advantageous embodiment, the analog measurement channel further comprises an amplifier to increase the gain and scale the offset of the measured value of the signal fed into the ADC module.

[0032] In a preferred embodiment, the control circuit further comprises a first resistor connected to a high side or a low side of the fluxgate device, the control circuit further comprises a third switch S3 arranged in a bypass circuit in parallel with the first resistor, the third switch being configured to short-circuit the first resistor when the switch is switched to a closed position.

[0033] In an advantageous embodiment, the analog measurement channel comprises a reference voltage source to control a supply reference voltage of the ADC module.

[0034] In an advantageous embodiment, the converter further comprises a test winding wound on the magnetic core, the test winding being connected to a connection interface of the fluxgate converter for connection to an electronic circuit of a user system.

[0035] In an advantageous embodiment, the test winding is configured to receive a test current from the user system and output a voltage to the user system in a second operating mode corresponding to a measurement signal of the magnetic field in the fluxgate device, so that the test winding provides a third measurement channel M3.

[0036] In a preferred embodiment, the fluxgate converter is connected to a user system to form together a current measurement system, the current measurement system having at least three measurement channels (a first PWM channel M1, a second analog channel M2 and a third channel M3) provided by the test winding, the user system being configured to calculate a comparison of the measurement signals from the three measurement channels in order to determine correct or incorrect operation.

[0037] In an advantageous embodiment, the electronic circuit of the user system is configured to use two of the three measurement signals having values ​​closest to each other in the event of a fault affecting one of the measurement channels and to discard the other measurement signal.

[0038] Also disclosed herein is a method of operating a fluxgate converter to perform a demagnetization process, the method comprising:

[0039] Step a) stopping the oscillation of the fluxgate device;

[0040] step b) after step a, restarting the oscillation of the fluxgate device at a fixed frequency and a high first saturation depth for a duration of a limited number of half cycles (preferably less than twenty half cycles, more preferably less than ten half cycles, for example between one and five half cycles);

[0041] Step e) after step b, stopping the oscillation of the fluxgate device;

[0042] Step i), after step e, operating the fluxgate device in an automatic oscillation sequence at a second intermediate saturation depth less than the first saturation depth;

[0043] Step j) after step i, gradually reducing the saturation depth to a nominal value.

[0044] Other objects and advantageous features of the present invention will be apparent from the claims, detailed description and accompanying drawings, in which:

[0045] Figure 1a is a schematic circuit block diagram of a fluxgate converter according to an embodiment of the present invention;

[0046] Figure 1b is a schematic circuit block diagram of a fluxgate converter according to another embodiment of the present invention;

[0047] Figure 2 is a schematic block diagram of a current measurement system having a fluxgate converter according to an embodiment of the present invention;

[0048] Figure 3a is a schematic simplified block diagram of a fluxgate converter measurement system according to an embodiment of the present invention, illustrating a first configuration for enhancing safety requirements;

[0049] Figure 3b is similar to Figure 3a A diagram illustrating a second configuration according to an embodiment of the present invention;

[0050] Figure 4a A process for resetting the magnetic circuit remanence to a defined magnetic offset value (eg, corresponding to a value set during initial calibration of the converter) is illustrated;

[0051] Figure 4b Graphs of magnetic excursion levels of a magnetic fluxgate converter are illustrated without and with a startup sequence and process for resetting magnetic circuit remanence in accordance with an embodiment of the present invention.

[0052] Referring to the drawings, a fluxgate current transducer for measuring a primary current flowing in one or more primary conductors 1 includes a fluxgate device 2 and a control circuit 4 connected to the fluxgate device.

[0053] The fluxgate device 2 includes a saturable magnetic core and an excitation coil 8 wound on the saturable magnetic core 6. The excitation coil 8 includes two windings (a first winding 8a and a second winding 8b) connected together at a common point 9, which forms an input of the first winding and an output of the second winding. The excitation coil 8 is connected to an excitation coil drive circuit, which includes a power supply controlled by a control circuit 4, and the control circuit is configured to supply an excitation current through the excitation coil so that the saturable magnetic core 6 is alternately saturated. More specifically, current is supplied to the first winding 8a and the second winding 8b in a sequential alternating manner, whereby the current in the first winding 8a generates a magnetic field in a first direction in the magnetic core, and the current in the second winding 8b generates a magnetic field in a second direction opposite to the first direction in the magnetic core, and the changing magnetic field in the magnetic core is switched sequentially, optionally with a short dead time period between switching from one winding to another.

[0054] The magnetic core is preferably made of a high permeability nanocrystalline magnetic material (e.g., FeSi or FeCo based nanocrystalline material) due to its low temperature drift, high permeability, low coercive field, and adaptable permeability value. The magnetic material can have the following BH loop shape characteristics: rectangular (longitudinal field annealing), circular (no field annealing), flat (transverse field annealing). Depending on the BH loop shape, the control circuit is configured to achieve alternating saturation of the magnetic material as the BH characteristics of the material change.

[0055] The switching of the excitation current is controlled by the control circuit 4 to supply the first winding or the second winding.

[0056] The control circuit 4 includes a microcontroller 22, which controls the current flowing in the excitation coil windings 8a, 8b via a first switch S1 and a second switch S2 connected in series with the respective first winding and the second winding. For example, the switch may include a transistor, or be composed of a transistor. The term "microcontroller" is intended to include any IC processor, MCU, ASIC, FPGA or other processing circuit capable of controlling the operation of the switches S1, S2.

[0057] The control circuit may include a first resistor 12 connected to the common point 9. Figure 1a In the embodiment of FIG. 1 , the first resistor 12 is connected to the common point 9 on the high side of the fluxgate device. Figure 1b In the embodiment of FIG. 1 , the first resistor 12 is connected to the common point 9 of the low side of the fluxgate device. The first resistor 12 can be used to set the sensitivity of the fluxgate converter for a pulse width modulation (PWM) measurement mode.

[0058] The control circuit may further comprise a second resistor 20 connected to the field coil windings 8a, 8b after the switches S1, S2. The second resistor may be used to set the sensitivity of the fluxgate converter for the analog measurement mode.

[0059] The control circuit may include a third switch S3 arranged in the bypass circuit 14 in parallel with the first resistor 12 to bypass the first resistor and increase the current through the excitation coil. The increased current may be used, for example, to perform deep saturation of the magnetic core, for example during a demagnetization process.

[0060] The control circuit includes an automatic oscillation circuit part including a high-speed comparator 24 in the microcontroller 3. The comparator 24 has a first input terminal 16a, which is connected to the excitation coil between the switches and the second resistor 20 after the first switch S1 and the second switch S2. The comparator is configured to detect the peak current in the excitation coil when the magnetic core reaches saturation to control the automatic oscillation switch signal that opens and closes the switches S1, S2 in an alternating manner.

[0061] The comparator may be connected at a second input 16b to a fixed reference voltage (such as ground or a fixed voltage level) or, in a preferred embodiment, to a DAC 26 (digital-to-analog converter) which supplies an adjustable reference voltage for adjusting the comparator output voltage. The comparator may implement selectable hysteresis values ​​and / or digital filtering to avoid glitches during switching of the winding switches S1, S2.

[0062] The output of the comparator can advantageously be connected to a high-speed PWM module 28 for managing the timing control of the two winding switches S1, S2, in particular controlling the dead time for switching the excitation supply current from one winding to the other. The PWM module is used to synchronize the control of the winding switches S1, S2. It limits the maximum oscillation frequency (for example, in the case of overcurrent), which may cause damage to the winding switches S1, S2 due to high switching losses. It is used to accurately generate the dead time (in the nanosecond range) between the winding switches S1, S2 to prevent shoot through during the switching of the winding switches S1, S2. The PWM module is also used to reduce EMI noise by distributing the operating frequency of the fluxgate to the nominal frequency.

[0063] The output of the PWM module can advantageously be connected to a combinatorial and sequential logic unit 30, which is used to control the winding switches S1 and S2 alternately and insert dead time at appropriate timing. It can basically include a D flip-flop (for S1 and S2 sequence) with an AND gate and a NOT gate for dead time insertion. Therefore, this circuit part acts as a filtering and synchronization unit, including the control of the dead time between switches.

[0064] The control circuit includes a capture module 32 having a first input capture section 32a and a second input capture section 32b connected to the output of the PWM module 28 and the logic unit 30 circuit portion, the input capture section being used to capture the timing of the switching signal to measure the duty cycle of the fluxgate for the PWM measurement mode. It is more convenient and reliable to measure the synchronization signal from the microcontroller 22 rather than using the transistor gate or drain signal (for example, for MOSFETs), because the amplitude and slope of these signals will drift relative to the temperature, supply voltage and drain current to be measured. The signal processing module 43 is configured to calculate the current from the input capture module 32 and the signal output by the ADC 18.

[0065] For analog measurement outputs, the control circuit may also advantageously include a reference voltage source 38 to compensate for ratiometric errors when the ADC is not referenced to an accurate / low drift voltage source.

[0066] The reference voltage source may be external to the microcontroller and used as a ratiometric low drift reference voltage for an analog-to-digital converter (ADC) in the microcontroller 22 .

[0067] The control circuit may also advantageously include a temperature measurement input for receiving the value of a temperature measurement performed within the microcontroller or external to the microcontroller to perform temperature compensation on the fluxgate measurement output if necessary.

[0068] The control circuit may also advantageously include an amplifier 42 to increase the gain and scale offset of the measurement signal taken across the second resistor 20 and supplied to the microcontroller 18 .

[0069] The fluxgate converter according to an embodiment of the present invention includes more than one measurement channel, including a first measurement channel provided by a PWM signal controlling the excitation coil switch and a second measurement channel provided by a connection to the excitation coil, the second measurement channel measuring the current in the excitation coil and processing the analog measurement signal in a microcontroller after conversion by an analog-to-digital (ADC) circuit 18.

[0070] The operation of the PWM module can include the following modes:

[0071] - are not synchronized with a fixed frequency by a comparator (mainly used for degaussing sequences), and

[0072] - Synchronized by comparator, where the frequency depends on the comparator trigger signal (for measurement sequences and also during demagnetization)

[0073] In both modes of operation of the PWM module, the duty cycle applied by the PWM module can be fixed or variable.

[0074] The arrangement of the fluxgate converter according to embodiments of the present invention advantageously allows:

[0075] - Manage the fluxgate operation mode (fixed frequency or auto-oscillation), whereby fixed frequency operation is used to perform the degaussing sequence. Low frequency operation enhances the efficiency of the degaussing sequence compared to high frequency operation;

[0076] - Manage deterministic stopping and starting of the converter (e.g. for demagnetization sequences or for putting the converter in sleep mode), whereby the combinatorial and sequential logic unit can control starting and stopping by putting both switch S1, S2 drive signals at low level;

[0077] - Increase or decrease magnetic field saturation value without additional hardware;

[0078] - Local analog and PWM redundant measurements for safety applications;

[0079] -Limit the number of components (passive, active and MCU IO).

[0080] Therefore, the fluxgate converter according to the embodiment of the present invention can:

[0081] - Provide different operating modes, including an automatic oscillating operating mode with variable duty cycle, or a fixed frequency mode that can be used, for example, during demagnetization;

[0082] -Mitigates EMI noise by spreading the frequency within a predetermined range;

[0083] - enables the magnetic field saturation level to be adjusted (increased or decreased) directly in the converter (e.g. for demagnetization sequences)

[0084] - enables the control circuit to be stopped without completely powering it off, in order to perform a check sequence (e.g. for safety functions or for putting a fluxgate sensor into sleep mode)

[0085] - Provides several measurement channels, including analog and PWM channels, for example for inherent redundancy and safety functions.

[0086] In an embodiment of the present invention, in addition to the winding of the excitation coil, the fluxgate device 2 may also include a test winding 10 wound on the magnetic core 6. The test winding increases the diagnostic range of the sensor, thereby enabling high safety standards, such as the highest ASIL level, to be achieved in a simple architecture. One of the applications that an embodiment of the present invention may advantageously implement is measuring DC and AC currents if used in an automotive on-board charger application.

[0087] The test winding can be connected to a connection interface 44 of the fluxgate converter for connection to the electronic circuit of the user system 3. The electronic circuit of the user system is configured to inject a test current into the test winding and verify the measurement output of the fluxgate converter to check the accuracy and normal function of the converter. The measurement outputs from both the PWM measurement channel (first measurement channel M1) and the analog measurement channel (second measurement channel M2) can be checked relative to the expected measurement value corresponding to the injected test current and / or compared to each other.

[0088] Injecting a test current in the test winding is a good way to verify the normal operation of the sensor while testing. When the test is completed and a problem occurs during normal operation (for example, loss of accuracy due to component drift), it is necessary to wait for a new test cycle to verify the incorrect behavior of the sensor, and additional mechanisms are often required to diagnose the problem online. Injecting current in the test winding is not a redundant path because it is only used during specific time periods and intervals (before startup) and not during nominal operation of the sensor. The test winding can be operated and controlled by the user system to check the linearity of the fluxgate converter measurement output within the operating range, however, this test procedure must be performed faster than the reaction time required by the fluxgate converter. The larger the operating range, the longer the duration of the linearity check, which will require reducing the reaction time of the fluxgate converter to enable the check.

[0089] The user system may also or alternatively use a test winding to verify the DC trip curve and the AC trip curve, but not necessarily linear. The user system injects current in the test winding and measures the duration required for the sensor to trip.

[0090] According to one aspect of the invention, the test winding is configured to be used during operation of the fluxgate converter to provide another measurement channel (third measurement channel M3). The test winding can be connected to a signal processing circuit in the user system 3 that performs PWM measurements (similar to the PWM measurements included in the fluxgate converter). The test winding voltage has a square wave shape, and its duty cycle varies proportionally with the leakage current. The user system measures the duty cycle, for example, using the zero crossing of the test winding voltage, whereby a comparator can be used to detect a 0V transition.

[0091] The user system 3 may perform a comparison of three measurement channels M1, M2, M3, two of which are processed within a fluxgate converter that outputs two measurement signals to the user system 3, and one of which (from the test winding) is processed in the user system to calculate a third measurement value. A difference (e.g., a difference greater than a predetermined threshold) in the measurement values ​​output by the three measurement channels may be used to detect a fault in the measurement system. For safety functions, in the event of a fault in the measurement system, the current measurement system may be configured to rely on two of the three measurement signals having values ​​closest to each other and to discard the other measurement signal.

[0092] Additionally, in an embodiment, the fluxgate converter may be configured to internally calculate comparison results of the analog and PWM measurement channels M1, M2, which already provides a first level of safety, and then the measurement values ​​output by the fluxgate converter may be compared with the measurement values ​​calculated from the signal of the test winding in the user system, thereby providing a second level of safety.

[0093] Any common mode causes of failure or faulty operation of the fluxgate converter, caused by, for example, faults in the magnetic core or field coils, can be checked by other diagnostics such as:

[0094] - measure the oscillation frequency of the fluxgate using a test winding (a wrong frequency means a wrong operation (e.g. the magnetic circuit is damaged, the automatic oscillation circuit is not working correctly);

[0095] -Measurement of the amplitude of the voltage on the test winding during operation (damage to the center-tapped bridge component, addition of resistors);

[0096] - Verify that the demagnetization sequence has taken place by monitoring the test winding and verifying the frequency (a good frequency sequence means in particular that the magnetic reset is good);

[0097] The channel measurement values ​​on the user side are preferably PWM measurement values, since these do not interfere with each other in contrast to analog measurement values.

[0098] The two windings forming the center-tapped excitation coil preferably have the same number of turns, but the number of turns of the test winding can be different from that of the excitation coil. In particular, the test winding can advantageously have more turns in order to have a higher voltage.

[0099] The proposed safety architecture can be integrated into providing a fluxgate converter with a winding accessible to a user system, which can be used to inject test current and test the converter internal measurement channel, or use it as an external measurement channel directly connected to the user system. The advantage of the test winding configuration of the present invention is that a single component is used for both: injecting current to test the converter, and measuring the primary current in an additional (redundant) channel to ensure safety. In the case of using the test winding to test the linearity over the entire operating range in real time, the bandwidth and reaction time of the measurement channel on the user system side are equivalent to the fluxgate converter compared to the reduced bandwidth. Typically at power-on, the circuit of the user system injects current in the test winding and verifies the measurement value. If the measurement is successful, the circuit of the user system activates the measurement of the test winding.

[0100] Typically at startup (or before live parts are energized), the user system uses a test winding to inject current into the fluxgate converter. The user system injects current into the test winding and verifies that the injected value corresponds to the value measured by the measurement channel inside the converter. The user system can also measure PWM directly on the test winding while injecting current. Finally, the user system performs a comparison of the injected current measured in the user system with the injected current measured in the fluxgate converter. If the comparison value is lower than a predetermined threshold, it means that the test winding and the measurement channel inside the fluxgate converter are working properly.

[0101] Only when this test passes does it mean that the fluxgate converter is operating properly (meaning the leakage current is measured correctly).The user system can then safely energize the live parts and the user system disconnects the injection circuit to prevent it from interfering with the operation of the fluxgate converter.

[0102] According to one aspect of the invention, a fluxgate converter may include a sequence for resetting the magnetic circuit to a deterministic magnetic circuit, which allows ensuring that the magnetic offset of the fluxgate converter remains at one setting during calibration. The sequence may advantageously operate in the following situations:

[0103] - Every time the fluxgate converter is powered on

[0104] -According to the user system's request

[0105] -After overcurrent

[0106] One of the main issues addressed concerns the way the fluxgate stops its operation when powered down, which affects the magnetic offset when restarting. Resetting is also important when the current through the fluxgate converter exceeds the measurement range (overcurrent).

[0107] In the stop sequence of the fluxgate converter 1, when the power is cut off, the voltage supplied to the fluxgate converter begins to decrease rapidly, whereby once the amplitude of the voltage drops below a certain level, the amplitude of the excitation current drops below a threshold value corresponding to the value of the DAC input terminal 16b of the comparator 24. Therefore, the comparator 24 is not triggered, the high-speed PWM module 28 is not triggered, and the PWM module 28 will operate the fluxgate converter in a fixed frequency mode (e.g., set to 5kHz in the illustrated example).

[0108] In this example, before power is removed, the fluxgate core oscillates automatically at a certain frequency (e.g., at 20kHz) with substantially constant flux densities +Hsat and -Hsat at saturation. After power is removed, the fixed frequency controlled by the PWM module 28 is set to a lower value (e.g., 5kHz in this example), which results in the fluxgate core having variable (uncontrolled) flux densities +Hsat and -Hsat. From a magnetic point of view, this corresponds to switching from a high frequency hysteresis cycle with a defined saturation depth (where the flux densities +Hsat and -Hsat are set by the DAC value input of the comparator 24) to a low frequency hysteresis cycle with an undefined saturation depth. One reason for this undefined saturation depth is that the lower the frequency, the deeper the saturation, first, because the longer the half cycle, the higher the current can rise if the supply voltage is high enough to overcome the resistance of the current path, and second, because the core will stay in a saturated state longer even after the finite voltage cannot increase the current further.

[0109] At the end of the shutdown sequence, the magnetic material will stop in an unknown state initiated by the low frequency hysteresis cycle.

[0110] At the next power-on, the fluxgate converter will start from this unknown state and quickly stabilize in the high frequency auto-oscillation state, but the transition to the high frequency state does not completely restore the magnetic state of the magnetic material. The magnetic excursion is partially affected by the shutdown sequence. The difference comes from the fact that during normal auto-oscillation operation, the hysteresis period is swept at a high frequency, and during shutdown, the hysteresis period is swept at a low frequency. When the frequency is changed, the magnetic material exhibits different magnetic properties and hysteresis periods. This is also reinforced by the fact that the saturation depth is different in the two cases.

[0111] The first solution to avoid starting from an unknown magnetization state of the core is to prevent the fluxgate converter from operating at low frequencies and stop the oscillations as soon as the excitation signal voltage input to the comparator drops below a certain level above the DAC input value. The instant at which the fluxgate converter should stop should be determined in order to ensure that the magnetic circuit always remains in the same state.

[0112] A second preferred solution consists in executing a specific start-up sequence without verifying the stop state of the fluxgate converter in order to reset the magnetic state of the magnetic material to a defined state which will be definitive.

[0113] exist Figure 4a An example of an advantageous sequence is illustrated in . The sequence can be summarized into two sequences labeled Degauss (Part 1) and Degauss (Part 2). The label Degauss in the sequence is not intended to refer to a conventional degaussing process in which the magnetic field and flux density are equal to 0, but is instead intended to refer to a degaussing process in which a deterministic magnetic state of the magnetic material (i.e., with a known or predetermined magnetic offset) is applied. The method of reinitializing the magnetic state of the magnetic circuit can be used after power-on, overload, or any situation that may have an adverse effect on the magnetic state of the magnetic material (e.g., closing a relay that charges a capacitor with a high peak current).

[0114] Normal Operation ("Measurement"):

[0115] The switch S3 is opened to limit the fluxgate saturation; when the comparator detects that the current through the second resistor R2 has increased to a value higher than the output voltage of the DAC supplied to the comparator input 16b, S1 and S2 are alternately closed and opened. After each opening of the switch, a certain dead time determined by the PWM module passes, after which the switch that was opened during the first half cycle is closed via the logic 30 (for example, using a flip-flop).

[0116] Degaussing (Part 1):

[0117] Step (a) - The control circuit begins the demagnetization process by disconnecting the excitation coil switches S1, S2 (i.e., setting them to off) at point (a) to stop the oscillation of the fluxgate device 2. The bypass circuit switch S3 may also be actuated by the control circuit to close the switch (i.e., set to on), however, in some embodiments, the bypass circuit switch S3 may remain open (i.e., remain set to off). Whether the bypass switch is actuated to close or remain open will depend on the magnetic properties of the material used for the fluxgate core. When the excitation coil switches S1, S2 are disconnected, the signal to the first input terminal 16a of the comparator 24 also remains constant, thereby cutting off the comparator output signal.

[0118] Step (b) - The DAC 26 is then controlled by the signal processing module 43 to output a high amplitude first voltage signal, for example, close to its maximum value (the corresponding fluxgate turn-off current is too high to be reached). Starting from time d (the end of the comparator delay point (d)), the fluxgate converter operates at a fixed frequency defined by the configuration value in the PWM module, the number of half cycles of this sequence being defined by the signal processing unit 43. The excitation current and thus the saturation depth are naturally limited by the supply voltage and the first resistor 12 and the second resistor 20 of the circuit.

[0119] Typically, the frequency must be lower than the minimum oscillation frequency reached during the turn-off sequence, and the lower the better. Since this sequence can be time-consuming for the start-up of the fluxgate converter, the number of half cycles and the duration of the entire sequence should be minimized, for example, less than ten half cycles, for example, between one and five half cycles. During this sequence, the current consumption of the fluxgate converter is maximum or close to maximum. After this sequence, the fluxgate core will be in a high magnetization state in one polarity or the other, depending on the polarity of the last half cycle, which is well defined because the sequence is the same every time it is applied.

[0120] Step (e) - The fixed frequency step is ended by the control circuit, thereby stopping the oscillation of the fluxgate device 2 by opening the field coil switches S1, S2 (ie, setting them to OFF) at point (e).

[0121] Step (f) - The high amplitude first DAC voltage signal supplied to the second input terminal 16b of the comparator is set to a nominal voltage amplitude, which may be a zero or non-zero nominal voltage amplitude lower than the high amplitude voltage signal.

[0122] Degaussing (Part 2): During or after the DAC delay (point (g)) and the comparator delay (point (h)), the excitation coil switches S1, S2 are closed, and the bypass circuit switch S3 may also be actuated by the control circuit to change state (i.e., set to open) by closing the switch. Whether the bypass switch S3 is actuated depends on whether it has been closed in the previous sequence.

[0123] Step (i) - The DAC voltage signal supplied to the second input terminal 16b of the comparator is set to a medium-high second voltage amplitude, which is greater than the nominal voltage amplitude and lower than the high amplitude first voltage signal. Therefore, the fluxgate converter operates in an automatic oscillation sequence, in which the saturation depth is slightly reduced from a high value to a nominal value. This process is available when the first resistor 12 is short-circuited (switch S3 is closed) and the saturation depth increases. Since the fluxgate converter operates in an automatic oscillation, the excitation current increases only during the saturation phase of the magnetic core, and the power consumption is reduced compared to the previous sequence (although still higher than the nominal operating mode).

[0124] Step (j) - After a short sequence of medium to high second voltage amplitudes (point (j)), the amplitude of the DAC voltage signal supplied to the second input 16b of the comparator gradually decreases back to the nominal voltage (point (k)) over the slope duration.

[0125] Step (k) - Bypass circuit switch S3 may also be actuated by the control circuit to open the switch (i.e., set to OFF), however in certain embodiments, bypass circuit switch S3 may remain closed (i.e., remain set to ON). Whether the bypass switch is actuated to open or remain closed will depend on the magnetic properties of the material used for the fluxgate core.

[0126] Step (m) - After a certain bypass switch change delay, the converter measurement process can begin.

[0127] exist Figure 4a middle:

[0128] - DAC Latency: The delay in microseconds required for the DAC to reach the desired setting. This parameter should be higher than the settling time of the DAC itself;

[0129] - Comparator Delay: The delay in microseconds necessary for the comparator to reach its actual value after activation. This parameter should be higher than the comparator power-on / activation delay or the comparator filter tempering time delay;

[0130] - Low frequency duration: The duration is expressed in milliseconds and corresponds to the desired duration that the fluxgate should oscillate at a predetermined fixed frequency;

[0131] - S3 change delay: The delay necessary for the fluxgate to reach the steady-state auto-oscillation frequency after the state of switch S3 changes, expressed in microseconds. State configurable means that switch S3 can be open or closed depending on the desired setting;

[0132] - TOP Duration: This duration is expressed in microseconds and corresponds to the expected duration that the fluxgate should automatically oscillate at high current (DAC high);

[0133] - Slope duration: This duration is expressed in microseconds and corresponds to the slope of the DAC falling from high to nominal. The fluxgate is in auto-oscillation mode and the frequency will increase slightly while the DAC value decreases.

[0134] Figure 4bThe magnetic offset of the fluxgate converter without using the above sequence (upper curve) and with using the above sequence (lower curve) is illustrated. As can be seen from the upper curve, without using the proposed startup sequence, the range of magnetization values ​​is larger, resulting in a magnetic offset that is uncertain and cannot be accurately compensated in the measurement output. In the lower curve using the proposed startup sequence, the range of magnetization values ​​is much smaller, making it possible to define a magnetic offset that can be compensated with higher accuracy in the measurement output.

[0135] Features List

[0136] Current measurement system

[0137] Fluxgate Current Converter1

[0138] Fluxgate Device 2

[0139] Saturable soft magnetic core 6

[0140] Excitation coil 8

[0141] First coil 8a

[0142] Second coil 8b

[0143] Test winding 10

[0144] Testing the winding interconnection interface 44

[0145] Control circuit 4

[0146] The first resistor 12

[0147] First resistor bypass circuit 14

[0148] Bypass switch S3

[0149] Field coil switch

[0150] First switch S1

[0151] The second switch S2

[0152] The second resistor 20

[0153] Microcontroller (MCU) 22

[0154] Comparator 24

[0155] DAC 26

[0156] PWM module 28

[0157] Combinational and sequential logic units 30 (configurable)

[0158] Capture module 32

[0159] First input capture unit 32a

[0160] Second input capture unit 32b

[0161] The first resistor switch controls the output terminal (IO) 34

[0162] Excitation coil control output terminal 36

[0163] ADC 18

[0164] Amplifier 42

[0165] Reference voltage source 38

[0166] Temperature measurement module 40

[0167] Signal processing module 43

[0168] The first measurement output channel M1

[0169] Second measurement output channel M2

[0170] The third measurement output channel M3

[0171] User System 3

[0172] Interconnection Circuit

[0173] Microcontroller 5

Claims

1. A fluxgate current converter, the fluxgate current converter comprising a control circuit (4) and a fluxgate device (2), the fluxgate device comprising a saturable soft magnetic core (6) surrounded by an excitation coil (8), the control circuit comprising an excitation coil drive circuit connected to the excitation coil, the excitation coil being configured to generate an alternating magnetic field to alternately saturate the soft magnetic core, the excitation coil comprising a first winding (8a) and a second winding (8b) connected together at a common point (9), the common point forming an input of the first winding and an output of the second winding, the first winding being connected in series with a first switch (S1) and the second winding being connected in series with a second switch (S2), the first switch and the second switch being controlled by the control circuit to provide an excitation coil current in the first winding, thereby generating a magnetic field in a first direction in the magnetic core, and the excitation coil current in the second winding generating a magnetic field in the magnetic core in a second direction opposite to the first direction, wherein The fluxgate current converter is connected to a user system (3) to form a current measurement system together, wherein the current measurement system has at least three measurement channels, namely a first PWM channel M1, a second analog channel M2 and a third channel M3 provided by a test winding (10), and the user system is configured to calculate a comparison result of the measurement signals from the three measurement channels in order to determine correct or incorrect operation.

2. The fluxgate current converter according to claim 1, wherein: The control circuit includes a comparator (24) having a first input terminal (16a) connected to the excitation coil after the first switch and the second switch.

3. The fluxgate current converter according to claim 2, wherein: The second input of the comparator is connected to a DAC (16b), which supplies an adjustable reference voltage for adjusting the output level of the comparator.

4. The fluxgate current converter according to claim 2, wherein: The control circuit includes a PWM module (28) connected to the output of the comparator and configured to control the timing of operation of the first switch and the second switch, including introducing a dead time during switching from one field coil winding to the other field coil winding.

5. The fluxgate current converter according to claim 4, further comprising a combinational and sequential logic unit (30) connected to the output of the PWM module (28) to generate a complementary signal including a dead time provided by the PWM module.

6. The fluxgate current converter according to claim 4, comprising a capture module (32) connected to the output of the PWM module (28) to capture the timing of the switching signal.

7. The fluxgate current converter according to claim 1, wherein: The fluxgate current converter comprises an analog measurement channel (M2) connected to the excitation coil after the first switch and the second switch, the analog measurement channel comprising an ADC module (18).

8. The fluxgate current converter according to claim 7, wherein: The analog measurement channel also includes a DAC (26) connected to the ADC module and also connected to one input terminal (16b) of a comparator (24) having a first input terminal (16a) connected to the excitation coil.

9. The fluxgate current converter according to claim 8, wherein: The analog measurement channel also includes an amplifier (42) to increase the gain and scale the offset of the measured value of the signal supplied to the ADC module.

10. The fluxgate current converter according to claim 1, wherein: The control circuit also includes a first resistor connected to the high side of the fluxgate device, and the control circuit also includes a third switch S3 arranged in a bypass circuit (14) in parallel with the first resistor, and the third switch is configured to short-circuit the first resistor when the switch is switched to a closed position.

11. The fluxgate current converter according to claim 7, wherein: The analog measurement channel includes a reference voltage source to control a power supply reference voltage of the ADC module.

12. The fluxgate current converter according to claim 1, wherein the test winding (10) is wound on the magnetic core (6), and the test winding is connected to a connection interface (44) of the fluxgate current converter to be connected to an electronic circuit of a user system (3).

13. The fluxgate current converter according to claim 12, wherein: The test winding is configured to receive a test current from the user system (3) and output a voltage to the user system (3) in a second operation mode corresponding to a measurement signal of a magnetic field in the fluxgate device, so that the test winding provides a third measurement channel.

14. The fluxgate current converter according to claim 1, wherein: The electronic circuit of the user system (3) is configured to use two of the three measurement signals having values ​​closest to each other in the event of a fault affecting one of the measurement channels and to discard the other measurement signal.

Images