Current transducer

Abstract

An open loop current transducer (1) for measuring an electric current flowing in a primary conductor (19) comprises a magnetic circuit core (3) having an air gap (4), and a magnetic field sensing device (5) positioned at least partly in the air gap, the magnetic field sensing device comprising a circuit board, a first magnetic field detector (8) in the form of an ASIC mounted on the circuit board, and a second magnetic field detector (12) in the form of a conductive pick-up coil formed on two or more layers in or on the circuit board. The outputs of the first and second magnetic field detectors are connected to a signal processing circuit (6) producing an output signal representative of the electric current, the outputs of the ASIC representing a low frequency (LF) channel, and the outputs of the pick-up coil representing a high frequency (HF) channel. The signal processing circuit comprises an adding circuit configured to sum the signals output by the LF channel and the signals output by the HF channel, and comprises one or more circuit components to adjust the gain of the output signals of the pick-up coil to the gain of the output signals of the ASIC.

Description

[0001] This invention relates to an open-loop current transducer for measuring current in the high-frequency range.

[0002] Wide bandgap semiconductors have opened up the possibility of significantly faster switching frequencies and steeper state transitions, resulting in switching currents with higher di / dt and switching voltages with higher dv / dt. Many applications use current measurements as inputs to their control loops, and fast, accurate current measurements are required to achieve advanced control schemes, leading to bandwidth requirements that are one to two orders of magnitude higher than previous generations of current transducers (100kHz -> 5MHz).

[0003] Open-loop current transducers are widely used to measure current flowing in a primary conductor. Such sensors typically include a magnetic core surrounding the primary conductor, with an air gap in which a magnetic field detector is positioned. The magnetic field detector can typically be a Hall effect sensor or a magnetoresistive magnetic field sensor. However, Hall sensors have limited bandwidth. Magnetoresistive sensors may have high bandwidth, but they have limited offset stability and 1 / f noise, and they are also susceptible to perming and hysteresis.

[0004] It is known that coreless current transducers are advantageous for measuring large currents, but for lower currents, the magnetic signal is small, and such transducers are therefore inferior to current sensors with gapped magnetic cores for applications requiring accurate current measurement over a large current range.

[0005] The open-loop transducer for high-bandwidth and high-current-range measurements, as described in EP1965217, combines the high-frequency measurement capabilities of a planar pickup coil with a Hall sensor. The pickup coil is implemented as a circuit trace on a PCB, connected in series with the output of an amplifier connected to the Hall sensor. One drawback of this known transducer is that the crossover frequency at which the coil begins to function is approximately an order of magnitude lower than the natural roll-off of the Hall sensor unit, resulting in a large deviation from the ideal flat frequency response over the high-current measurement range.

[0006] Other conventional current transducers with large operating frequency bandwidths typically suffer from limitations in amplitude range for measurements requiring desired accuracy. Furthermore, conventional open-loop current sensors with large operating frequency bandwidths typically have response times exceeding 2µs (2 x 10⁻⁶). -6 (seconds), which may be too slow for some applications.

[0007] Therefore, the object of the present invention is to provide an open-loop current transducer having: a wide bandwidth for measuring current with a short response time, and a large amplitude range with high offset stability and low deviation from the ideal flat frequency response within the measurement range.

[0008] Wide bandwidth is available, specifically from 0 (DC) to approximately 1-5 MHz. High current amplitude range can be from approximately 0 A to 50 A to a maximum of 300 A.

[0009] It is advantageous to provide a compact and economically manufactured current transducer.

[0010] It is advantageous to provide a compact current transducer that employs a simple signal processing circuit.

[0011] A further advantage is the provision of a current transducer with low power consumption, particularly for applications related to autonomous devices.

[0012] The object of the present invention has been achieved by providing an open-loop current transducer according to one aspect of this disclosure.

[0013] This document discloses an open-loop current transducer for measuring current flowing in a primary conductor, comprising a magnetic core with an air gap and a magnetic field sensing device including a circuit board, a first magnetic field detector in the form of an ASIC mounted on the circuit board, and a second magnetic field detector in the form of a conductive pickup coil formed on or overlaid on the circuit board below and to the ASIC, wherein the ASIC and the pickup coil are positioned in the air gap. The outputs of the first and second magnetic field detectors are connected to a signal processing circuit that generates an output signal representing the current, the output of the ASIC representing a low-frequency (LF) channel and the output of the pickup coil representing a high-frequency (HF) channel. The signal processing circuit includes an adder circuit configured to sum the signal output from the LF channel with the signal output from the HF channel, and one or more circuit components, including at least a gain adjustment resistor (Rac) connected in series with the output of the pickup coil to adjust the gain of the output signal of the pickup coil to the gain of the output signal of the ASIC.

[0014] In an advantageous embodiment, the adder circuit includes an operational amplifier, a resistor connected across the operational amplifier between the negative input and the output of the operational amplifier, and a capacitor connected across the operational amplifier between the negative input and the output of the operational amplifier, thus forming an inverting summing circuit with low-pass filtering.

[0015] The transducer according to embodiments of the invention advantageously allows for primary current measurement over a bandwidth from DC to several MHz, exhibiting frequency response flatness better than ±1 dB and highly stable offset, typically less than 0.1% of full scale. Furthermore, the open-loop response time achievable with the transducer according to embodiments of the invention can be approximately or less than 10 ns (10 x 10⁻⁶). -9 Second).

[0016] The transducer according to an embodiment of the invention also advantageously has a good signal-to-noise ratio (SNR) because the output noise from the ASIC is filtered by a low-pass filter of the operational amplifier circuit.

[0017] In an advantageous embodiment, the positive input of the operational amplifier is connected to a reference voltage source of the signal processing circuit.

[0018] In an advantageous embodiment, the positive input of the operational amplifier is also connected to an external voltage reference connection via a resistor.

[0019] In an advantageous embodiment, the reference voltage source is connected to the positive input of the operational amplifier via a series resistor and a parallel capacitor configured to adjust the impedance and reduce noise at the voltage reference input.

[0020] In an advantageous embodiment, the gain adjustment resistor is adjustable: by laser fine-tuning of the gain adjustment resistor; by setting the gain adjustment resistor as a programmable resistor implemented on an integrated circuit; or by adding or removing one or more additional resistors connected in parallel with the gain adjustment resistor.

[0021] In an advantageous embodiment, the signal processing circuit includes a capacitor connected in series with the output of the pickup coil.

[0022] In an advantageous embodiment, the magnetic field sensing device includes an electrostatic shield connected to ground and disposed on both sides of and overlapping the pickup coil, configured to reduce capacitive coupling between the primary conductor and the pickup coil. The electrostatic shield includes a plurality of conductive circuit traces on a circuit board, each of which is separated by a non-conductive gap configured to reduce eddy currents.

[0023] In an advantageous embodiment, the pickup coil includes at least two coil portions located on different layers embedded within a circuit board substrate and connected in series via conductive connection paths through these layers.

[0024] In an advantageous embodiment, an electrostatic shield is formed on the opposite outer side of the circuit board.

[0025] Other objects and advantageous features of the invention will become clear from the claims, detailed descriptions, and drawings, wherein:

[0026] Figure 1a This is a perspective view of a portion of a current transducer (with the housing removed) according to an embodiment of the present invention;

[0027] Figure 1b yes Figure 1a A side view of the portion of the current transducer;

[0028] Figure 1c Is it through Figure 1b A cross-sectional view of line 1c-1c;

[0029] Figure 2 This is a schematic perspective view of the pickup coil of a current transducer according to an embodiment of the present invention;

[0030] Figure 3 This is a block diagram of the signal processing circuit of a current transducer according to an embodiment of the present invention;

[0031] Figure 4 This is a circuit diagram of the signal processing circuit of a current transducer according to an embodiment of the present invention.

[0032] Referring to the figures, a current transducer 1 for measuring the current flowing in the primary conductor 19 passing through the central aperture 2 of the transducer includes: a magnetic circuit 3 having an air gap 4 and a magnetic field sensing device 5 at least partially positioned in the air gap. As is known in the art, the magnetic circuit 3 can be made of stacked sheets or a solid core with different shapes and sizes and different magnetically conductive materials. As is known in the art, the magnetic circuit can also be divided into two parts to allow the sensor components to surround the primary conductor. Additionally, as is known in the art, a portion of the primary conductor 19 can be integrated into the sensor and has terminals for connection to the primary conductor.

[0033] In a preferred embodiment, the magnetic core is made of a ferrite material to better support high-frequency primary currents compared to a soft iron core.

[0034] The current sensor may include a second air gap and a second magnetic field sensing device at least partially located in the second air gap.

[0035] The magnetic field sensing device includes a circuit board 7, a first magnetic field detector 8, a second magnetic field detector 12, and a signal processing circuit 6 mounted on the circuit board.

[0036] The signal processing circuit 6 is located on the circuit board 7 and connected to the magnetic field detectors 8 and 12.

[0037] Connection terminal 20 connected to the circuit board allows connection to an external circuit system (not shown) of a device (e.g., an electric motor), where the transducer is implemented to measure primary current (e.g., the phase current of the electric motor), typically used to control the function of the device (e.g., the operation of the motor).

[0038] In a preferred embodiment, the first magnetic field detector 8 is in the form of a Hall effect sensor included in an integrated circuit (ASIC), having connection terminals 10, for example, connected to circuit traces on a circuit board 7 by soldering. However, the first magnetic field detector may also include other types of known magnetic field sensors, such as magnetoresistive magnetic field sensors or fluxgate magnetic field sensors. In a preferred embodiment, the Hall effect sensor may operate using a known current spin technique to reduce offset and 1 / f noise.

[0039] The second magnetic field detector 12 of the magnetic sensing device includes a conductive coil comprising at least two coil portions 12a, 12b formed by circuit traces on or within a circuit board. The coil portions may be connected in series to form a single pickup coil. The coil portions may be connected together by means of conductive plated vias 14 or interconnecting pins or paths through the board to interconnect the coil portions. The coil can therefore be formed on the circuit board using conventional techniques for generating conductive traces on the circuit board, thus benefiting from the presence of a circuit board for connecting the first magnetic field detector and the signal processing or preprocessing circuitry 6. The coil is preferably connected to the ASIC's reference voltage VREF rather than ground to reduce noise at the coil output.

[0040] The circuit board may advantageously also include electrostatic shielding elements 13a, 13b on the top and bottom layers, such that the pickup coil 12 is sandwiched between the electrostatic shielding elements forming an electrostatic shielding layer around the pickup coil. The shielding layer reduces direct or capacitive coupling between the pickup coils 12a, 12b and the primary conductor 19, or via the magnetic core 3. This provides protection against electrical interference, particularly that generated by primary currents with high voltage change rates (high dV / dt). The electrostatic shielding elements can be formed on the circuit board using conventional techniques for creating conductive traces on the circuit board and can be connected to ground or another reference voltage connection. The electrostatic shielding elements can be formed on the pickup coil on opposite outer surfaces of the circuit board and on an internal embedded layer, but they can also be arranged on a layer embedded within the circuit board substrate. The electrostatic shielding elements may advantageously have a comb-like structure with multiple conductive traces separated by non-conductive traces to reduce the formation of eddy currents in the shielding layer. The conductive tracks are preferably electrically interconnected only at one end to prevent eddy currents from circulating in the loop; otherwise, the loop will be formed by the tracks if both ends are connected.

[0041] Multilayer pickup coils have increased gain compared to single-layer coils and advantageously allow for a reduction in the cutoff frequency of the low-pass filter to obtain a low-noise output of the signal from the pickup coil.

[0042] The signal processing circuit 6 according to an embodiment of the invention includes a circuit system for combining an output signal representing a low-frequency (LF) channel from a first magnetic field detector 8 and an output signal representing a high-frequency (HF) channel from a pickup coil 12. These two output signals are advantageously combined by summing the LF and HF output signals using an adder circuit 24, which may advantageously include a low-pass filter 25.

[0043] In a preferred embodiment, the adder circuit 24 with a low-pass filter 25 includes an operational amplifier 26 and a resistor R1 and a capacitor C1 connected across the operational amplifier between its first input (negative input) and output, thus forming an inverting summing circuit with a low-pass filter. The operational amplifier 26, resistor R1, and capacitor C1 form an active first-order low-pass filter. The second input (positive input) of the operational amplifier is connected to a reference voltage.

[0044] The reference voltage input can be connected to a reference voltage source VREF (e.g., a 2.5 V voltage source) of the signal processing circuit, optionally via a series resistor Rr2 and a parallel capacitor Cr, to adjust impedance and reduce noise. The level of the input reference voltage can optionally be adjusted via an external connection VREF_IN connected to the second input of the operational amplifier via resistor Rr1. Resistor Rr1 allows the user of the current transducer to adjust the voltage level of the transducer output measurement signal Uout to user requirements, particularly the requirements of the external circuitry to which the transducer is connected via connection terminal 20. For example, this allows the user to change the voltage level of the output measurement signal so that it remains positive current throughout the transducer's measurement range, which facilitates device control in many applications.

[0045] The gain of the first magnetic field sensor included in the ASIC can be adjusted by setting the current output level of the ASIC through a resistor Rdc connected to the ASIC output terminal OUT and / or by adjusting a register in the ASIC's integrated circuit.

[0046] In the HF channel connected to the pickup coil 12, the high-pass filter 21 can be formed by a capacitor Cac that removes any DC or low-frequency signals (preferably signals with frequencies below 10 Hz). The capacitor Cac helps reduce the gain of the HF path at DC or low frequencies and avoids offset of the amplified operational amplifier. The pickup coil gain can be advantageously adjusted to match the gain of the LF channel, such that the sum of the two signals has a frequency response close to an ideal flat frequency response, preferably better than + / -1 dB, while benefiting from a highly stable offset. This stable offset arises from a combination of providing a Hall effect sensor in an ASIC driven by high-frequency current spin technology but operating under low-frequency primary current measurement, and on the other hand, providing a multilayer pickup coil formed on a circuit board (thus allowing for a larger magnetic field pickup surface area than the ASIC), operating at higher frequencies, which inherently has no offset drift. Compared to the series connection found in the prior art, the signal summation according to the invention allows for a flattened frequency response, whereas in a series connection, the crossover frequency at which the coil begins to function is about an order of magnitude lower than the natural roll-off of the Hall sensor unit, which results in a large deviation from the ideal flat frequency response over a high current measurement range.

[0047] Gain adjustment of the HF channel can be advantageously achieved by connecting a resistor Rac in series with the output of the pickup coil. The resistance value of the HF channel gain adjustment resistor Rac can be adjusted in several ways, including: by laser fine-tuning of the resistor Rac; by setting Rac as a programmable resistor implemented on an integrated circuit; or by adding or removing one or more additional resistors connected in parallel with Rac.

[0048] The LF channel may also include a sensitivity adjustment component 23, for example, to allow calibration of the transducer, such as to compensate for variations in the length of the gap 4. The sensitivity adjustment component may be implemented as a programmable element in the ASIC for adjusting the output level OUT, or by using a programmable gain amplifier (PGA) (not shown) at the output of the ASIC.

[0049] The following is a further description Figure 4 Features of the working principle of the embodiments shown:

[0050] LF path Below the crossover frequency defined by the time constant R1×C1, the operational amplifier circuit amplifies the ASIC output voltage by a factor R1 / Rdc. Without other components, this circuit would be a simple active first-order low-pass amplifier with the same corner frequency mentioned above.

[0051] HF pathThe idealized output voltage of the pickup coil rises proportionally to the frequency of the measured current, with its phase leading by 90°. It should pass through an integrator within the frequency range where the signal serves as the image of the primary current. This is the circuit shown for frequencies above the crossover frequency, where the pickup coil voltage is higher than the ASIC output voltage. To achieve a smooth transition between the two frequency ranges, the HF gain should be adjusted so that the output voltage of the pickup coil at the crossover frequency has the same amplitude as the amplitude from the ASIC. In practice (due to parasitic elements not shown in the circuit diagram, nonlinearities, etc.), a slightly better response in the time domain can be obtained by adjusting the HF gain to be slightly higher than the LF gain.

[0052] Preferred values ​​for the component:

[0053] The capacitor Cac should have a high capacitance value, preferably between 5 and 20 µF, for example around 10 µF, to achieve a good cost-performance ratio.

[0054] The values ​​of resistor R1 and capacitor C1:

[0055] The lower the cutoff frequency, the lower the output noise (there are limitations given by the actual size of the pickup coil; small pickup coils with fewer turns can supply sufficiently high voltage at higher frequencies).

[0056] C1: Preferably, a capacitor between 1nF and 10nF is used;

[0057] R1: The resistance value can vary between 30 kQ and 200 kQ depending on the desired gain; a crossover frequency of around 1 kHz is preferred; for a given crossover frequency, increasing C1 improves the dv / dt perturbation effect (from 1 nF to 10 nF, an increase of 10 times).

[0058] However, as the value of capacitor C1 increases, the gain adjustment resistor Rac needs to be reduced in order to keep the crossover frequency at the desired value, such as 1 kHz, and the performance of di / dt degrades. Therefore, a trade-off needs to be found based on the desired transducer sensitivity.

[0059] The gain adjustment resistor Rac should preferably be higher than 1 kΩ, and even better if it is higher (from 1 kΩ to 30 kΩ; depending on the transducer gain).

[0060] Non-limiting but advantageous examples of approximations of other component values ​​may include, for example:

[0061] ●Rdc = R1,

[0062] ●Rr2 = R1 / 2,

[0063] ●Rr1 = R1,

[0064] ●Cr => Approximately 10 µF (as large as possible).

Claims

1. An open loop current transducer (1) for measuring an electric current flowing in a primary conductor (19), comprising a magnetic circuit core (3) having an air gap (4), and a magnetic field sensing device comprising a circuit board, a first magnetic field detector (8) in the form of an ASIC mounted on the circuit board, and a second magnetic field detector (12) in the form of a conductive pick-up coil formed on two or more layers in or on the circuit board below and in overlapping relationship with the ASIC, the ASIC and the pick-up coil being positioned in said air gap, output terminals of the first and second magnetic field detectors being connected to a signal processing circuit (6) producing an output signal representative of said electric current, output terminals of the ASIC representing a low frequency (LF) channel, and output terminals of the pick-up coil representing a high frequency (HF) channel, characterized in that, The signal processing circuit comprises an addition circuit configured to sum the signal output by the LF channel with the signal output by the HF channel, and one or more circuit components including at least a gain adjustment resistor (Rac) connected in series between the output of the pick-up coil and the HF channel input of the addition circuit to adjust the gain of the output signal of the pick-up coil to the gain of the output signal of the ASIC.

2. The transducer of claim 1, wherein the addition circuit comprises an operational amplifier (26), a resistance (Rl) connected across the operational amplifier between its negative input and output, and a capacitance (Cl) connected across the operational amplifier between its negative input and output.

3. The transducer of claim 2, wherein the positive input of the operational amplifier is connected to a reference voltage source (VREF) of the signal processing circuit.

4. The transducer of claim 3, wherein the positive input of the operational amplifier is also connected to an external connection (VREF_IN) via a resistor (Rrl).

5. The transducer of claim 3, wherein the reference voltage source (VREF) is connected to the positive input of the operational amplifier via a series resistance (Rr2) and a parallel capacitance (Cr) configured to adjust the impedance and reduce the noise on the voltage reference input.

6. The transducer of claim 1, wherein the gain adjustment resistor (Rac) is adjustable: by laser trimming of the gain adjustment resistor (Rac); by setting the gain adjustment resistor to a programmable resistor implemented on the integrated circuit; or by adding or removing one or more additional resistors connected in parallel with the gain adjustment resistor.

7. The transducer of claim 1, the signal processing circuit comprising a capacitance (Cac) connected in series to the output of the pick-up coil.

8. The transducer of claim 1, wherein the magnetic field sensing device comprises electrostatic shields (13a, 13b) connected to ground and arranged on both sides of the pick-up coil and overlapping the pick-up coil, configured to reduce the capacitive coupling between the primary conductor and the pick-up coil, the electrostatic shields comprising a plurality of conductive circuit traces on the circuit board, each of the plurality of conductive circuit traces being separated by a non-conductive gap configured to reduce eddy currents.

9. The transducer of claim 8, wherein the pick-up coil comprises at least two coil portions (12a, 12b) located on different layers embedded within the circuit board substrate and connected together in series by conductive connection paths that pass through these layers.

10. The transducer of claim 9, wherein the electrostatic shields are formed on opposite outer sides of the circuit board.

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