WO2018181089A1

WO2018181089A1 - Electric current sensor

Info

Publication number: WO2018181089A1
Application number: PCT/JP2018/011969
Authority: WO
Inventors: 竜麿堀
Original assignee: Tdk株式会社
Priority date: 2017-03-30
Filing date: 2018-03-26
Publication date: 2018-10-04
Also published as: JP2018169302A; JP6551448B2

Abstract

This electric current sensor is provided with: a magnetic core (20) magnetically coupled with an electric current path (P); an H-bridge circuit having first and second connection nodes (N1, N2) alternately inverted in polarity on the basis of an oscillation signal (Q), a power source node (N3) to which a power source voltage (Vcc) is applied, and an output node (N4); a detection coil (Lp) wound on the magnetic core (20) and connected between the first connection node (N1) and the second connection node (N2); a comparator (31) for comparing the voltage (Vc) of the output node (N4) and a reference voltage (Vcmp); a flip-flop circuit (32) for inverting the oscillation signal (Q) on the basis of the output of the comparator (31); and a reference voltage generating circuit (33) for changing the reference voltage (Vcmp) in accordance with the power source voltage (Vcc). Through the present invention, a change in the sensitivity of detection of electric current due to a change in the power source voltage (Vcc) is suppressed.

Description

Current sensor

The present invention relates to a current sensor, and more particularly to a current sensor provided with a self-excited oscillation circuit including an H bridge circuit.

Patent Document 1 discloses a current sensor including an H bridge circuit. The H-bridge circuit has first and second connection nodes whose polarities are alternately reversed, and is wound around a magnetic core between the first connection node and the second connection node. A detection coil is connected. Since the inductance of the detection coil changes depending on the magnetic permeability of the magnetic core, if the magnetic permeability of the magnetic core changes due to the current flowing through the primary coil, the time required for the magnetic core to saturate changes. Therefore, it is possible to know the amount of current to be measured by detecting the time until the magnetic core is saturated.

Special table 2012-526981 gazette

As a method of detecting the time until the magnetic core is saturated, a method of configuring a self-excited oscillation circuit with an H-bridge circuit and monitoring the frequency and duty of the oscillation signal can be considered. When the self-excited oscillation circuit is configured by the H bridge circuit, the output voltage of the H bridge circuit is compared with the reference voltage, and the polarity of the first and second connection nodes is reversed every time the output voltage exceeds the reference voltage. Good.

However, with this method, the current detection sensitivity varies greatly depending on the power supply voltage, so that the amount of current cannot be measured correctly when a power supply voltage different from the specification is used or in an environment where the power supply voltage fluctuates. was there.

Therefore, an object of the present invention is to suppress a change in detection sensitivity due to a change in power supply voltage in a current sensor including a self-excited oscillation circuit including an H bridge circuit.

A current sensor according to the present invention includes a magnetic core magnetically coupled to a current path, first and second connection nodes whose polarities are alternately inverted based on an oscillation signal, a power supply node to which a power supply voltage is applied, and an output node An H bridge circuit including: a detection coil that is wound around the magnetic core and connected between the first connection node and the second connection node; and a voltage and a reference voltage of the output node. A comparator for comparison; an oscillation signal generation circuit for inverting the oscillation signal based on an output of the comparator; and a reference voltage generation circuit for changing the reference voltage in accordance with the power supply voltage.

According to the present invention, since the reference voltage changes according to the power supply voltage, it is possible to suppress a change in current detection sensitivity due to a change in the power supply voltage.

In the present invention, the reference voltage generation circuit preferably makes the power supply voltage and the reference voltage proportional. In this case, it is preferable that the reference voltage generation circuit includes a voltage dividing resistor that divides the power supply voltage. According to this, it is possible to make the power supply voltage and the reference voltage proportional with a simple configuration. Furthermore, in this case, the voltage dividing resistor may include a variable resistor. This makes it possible to adjust the relationship between the power supply voltage and the reference voltage.

The current sensor according to the present invention includes a negative feedback coil that cancels a magnetic flux generated in the magnetic core due to a current flowing through the current path, and a negative feedback current output circuit that flows a negative feedback current to the negative feedback coil based on the oscillation signal. It is preferable to further comprise. According to this, since closed loop control is performed, more accurate measurement can be performed.

As described above, according to the present invention, a change in detection sensitivity due to a change in power supply voltage can be suppressed in a current sensor including a self-excited oscillation circuit including an H bridge circuit.

FIG. 1 is a block diagram showing a configuration of a current sensor 100 according to an embodiment of the present invention. FIG. 2 is a schematic diagram for explaining an example of the positional relationship among the primary coil 10, the magnetic core 20, the detection coil Lp, and the negative feedback coil Lc. FIG. 3 is a circuit diagram of the self-excited oscillation circuit 30. FIG. 4 is a waveform diagram of the oscillation signal Q. FIG. 5 is a graph for explaining the magnetic characteristics of the magnetic core 20 and shows a case where the external magnetic field H _ext is zero. FIG. 6 is a graph for explaining the magnetic characteristics of the magnetic core 20 and shows a case where an external magnetic field H _ext is present. FIG. 7 is a waveform diagram showing changes in the level Vc of the output node N4. FIG. 8 is a waveform diagram showing changes in the oscillation signal Q and the inverted oscillation signal / Q. FIG. 9 is a graph showing the relationship between the reference voltage Vcmp and the detection sensitivity. FIG. 10 is a graph showing the relationship between the reference voltage Vcmp and the detection sensitivity.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

FIG. 1 is a block diagram showing a configuration of a current sensor 100 according to an embodiment of the present invention.

As shown in FIG. 1, the current sensor 100 according to the present embodiment is a device that measures a current Ip flowing in a current path P, and is a magnetic core that is magnetically coupled to a primary coil (bus bar) 10 provided in the current path P. 20, a detection coil Lp wound around the magnetic core 20, and a self-excited oscillation circuit 30 connected to the detection coil Lp. As will be described later, the self-excited oscillation circuit 30 includes an H-bridge circuit, whereby the polarity of the voltage applied to both ends S1, S2 of the detection coil Lp is periodically inverted.

Furthermore, the current sensor 100 according to the present embodiment includes a negative feedback coil Lc, a negative feedback current output circuit 40 that supplies a negative feedback current Io to the negative feedback coil Lc, and a signal that generates a sensor output OUT based on the negative feedback current Io. And an output circuit 50.

FIG. 2 is a schematic diagram for explaining an example of the positional relationship among the primary coil 10, the magnetic core 20, the detection coil Lp, and the negative feedback coil Lc.

In the example shown in FIG. 2, the magnetic core 20 is inserted into the coil axis of the primary coil 10, and the detection coil Lp is wound around the magnetic core 20. Further, a negative feedback coil Lc is disposed in the vicinity of the detection coil Lp, whereby the detection coil Lp and the negative feedback coil Lc are magnetically coupled as indicated by a symbol M. The primary coil 10, the magnetic core 20, the detection coil Lp, and the negative feedback coil Lc are covered with a magnetic shield 70 for shielding an external magnetic field.

When the current Ip flows through the primary coil 10, the magnetic flux B 1 generated thereby flows through the magnetic shield 70, and a part thereof passes through the magnetic core 20. As a result, the magnetic permeability of the magnetic core 20 changes, so that the inductance of the detection coil Lp changes. On the other hand, the negative feedback coil Lc is given a negative feedback current Io that cancels the magnetic flux B1 generated in the magnetic core 20 by the current Ip flowing through the primary coil 10. Thereby, the negative feedback coil Lc generates the magnetic flux B2, and cancels the magnetic flux B1 generated in the magnetic core 20.

FIG. 3 is a circuit diagram of the self-excited oscillation circuit 30.

As shown in FIG. 3, the self-excited oscillation circuit 30 is an H-bridge self-excited oscillation circuit, and switches SW1 to SW4, resistors R1 to R3, a comparator 31, a flip-flop circuit 32, and a reference voltage generation circuit. 33 is provided. The switch SW1 and the switch SW3 are connected in series between the power supply node N3 and the output node N4, and the first connection node N1 that is a connection point thereof is connected to one end S1 of the detection coil Lp via the resistor R1. It is connected. Similarly, the switch SW2 and the switch SW4 are connected in series between the power supply node N3 and the output node N4, and the second connection node N2, which is the connection point, is connected to the detection coil Lp via the resistor R2. Connected to the other end S2. A power supply voltage Vcc is applied to power supply node N3. The output node N4 is grounded via the resistor R3.

The non-inverting input terminal (+) of the comparator 31 is connected to the output node N4, and the reference voltage Vcmp is applied to the inverting input terminal (−). Thereby, when the level Vc of the output node N4 exceeds the reference voltage Vcmp, the output of the comparator 31 changes to a high level. The reference voltage Vcmp is generated by the reference voltage generation circuit 33. The reference voltage generation circuit 33 is a voltage dividing resistor including resistors R4 and R5 connected in series between a power supply line to which a power supply voltage Vcc is applied and a power supply line to which a ground potential is applied. For this reason, the level of the reference voltage Vcmp is proportional to the level of the power supply voltage Vcc.

The output of the comparator 31 is input to the clock node of the flip-flop circuit 32. The oscillation signal Q output from the flip-flop circuit 32 controls the switches SW1 and SW4, and the inverted oscillation signal / Q controls the switches SW2 and SW3. The inverted oscillation signal / Q is fed back to the data node of the flip-flop circuit 32. As a result, the logic levels of the oscillation signal Q and the inverted oscillation signal / Q output from the flip-flop circuit 32 are inverted each time the output of the comparator 31 changes from the low level to the high level. Thus, the flip-flop circuit 32 constitutes an oscillation signal generation circuit that generates the oscillation signal Q and the inverted oscillation signal / Q. However, in the present invention, it is not essential to configure the oscillation signal generation circuit by a flip-flop circuit.

When the self-excited oscillation circuit 30 shown in FIG. 3 is turned on, the switches SW1 and SW4 are turned on and the switches SW2 and SW3 are turned off, the switches SW2 and SW3 are turned on, and the switches SW1 and SW4 are turned off. The second state appears alternately. In the first state, a current flows from the power supply line to which the power supply voltage Vcc is applied through the switch SW1, the resistor R1, the detection coil Lp, the resistor R2, the switch SW4, and the resistor R3. As a result, the level Vc of the output node N4 gradually increases, and when this exceeds the reference voltage Vcmp, the output of the comparator 31 changes from the low level to the high level.

When the output of the comparator 31 changes to a high level, the logic levels of the oscillation signal Q and the inverted oscillation signal / Q are inverted, and a transition is made to the second state. In the second state, a current flows from the power supply line to which the power supply voltage Vcc is applied through the switch SW2, the resistor R2, the detection coil Lp, the resistor R1, the switch SW3, and the resistor R3. As a result, the level Vc of the output node N4 gradually increases, and when this exceeds the reference voltage Vcmp, the output of the comparator 31 changes from the low level to the high level.

By repeating such an operation, the self-excited oscillation circuit 30 alternately enters the first state and the second state. As a result, the polarity of the voltage applied to both ends of the detection coil Lp is periodically reversed, so that the waveform of the oscillation signal Q is a waveform that alternately repeats a high level and a low level as shown in FIG. Here, reference numeral T in FIG. 4 shows the oscillation period of the self-oscillation circuit 30, reference numeral T ₁ represents a period in a first state, symbol T ₂ denotes a period in the second state. The oscillation period T of the self-excited oscillation circuit 30 and the duty of the oscillation signal Q change depending on the magnetic permeability of the magnetic core 20. Hereinafter, this phenomenon will be described in more detail.

5 and 6 are graphs for explaining the magnetic characteristics of the magnetic core 20. FIG. 5 shows a case where the external magnetic field H _ext is zero, and FIG. 6 shows a case where the external magnetic field H _ext exists. ing. In either case, the horizontal axis represents the magnetic field strength H, and the vertical axis represents the magnetic flux density B.

As shown in FIG. 5, when the external magnetic field H _ext is zero (when the current Ip does not flow in the current path P), the BH curve that appears when the magnetic field applied by the detection coil Lp changes in one direction ( Point 1 → point 2) and the BH curve (point 3 → point 4) appearing when the magnetic field applied by the detection coil Lp changes in the opposite direction are symmetrical. Here, the point 2 indicates a point where the magnetic flux density B becomes a predetermined value _Bth when the magnetic field applied by the detection coil Lp changes in one direction. Similarly, the point 4 indicates a point where the magnetic flux density B becomes a predetermined value −B _th when the magnetic field applied by the detection coil Lp changes in the reverse direction.

The case where the magnetic field applied by the detection coil Lp changes in one direction is a state where a current flows from the terminal S1 to the terminal S2 shown in FIG. 3, that is, the first state. On the other hand, the case where the magnetic field applied by the detection coil Lp changes in the reverse direction is a state where a current flows from the terminal S2 to the terminal S1 shown in FIG. 3, that is, the second state. When the external magnetic field H _ext is zero (when the current Ip does not flow in the current path P), the duty of the oscillation signal Q is 50% because the BH curve is symmetrical.

On the other hand, when the external magnetic field H _ext exists (when the current Ip flows in the current path P), the BH curve is shifted by the intensity of the external magnetic field H _ext as shown in FIG. As a result, a BH curve (point 1 → point 2) that appears when the magnetic field applied by the detection coil Lp changes in one direction and a BH curve (point that appears when the magnetic field applied by the detection coil Lp changes in the opposite direction are shown. 3 → Point 4) is asymmetric. For this reason, the duty of the oscillation signal Q deviates from 50%.

FIG. 7 is a waveform diagram showing changes in the level Vc of the output node N4, and FIG. 8 is a waveform diagram showing changes in the oscillation signal Q and the inverted oscillation signal / Q. In both figures, the solid line indicates the case where the external magnetic field H _ext is zero (when the current Ip does not flow in the current path P), and the broken line indicates the case where the external magnetic field H _ext exists (the current Ip in the current path P). Is shown).

As shown in FIG. 7, in any case, the polarity reverses every time the level Vc of the output node N4 reaches the reference voltage Vcmp with the passage of time, and instantaneously drops to −Vcmp. Level Vcmp corresponds to the value _{B th} in FIG. 5 and FIG. 6, the level of -Vcmp corresponds to a value _{-B th} shown in FIGS. When the external magnetic field H _ext is zero, since the BH curve is symmetrical, the duty of the oscillation signal Q is 50% (T ₁ = T ₂ ) as shown in FIG. On the other hand, when the external magnetic field H _ext is present, the BH curve is asymmetrical, so that the duty of the oscillation signal Q exceeds 50% (T ₁ ′> T ₂ ′) as shown in FIG. At the same time, the period T of the oscillation signal Q is shortened by the decrease in the inductance of the detection coil Lp due to the magnetic saturation of the magnetic core 20. That is, the oscillation frequency of the self-excited oscillation circuit 30 is increased.

The oscillation signal Q and the inverted oscillation signal / Q generated by the self-excited oscillation circuit 30 are supplied to the negative feedback current output circuit 40 as shown in FIG. The negative feedback current output circuit 40 monitors the duty or frequency of the oscillation signal Q and the inverted oscillation signal / Q, and generates a negative feedback current Io based on this. For example, the control is performed so that the amount of the negative feedback current Io increases as the duty of the oscillation signal Q and the inverted oscillation signal / Q increases from 50%. The negative feedback current Io is supplied to the negative feedback coil Lc, and generates a magnetic flux B2 that cancels the magnetic flux B1 generated by the primary coil 10. By such closed loop control, the magnetic flux B1 generated by the primary coil 10 is always canceled and the duty of the oscillation signal Q is controlled to be 50%.

The negative feedback current Io is converted into a voltage Vd by a resistor R6 connected in series with the negative feedback coil Lc, and the level thereof is detected by the signal output circuit 50. The signal output circuit 50 generates a sensor output OUT based on the voltage Vd and outputs it to the outside.

9 and 10 are graphs showing the relationship between the reference voltage Vcmp and the detection sensitivity, and the vertical axis indicating the sensitivity indicates the rate of change (% / A) of the sensor output OUT with respect to the current Ip flowing through the primary coil 10. The difference between FIG. 9 and FIG. 10 is the difference in material and shape of the magnetic core 20.

9 and 10, it can be seen that the sensitivity of the current sensor 100 varies depending on the reference voltage Vcmp and the power supply voltage Vcc. Here, assuming that the reference voltage Vcmp is a constant value, when the power supply voltage Vcc fluctuates, the sensitivity greatly changes accordingly. As an example, in the case of the characteristics shown in FIG. 9, when the reference voltage Vcmp is fixed at 500 mV, if the power supply voltage Vcc is 4.5 V, the sensitivity is about 4.1% / A and the power supply voltage Vcc is 5. If it is 0V, the sensitivity is about 3.2% / A, and if the power supply voltage Vcc is 5.5V, the sensitivity is about 2.7% / A. In the case of the characteristics shown in FIG. 10, when the reference voltage Vcmp is fixed at 600 mV, the sensitivity is about 3.4% / A and the power supply voltage Vcc is 5.0 V if the power supply voltage Vcc is 4.5 V. If so, the sensitivity is about 1.9% / A, and if the power supply voltage Vcc is 5.5V, the sensitivity is about 1.4% / A.

However, since the current sensor 100 according to the present embodiment generates the reference voltage Vcmp by dividing the power supply voltage Vcc, the level of the reference voltage Vcmp is not fixed and changes in proportion to the power supply voltage Vcc. To do. As a result, even if the power supply voltage Vcc fluctuates, a change in sensitivity associated therewith is suppressed. As an example, if the characteristics shown in FIG.
Vcmp = Vcc × 1/10
If the power supply voltage Vcc is 4.5 V, the reference voltage Vcmp is 450 mV, so the sensitivity is about 3.4% / A, and if the power supply voltage Vcc is 5.0 V, the reference voltage Vcmp is 500 mV. Therefore, the sensitivity is about 3.2% / A, and if the power supply voltage Vcc is 5.5V, the reference voltage Vcmp is 550 mV, so the sensitivity is about 3.1% / A, and the reference voltage Vcmp is fixed to 500 mV. It can be seen that the change in sensitivity is significantly suppressed compared to the case.

Similarly, if the characteristics shown in FIG.
Vcmp = Vcc × 3/25
If the power supply voltage Vcc is 4.5 V, the reference voltage Vcmp is 540 mV, so the sensitivity is about 1.5% / A, and if the power supply voltage Vcc is 5.0 V, the reference voltage Vcmp is 600 mV. Therefore, if the sensitivity is about 1.9% / A and the power supply voltage Vcc is 5.5V, the reference voltage Vcmp is 660 mV, so the sensitivity is about 2.1% / A, and the reference voltage Vcmp is fixed at 600 mV. It can be seen that the change in sensitivity is significantly suppressed compared to the case.

The higher the reference voltage Vcmp, the higher the sensitivity of the current sensor 100 tends to be. However, as shown in FIGS. 9 and 10, when the power supply voltage Vcc is low, the settable reference voltage Vcmp is also low. For example, with the characteristics shown in FIG. 9, the upper limit of the reference voltage Vcmp when the power supply voltage Vcc is 4.5V is about 620 mV, and the upper limit of the reference voltage Vcmp when the power supply voltage Vcc is 5.0V is about 710 mV. When the power supply voltage Vcc is 5.5V, the upper limit of the reference voltage Vcmp is about 830 mV. If a reference voltage Vcmp exceeding this is applied, correct oscillation operation cannot be performed. For this reason, when the reference voltage Vcmp is fixed, the level of the reference voltage Vcmp must be set to a certain level in consideration of fluctuations in the power supply voltage Vcc. As a result, it is difficult to obtain high sensitivity.

However, in this embodiment, since the level of the reference voltage Vcmp changes in proportion to the power supply voltage Vcc, even if the power supply voltage Vcc varies by appropriately setting the voltage dividing ratio of the resistors R4 and R5. The reference voltage Vcmp near the upper limit can always be used. For this reason, it is possible to obtain higher sensitivity than in the case where the reference voltage Vcmp is fixed.

The preferred embodiments of the present invention have been described above, but the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the spirit of the present invention. Needless to say, it is included in the range.

For example, in the above embodiment, the reference voltage Vcmp is generated using the voltage dividing resistor composed of the resistors R4 and R5. However, the voltage dividing ratio can be made variable by making one of the resistors R4 and R5 a variable resistor. Is also possible.

10 Primary coil 20 Magnetic core 30 Self-excited oscillation circuit 31 Comparator 32 Flip-flop circuit (oscillation signal generation circuit)
33 Reference Voltage Generation Circuit 40 Negative Feedback Current Output Circuit 50 Signal Output Circuit 70 Magnetic Shield 100 Current Sensor Lc Negative Feedback Coil Lp Detection Coil N1 First Connection Node N2 Second Connection Node N3 Power Supply Node N4 Output Node P Current Path R1 ~ R6 Resistance SW1 ~ SW4 switch

Claims

A magnetic core that is magnetically coupled to the current path;
An H-bridge circuit having first and second connection nodes whose polarities are alternately inverted based on an oscillation signal, a power supply node to which a power supply voltage is applied, and an output node;
A detection coil wound around the magnetic core and connected between the first connection node and the second connection node;
A comparator that compares the voltage of the output node with a reference voltage;
An oscillation signal generation circuit for inverting the oscillation signal based on the output of the comparator;
A current sensor comprising: a reference voltage generation circuit that changes the reference voltage in accordance with the power supply voltage.
The current sensor according to claim 1, wherein the reference voltage generation circuit makes the power supply voltage proportional to the reference voltage.
3. The current sensor according to claim 2, wherein the reference voltage generation circuit includes a voltage dividing resistor that divides the power supply voltage.
The current sensor according to claim 3, wherein the voltage dividing resistor includes a variable resistor.
A negative feedback coil that cancels magnetic flux generated in the magnetic core due to a current flowing through the current path; and a negative feedback current output circuit that causes a negative feedback current to flow through the negative feedback coil based on the oscillation signal. The current sensor according to any one of claims 1 to 4.