JP7430033B2 - current detection device - Google Patents

Description

The present invention relates to a current detection device suitable for detecting minute currents, for example.

For example, a switching power supply circuit is used in a charging device that uses a capacitive element such as a battery or a capacitor as a load (see Patent Document 1). The charging device performs PWM control on the rated line current supplied from the DC power supply section using a switching power supply circuit (switching element) to control the current output to the charging load. The voltage of the charging load is fed back to the control circuit via the isolation amplifier, and control is performed to increase the value of the charging current when the load voltage is low, and to reduce the charging current as the load voltage increases. Then, when the voltage of the charging load reaches a fully charged state, the switching operation is stopped and no further charging current is output. Even if the charging device remains connected to the DC power supply, the switching power supply circuit is considered to be cutting off line current conduction.

Japanese Patent Application Publication No. 2018-148663

However, even if the switching power supply circuit is controlled to be cut off, for example, if there is some kind of malfunction inside the circuit of the charging device (deterioration of the switching element over time, etc.), a very small amount of current may be conducted inside the circuit. There are things to do. When the switching operation is stopped, even if a small current is conducted, the voltage detection circuit cannot detect this, and the device cannot take any countermeasures. Of course, this does not pose a major problem as long as the minute current passes through the switching element, but there is a problem in that extra power loss is generated in addition to standby power. Furthermore, since the line current is conducting even when switching is stopped, the risk of accidents due to electrical leakage cannot be overlooked.

The above is an example of a charging device, but even when a switch or the like is used to turn on and off an industrial or commercial line that carries a large current, a minute current may continue somewhere after the line current is cut off. Since leakage current problems such as these can often occur, countermeasures are required.

Therefore, the present invention provides a technique for detecting minute currents that is useful as a countermeasure to the above.

In order to solve the above problems, the present invention employs the following solving means. Note that the parentheses in the following description are for reference only, and the present invention is not limited thereto.

The present invention provides a current detection device. The current detection device of the present invention performs a detection operation using a piezoelectric transformer and a circuit network connected thereto as a central element in current detection. The network may consist of diode bridges and resistors. Further, the piezoelectric transformer has four terminals polarized on the primary side and the secondary side. The primary side and secondary side of a piezoelectric transformer are insulated by the piezoelectric body (piezoelectric element, piezoelectric ceramics) itself.

A piezoelectric transformer is caused to resonate by applying a drive signal in a region near the resonant frequency where the input impedance becomes inductive to one side (for example, the primary side), while the other side (for example, the secondary side) is connected to the termination circuit. Terminates with A network is connected in parallel to this termination circuit to the other side of the piezoelectric transformer. Note that a termination circuit may be included in the configuration of the circuit network.

The electrical characteristics of the circuit network change as follows depending on the conduction state of the current to be detected.
(1) When the current to be detected is a relatively large current value (for example, several tens to several hundred A), the current to be detected flows through the diode bridge, so each diode becomes conductive, and the other side of the piezoelectric transformer (opposite The impedance of the entire circuit network as seen from the electrodes (between the electrodes) maintains a relatively low impedance state.
(2) When the current to be detected is a small current value (for example, about several mA), each diode in the diode bridge is in a cutoff state (the small current Icut is assumed to be a voltage that does not eliminate the cutoff, Vcut), Icut≦Vcut/R1 (resistance R1), and the current to be detected flows only through the resistance, so the impedance of the entire circuit network seen from the other side of the piezoelectric transformer (between the opposing electrodes) is compared to when a large current is conducting. resulting in high impedance.

As a result, when the current to be detected in (1) above is a relatively large current, the impedance terminated on the other side of the piezoelectric transformer (between the opposing electrodes) generally becomes a low impedance. Note that the impedance of the termination circuit is set to be sufficiently higher than the low impedance value that the diode bridge appears in the conductive state.
On the other hand, when the current to be detected in (2) above is a small current value, the impedance terminated on the other side of the piezoelectric transformer (between the opposing electrodes) is approximately equal to the impedance of the termination circuit.

In this way, the terminal impedance that appears on the other side of the piezoelectric transformer (between the opposing electrodes) changes depending on the conduction state of the current to be detected (relatively large current or small current), so this change is captured and the detection signal is can be output.

For this reason, the current detector of the present invention includes a detector. The detector outputs a detection signal on one side of the piezoelectric transformer in accordance with the conduction state of the current to be detected, with the circuit network connected in parallel with a termination circuit that terminates the other side of the piezoelectric transformer. For example, if the conduction state of the current to be detected changes to state 1, which is a relatively large current, and state 2, which is a very small current, the detection signal becomes a unique signal in state 1 and state 2, respectively.

As a result, for example, if the current to be detected is the line current (large current) of power supply equipment or power generation equipment, if the detection signal indicates state 1, it can be known that the line current is flowing normally; When the signal indicates state 2, it can be known that a minute current different from the line current is flowing. At this time, the threshold value of the current Icut to be detected as a minute current can be arbitrarily set based on the above relational expression [Vcut/R1]. In other words, when setting an extremely small current that is acceptable as a line current (a weak value that cannot be considered as a leakage current) as a threshold value, and detecting a current value higher than this as an unacceptable microcurrent (a value in a current leakage state), a diode bridge and a resistor are used. By appropriately selecting and setting a threshold value, it is also possible to detect whether the current is a minute current within the allowable value range (state 2) or a minute current that is greater than the allowable value (state 1).

According to the present invention, a technique for detecting minute current can be provided.

FIG. 1 is a block diagram schematically showing the configuration of a current detection device according to an embodiment. It is a figure showing a piezoelectric transformer alone. FIG. 3 is a diagram showing frequency characteristics of input impedance of a piezoelectric transformer. FIG. 3 is a diagram showing an equivalent circuit of a piezoelectric transformer. This is an equivalent circuit of the piezoelectric transformer seen from between the detection side counter electrodes P3 and P4 when the resistor Rb is terminated between the detection side counter electrodes P1 and P2 in FIG. FIG. 7 is a diagram showing an equivalent circuit when the piezoelectric transformer is driven in a frequency range in which the impedance on the detection side becomes inductive. 7 is an equivalent circuit diagram when driving the piezoelectric transformer shown in the equivalent circuit of FIG. 6. FIG. FIG. 3 is a diagram showing a specific example of a circuit configuration of a current detection device. FIG. 7 is a diagram showing the relationship between the output waveform of the oscillator and the voltage waveform of the voltage between the terminals, according to conditions (A) to (C).

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram schematically showing the configuration of a current detection device 100 according to an embodiment.
The current detection device 100 performs a detection operation by conducting, for example, a DC current to be detected (line current IL) from a terminal IN+ to a terminal IN-, and outputs the result to an output terminal OUT. The configuration of the current detection device 100 will be described below.

[Piezoelectric transformer]
The current detection device 100 includes a piezoelectric transformer PZT1. The piezoelectric transformer PZT1 is, for example, a piezoelectric body (piezoelectric ceramics) that has been polarized into a primary side and a secondary side, and has a primary side electrode pair (counter electrodes P3, P4) and a secondary side electrode pair (counter electrodes) on the outer surface. P1, P2) are formed. Such a piezoelectric transformer PZT1 is constructed by forming a piezoelectric material such as lead zirconate titanate into a plate shape and subjecting it to polarization treatment to form a primary side and a secondary side. The external shape is schematically shown in a side view. On the surface of the piezoelectric body, a pair of primary electrodes (counter electrodes P3, P4) and a pair of secondary electrodes (counter electrodes P1, P2) are formed as thick films using, for example, silver paste. These counter electrodes P3, P4 and counter electrodes P1, P2 form a pair in the thickness direction of the piezoelectric body, and there is a sufficient space between the primary electrode pair and the secondary electrode pair on the outer surface of the piezoelectric body. Insulation distance is ensured. Note that the piezoelectric transformer PZT1 may be constituted by a laminate in which a plurality of piezoelectric layers are stacked, and electrodes may be arranged in the inner layer.

The current detection device 100 also includes an oscillator 110, a detector 120, and a circuit network 130. Of these, the oscillator 110 and the detector 120 are connected, for example, to the primary side of the piezoelectric transformer PZT1, and the circuit network 130 is connected to the secondary side of the piezoelectric transformer PZT1. Further, in this embodiment, the circuit configuration is such that the oscillator 110 is connected to the primary side electrode pair (counter electrodes P3, P4).

[Circuit network]
The circuit network 130 has a circuit configuration in which, for example, a diode bridge composed of four diodes D1 to D4 and a resistor R1d are arranged. The diode bridge is a bridge configuration in which each of the diodes D1 to D4 conducts the current to be detected in the forward direction , and the resistor R1d determines the current threshold when the diodes D1 to D4 become conductive or cut-off. There is.

That is, as described above, the current to be detected (hereinafter referred to as "line current IL") flows from the terminal IN+ to the terminal IN-. At this time, if the resistance value R1d and the cutoff voltage of the diodes D1 to D4 are Vcut, then when the line current IL is in a conductive state that satisfies the relationship [IL>2×Vcut/R1d], part of the line current IL is The current flows through the diode bridge, and the four diodes D1 to D4 become conductive. Such an energization state can be assumed, for example, when the line current IL is flowing at a normal large current.

On the other hand, when the line current IL is in an energized state that satisfies the relationship [IL≦2×Vcut/R1d], all of the line current IL flows through the resistor R1d, so each of the diodes D1 to D4 is in a cutoff state. Such an energized state can be assumed, for example, when a minute current (extremely weak current) continues to flow in the line current IL even after the line current IL path is cut off. Note that this condition is also satisfied when the line current IL is in a non-energized state (0 A). Therefore, in this embodiment, a minute current that satisfies the above relational expression is considered as a permissible range, but even if it is not a large current, a minute current that does not satisfy the above relational expression is detected as outside the allowable range. Is possible. Note that such a usage pattern will be further described later.

[Termination circuit]
Further, the secondary side opposing electrodes P1 and P2 of the piezoelectric transformer PZT1 are terminated by a terminating resistor (impedance) Z1. The diode bridge (diodes D1 to D4) and the resistor R1d of the circuit network 130 are connected to the secondary side counter electrodes P1-P2 in parallel with this terminating resistor Z1. Note that, in FIG. 1, the terminating resistor Z1 is included as part of the configuration of the circuit network 130.

[Change in terminal impedance]
At this time, the terminal impedance seen from between the secondary side opposing electrodes P1 and P2 of the piezoelectric transformer PZT1 changes according to the switching between the conduction state and the cutoff state of the diodes D1 to D4 according to the conduction state of the line current IL as described above. and change.

[At low impedance]
For example, when the line current IL is in an energized state that satisfies the relationship [IL>2×Vcut/R1d], the impedances of the diodes D1 to D4 maintain a low impedance state. As a result, the impedance terminated between the secondary opposing electrodes P1 and P2 of the piezoelectric transformer PZT1 becomes approximately low impedance. Note that the impedance of the terminating resistor Z1 is set to an impedance value sufficiently higher than the low impedance value produced by the diode bridge constituted by the diodes D1 to D4. Furthermore, since the diodes D1 to D4 form a bridge, the resistor R1d does not affect the impedance of the terminating resistor Z1 terminated between the secondary opposing electrodes P1 and P2 of the piezoelectric transformer PZT1.

[At high impedance]
On the other hand, when the line current IL is in an energized state that satisfies the relationship [IL<2×Vcut/R1d], the impedance of the diode bridge in the cut-off state as seen from between the secondary side opposing electrodes P1 and P2 of the piezoelectric transformer PZT1 is high impedance. Therefore, the impedance terminated between the secondary opposing electrodes P1 and P2 of the piezoelectric transformer PZT1 is approximately the impedance of the terminating resistor Z1.

As described above, in the current detection device 100 of the present embodiment, the value of the line current IL to be detected is set between the secondary side opposing electrodes P1 and P2 of the piezoelectric transformer PZT1 with [2×Vcut/R1d] as the threshold value. It can be seen that the terminal impedance can be varied from low to high.

Therefore, in the present embodiment, detection according to the conduction state of the line current IL is performed using the characteristics of impedance changes that appear between the secondary side opposing electrodes P1 and P2 of the piezoelectric transformer PZT1 according to the conduction state of the line current IL. The operation is realized. The configuration for this purpose is the oscillator 110 and detector 120 described above. The oscillator 110 and the detector 120 will be explained below.

[Oscillator]
The oscillator 110 applies a drive signal between the primary side opposing electrodes P3 and P4 of the piezoelectric transformer PZT1. At this time, the drive signal oscillates in a frequency region (near the resonance frequency) where the impedance between the primary side opposing electrodes P3 and P4 of the piezoelectric transformer PZT1 becomes inductive. Note that the impedance-frequency characteristics of the piezoelectric transformer PZT1 will be described later.

〔Detector〕
The detector 120 monitors a part of the voltage waveform (voltage Vca between capacitive terminals) based on the resonance state of the piezoelectric transformer PZT1 in a driving state where the impedance is inductive, and outputs a detection signal to the output terminal OUT based on the result. Output. Note that the internal equivalent circuit and some voltage waveforms of the piezoelectric transformer PZT1 will be further described later.

[Insulation by piezoelectric transformer]
In this way, the oscillator 110 and the detector 120 are connected to the primary side of the piezoelectric transformer PZT1, while the circuit network 130 through which the line current IL conducts is connected to the secondary side, so that in the current detection device 100, the primary It can be seen that the side circuit and the secondary side circuit are insulated by the piezoelectric transformer PZT1. The vertical dotted line shown in FIG. 1 is the insulation line.

In this embodiment, the side to which the drive signal is applied to the piezoelectric transformer PZT1 is referred to as the "primary side," and the opposite side is referred to as the "secondary side." However, the side to which the applied voltage is relatively large is referred to as the "primary side." '', and the opposite side is sometimes considered the ``secondary side''. Although the line current IL is not a circuit configuration applied to the piezoelectric transformer PZT1, since the circuit network 130 is connected to the piezoelectric transformer PZT1, if the voltage of the line current IL conducting the circuit network 130 is higher than the driving voltage, the circuit The side to which the network 130 is connected can also be considered the "primary side."

[Unification of names]
In this way, when referring to the piezoelectric transformer PZT1 as "primary side/secondary side", there are differences depending on the conditions of use, technical customs, etc., and it may not be unambiguous.
Therefore, in this embodiment, in view of the relative nature of the names such as "primary side/secondary side", the names are unified as follows.

[Detected side/detecting side]
That is, the side to which the circuit network 130 is connected to the piezoelectric transformer PZT1 is defined as the "detected side", and the side to which the oscillator 110 and the detector 120 are connected is defined as the "detection side". Note that these terms "detected side/detection side" are for convenience of explanation, and the piezoelectric transformer PZT1 does not have "primary side/secondary side" in the original meaning. Needless to say.

[Detection method using impedance change]
FIG. 2 is a diagram showing the piezoelectric transformer PZT1 alone. The piezoelectric transformer PZT1 used in this embodiment has an insulated four-terminal structure as described above.

The impedance Za seen from between the detecting side counter electrodes P3 and P4 of the piezoelectric transformer PZT1 changes depending on the value of the impedance connected between the detected side counter electrodes P1 and P2. In particular, in the frequency range where the impedance Za is inductive, the impedance terminated between the detection side opposing electrodes P1 and P2 of the piezoelectric transformer PZT1 affects the Q value of the impedance Za as seen from the detection side.

On the other hand, as described above, the impedance terminated between the opposite electrodes P1 and P2 on the detection side changes from high to low due to the influence of the impedance change of the diode bridge in the circuit network 130. Since the impedance of the diode bridge changes greatly depending on the conduction state of the line current IL, the circuit network 130 connected between the counter electrodes P1 and P2 on the detection side has a threshold value (2×Vcut/R1d) of the minute current. At the same time, the value of line current IL can be converted into a change in impedance.
The current detection device 100 of this embodiment can detect minute currents by utilizing such characteristics.

[Impedance frequency characteristics of piezoelectric transformer]
Here, FIG. 3 is a diagram showing the frequency characteristics of the input impedance of the piezoelectric transformer PZT1. Normally, the input impedance of the piezoelectric transformer PZT1 is inductive in a region where the phase is close to +90 degrees (region from the resonant frequency fr to the anti-resonant frequency frr), and capacitive in other regions. In the region from the resonant frequency fr to the anti-resonant frequency frr, the absolute value of impedance increases as the frequency increases.

[Explanation using equivalent circuit]
FIG. 4 is a diagram showing an equivalent circuit of the piezoelectric transformer PZT1. This equivalent circuit is for a frequency region near the resonance frequency of the piezoelectric transformer PZT1. Therefore, although the opposing electrodes P1 and P2 on the detection side appear to be short-circuited in terms of direct current due to the winding of the ideal transformer T1, they are actually open in terms of direct current. For this reason, the equivalent circuit between the detection side counter electrodes P1 and P2 and the detection side counter electrodes P3 and P4 of the piezoelectric transformer PZT1 is Note that the impedance will be high regardless of the

FIG. 5 is an equivalent circuit of the piezoelectric transformer PZT1 seen from between the detection side counter electrodes P3 and P4 when the resistor Rb is terminated between the detection side counter electrodes P1 and P2 in FIG.
The conversion from the equivalent circuit of FIG. 4 to the equivalent circuit of FIG. 5 can be explained by the following equation. That is, the resistance R1' and the capacitance C1' in the equivalent circuit of FIG. 5 are expressed by the following equation [Equation 1].

FIG. 6 is a diagram showing an equivalent circuit when the piezoelectric transformer PZT1 is driven in a frequency range in which the impedance on the detection side becomes inductive. That is, if the impedance seen from the detection-side counter electrodes P3-P4 is Za, this impedance Za decreases in capacitance as the frequency increases as shown in FIG. 3 in the vicinity of the resonance frequency fr of the piezoelectric transformer PZT1. It has a frequency characteristic in which it changes from an inductive impedance region to a capacitive impedance region again.

At this time, if the impedance Za seen from the detection side counter electrodes P3-P4 is limited to the frequency range where it becomes an inductive impedance, the equivalent circuit in FIG. 5 can be expressed as the equivalent circuit in FIG. That is, under the conditions of the inductive impedance region, the resistance Ra and inductance La associated with the conversion from the equivalent circuit of FIG. 5 to the equivalent circuit of FIG. 6 are expressed by the following formula [Equation 2].

[Drive model]
FIG. 7 is an equivalent circuit diagram when the piezoelectric transformer PZT1 shown in the equivalent circuit of FIG. 6 is driven. That is, with the oscillator OSC connected between the detection side counter electrodes P3 and P4 and the capacitor Ca arranged in series with the oscillator OSC, the piezoelectric transformer PZT1 is driven between the detection side counter electrodes P3 and P4.

The oscillation frequency fosc of the oscillator OSC is set in a frequency range in which the impedance Za between the detection-side opposing electrodes P3 and P4 of the piezoelectric transformer PZT1 exhibits inductive properties, and in the vicinity of the resonance frequency fr. Further, during driving by the oscillator OSC, control is performed to maintain the value of the inductance La of the equivalent circuit constant. Note that control of the oscillation frequency fosc by the oscillator OSC will be described later along with a specific circuit configuration example.

In addition, in the drive model of FIG. 7, the value of the capacitance Ca is selected as a value that satisfies fosc≒1/(2π√(La×Ca). At this time, the voltage Vca between the terminals of the capacitance Ca is calculated using the following formula [Math. 3].

Here, as shown in the above equation [Equation 2], the inductance La changes with the change in the terminating resistance Rb, so the oscillation frequency that maintains the inductance La constant during driving by the oscillator OSC as described above. control to fosc. However, as shown in FIG. 3, since the phase angle changes rapidly near the resonance frequency fr, the value of the oscillation frequency fosc practically does not change even if the inductance La is kept constant through control. Therefore, the change in the terminal voltage Vca due to the change in the termination resistance Rb depends only on the change in the internal equivalent resistance Ra, and the change is inversely proportional to the value of the equivalent resistance Ra from the above formula [Equation 3]. I understand.

Therefore, the value Ras of the equivalent resistance Ra when the terminating resistor Rb≒0Ω and the value Ras when the terminating resistor Rb is the matching impedance value of the piezoelectric transformer PZT1 [Rb≒1/(2πfosc×C02)] Calculate the ratio with Rarb. At this time, if the oscillation frequency fosc is approximated to be near the resonance frequency fr, the relationship between them is expressed by the following equation [Equation 4].

Therefore, when the terminating resistance Rb changes from a low resistance value near 0Ω to a value [1/(2πfosc×C02)] which is the matching impedance of the piezoelectric transformer PZT1, the value of the terminal voltage Vca from the above equation [Equation 4] becomes 2R1 /Rb times.

From the above considerations, in the circuit configuration shown in FIG. 1, if the impedance value of the terminating resistor Z1 is selected as 1/(2πfosc×C02), the terminating equivalent resistance Rb between the detection side counter electrodes P1 and P2 is The value is as follows.
That is, in the circuit configuration of FIG. 1, when the diodes D1 to D4 of the diode bridge are conductive, the terminal equivalent resistance Rb≈0Ω.
On the other hand, when the diodes D1 to D4 of the diode bridge are cut-off, the terminal equivalent resistance Rb=1/(2πfosc×C02).

From the above, the current detection device 100 of this embodiment can perform a minute current detection operation as follows.

The oscillator 110 oscillates at a frequency whose oscillation frequency fosc is near the resonance frequency of the piezoelectric transformer PZT1 and at which the impedance Za between the detection side counter electrodes P3 and P4 becomes inductive. At this time, the oscillator 110 controls the frequency so that the inductance La of the equivalent circuit shown in FIG. 6 always maintains a constant value.

Further, a capacitor that resonates integrally with the piezoelectric transformer PZT1 is mounted in the circuit of the detector 120. The capacitance of this capacitor corresponds to the capacitance Ca shown in the equivalent circuit of FIG. 7, and is selected to be a constant such that the resonance frequency with the inductance La is the oscillation frequency fosc.

Further, it is assumed that the terminating resistor Z1 connected to the detection side counter electrodes P1-P2 has a value of 1/(2πfosc×C02), which is the matching impedance of the piezoelectric transformer PZT1.
The resistor R1d is selected to have a resistance value R1d=2Vcut/Idet in correspondence with the microcurrent detection threshold Idet. Here, the forward cutoff voltage Vcut of the diodes D1 to D4 is used as described above.

The detector 120 monitors the inter-terminal voltage Vca of the capacitor Ca, which is equivalently shown in FIG. do. For example, when it is detected that the value of the voltage Vca between the terminals observed when the line current IL is in a relatively large current conduction state (for example, several tens of A or more) has decreased to about 2R1/Rb times. , it is determined that the line current IL has changed to a conductive state below the threshold value Idet, and a detection signal indicating this (a microcurrent conduction state) is output to the output terminal OUT. On the other hand, even when the line current IL is a relatively large current, a detection signal representing that fact (large current energization state) is outputted to the output terminal OUT.

[Usage form]
The current detection device 100 of this embodiment can be used, for example, in the following usage mode.
For example, the line current IL is conducted from the terminal IN+ to the terminal IN- on the power supply path of the charging device. A relatively large current (for example, about several tens of amperes) is used as the line current IL, and the detection target minute current (for example, about several mA) is appropriately set to set the cutoff voltage Vcut of the diodes D1 to D4. Select the impedance of the resistor R1d and the terminating resistor Z1. Since the threshold value Idet is determined from each of these selected values, it is possible to set an allowable range of minute current in the power supply path.

When the line current IL is in a relatively large current conduction state, the diode bridge is conductive as described above and enters a low impedance state. In this case, the detector 120 outputs, for example, a High signal from the output terminal OUT, which is used as a detection signal indicating that a large current is in a conductive state. However, if the line current IL is equal to or greater than the threshold value Idet, a High signal will be output even for a current that is not a relatively large current (a minute current that exceeds the allowable range).

On the other hand, when the line current IL is in a conductive state with a minute current (below the threshold value Idet), the diode bridge is cut off as described above and enters a high impedance state. In this case, the detector 120 outputs, for example, a Low signal from the output terminal OUT, which is used as a detection signal indicating that a minute current is being conducted that is below the allowable range.

At this time, if the charging device is equipped with a power switch, for example, when the power switch is turned off, the path of the line current IL will be cut off and the large current will not be flowing. In this case, if the current level is a minute current (extremely weak current) below the allowable range, a Low signal will be output from the detector 120, so if the conditions of "power switch OFF and Low signal" are met, it is determined to be normal. be able to.
However, if the detector 120 is still outputting a High signal even after the power switch is turned off, this means that a conduction state of a minute current exceeding the allowable range has been detected. Therefore, if the conditions of "power switch OFF and High signal" are met, this means that a leakage current is occurring somewhere inside the charging device, so it can be determined that there is an abnormality.
Note that when the power switch is ON, the line current IL is in a large current conduction state, so if the condition of "power switch ON and High signal" is satisfied, it can be determined to be normal.
In this way, the current detection device 100 of this embodiment can be used for detecting minute currents (earth leakage detection).

[Circuit configuration example]
FIG. 8 is a diagram showing a specific example of the circuit configuration of the current detection device 100. The correspondence relationship with the circuit configuration shown in FIG. 1 will be explained below.

In the circuit of FIG. 8, diodes D1 to D4 and resistors R1 and R2 constitute a network 130. The arrangement of diodes D1 to D4 and resistor R1 is the same as the circuit configuration in FIG. The resistor R1 corresponds to the resistor R1d shown in FIG. The resistor R2 constitutes an impedance equivalent to the terminating resistor Z1 shown in FIG.

The operational amplifier IC1 and its peripheral circuits (resistors R13 to R16, capacitors C3 and C4), and the transistors Q1, Q2, Q3 and their peripheral circuits (resistors R3 to R11) constitute the oscillator 110 shown in FIG. The output of the oscillator 110 corresponds to the collectors of the transistors Q1 and Q2.

The two capacitors C1 and C2 constitute the capacitance Ca shown in the equivalent circuit of FIG. 7, and these capacitors C1 and C2 resonate together with the piezoelectric transformer PZT1. Therefore, capacitors C1 and C2 are included in detector 120 in the circuit configuration of FIG. When the collector outputs of the transistors Q1 and Q2 operate as current sources, the value of the capacitance Ca seen in the equivalent circuit is Ca=C1×C2/(C1+C2). On the other hand, when the collector outputs of the transistors Q1 and Q2 operate close to a voltage source, the value of the capacitance Ca seen in the equivalent circuit becomes Ca=C1.

Further, the configuration of the detector 120 includes the following.
The operational amplifier IC2, the diode D5, and the capacitor C5 rectify the voltage between the terminals of the capacitor C1 into a DC voltage. The inter-terminal voltage of the capacitor C1 corresponds to the inter-terminal voltage Vca explained above. Comparator IC3 monitors the DC voltage obtained by rectifying the voltage between the terminals of capacitor C1 (voltage between terminals of capacitor C5) using the divided voltage value Vcc×R19/(R19+R20) of drive voltage Vcc as a threshold value, and the line current IL is below the threshold value. When this happens, a Low signal is output from the output terminal OUT.

[Oscillator operation]
The oscillation operation of the oscillator 110 shown in the circuit of FIG. 8 is as follows.
That is, the operation of the oscillator 110 is the same as the basic operation of the multivibrator in which the charging/discharging time constant is phase-controlled to the target frequency. Phase control is control that detects the error between the frequency due to the basic operation of the multivibrator and the target frequency as a phase difference, converts the phase difference into voltage, and applies feedback. The target frequency here is the resonance frequency (1/2π√(La×Ca)) in the equivalent circuit of FIG.

In the circuit of FIG. 8, the operational amplifier IC1, resistors R13 to R16, and capacitor C3 constitute the basic circuit of the multivibrator. The impedance between the opposing electrodes P4 and P3 of the piezoelectric transformer PZT1, the capacitors C1 and C2, and the resistor R12 are a circuit for controlling the phase of the charge/discharge time constant to a target frequency. In the case of the circuit shown in FIG. 8, the target frequency is the resonant frequency of the resonant circuit constituted by the capacitors C1 and C2 and the impedance between the opposing electrodes P4 and P3 of the piezoelectric transformer PZT1. However, the target frequency needs to be limited within the range of the inductive region in which the impedance between the opposing electrodes P4 and P3 of the piezoelectric transformer PZT1 is shown in FIG. By limiting the target frequency range in this way, the equivalent circuit between the opposing electrodes P4 and P3 of the piezoelectric transformer PZT1 can be considered as shown in FIG.

The oscillation frequency of the multivibrator constituted by the oscillator 110 is determined by the charging and discharging time of the capacitor C3. The charging and discharging characteristics of capacitor C3 are determined by the sum of the current that enters from the output voltage of operational amplifier IC1 via resistor R13, the current that feeds back from the voltage between the terminals of capacitor C1 via resistor R12, and the sum of the current that enters from the output voltage of operational amplifier IC1 via resistor R12, It is determined by the amount of charge accumulated in . Here, the amount of charge stored and released in the capacitor C3 is q, the charging time of the capacitor C3 is tc, and the discharging time is td. At this time, the charging and discharging times tc and td are expressed by the following formula.
tc=q/Ic
td=q/Id

Ic in the above equation is the average value within the charging time of the total current of the current flowing from the output voltage of the operational amplifier IC1 via the resistor R13 and the current flowing from the terminal voltage of the capacitor C1 via the resistor R12. .

Similarly, Id in the above is the average value within the discharge time of the total current of the current flowing to the output voltage of the operational amplifier IC1 via the resistor R13 and the current flowing to the terminal voltage of the capacitor C1 via the resistor R12. do.

Normally, the oscillation waveform of the oscillator 110 is oscillated with a duty ratio of 1:1, so in the following description, it is assumed that the condition tc=td=ta holds true. Then, the current flowing in through the resistor R13 and the current flowing out will be equal, and both can be made into the current Ir13. Further, the current flowing in through the resistor R12 and the current flowing out are equal to each other, and both can be the current Ir12. When defined in this way, the charging/discharging time ta is expressed by the following expression, and the oscillation frequency f of the oscillator 110 is expressed by the following expression.
ta=q/Ic=q/(Ir13+Ir12)
f=(Ir13+Ir12)/(2q)

Assuming that the oscillation frequency f of the oscillator 110 is within a range where the impedance between the opposing electrodes P4 and P3 of the piezoelectric transformer PZT1 becomes inductive, its operation can be expressed by the equivalent circuit shown in FIG. At this time, the output of the oscillator OSC in the equivalent circuit of FIG. 7 corresponds to the collectors of transistors Q1 and Q2 in the circuit of FIG. Furthermore, the resistance Ra and inductance La in the equivalent circuit of FIG. 7 indicate the impedance between the opposing electrodes P4 and P3 of the piezoelectric transformer PZT1. Capacitance Ca in the equivalent circuit of FIG. 7 corresponds to the series capacitance of capacitor C1 and capacitor C2 in the circuit of FIG. However, the inter-terminal voltage of the capacitor C1 in the circuit of FIG. 8 is actually a voltage obtained by dividing the inter-terminal voltage Vca appearing in the equivalent circuit of FIG. 7 by the capacitor C1 and the capacitor C2. Therefore, when the output of the oscillator OSC is set to 1 in the equivalent circuit of FIG. 7, the capacitance Ca and the voltage Vca between its terminals are each expressed by the following formula [Equation 5]. In addition, in the formula [Equation 5] below, in order to distinguish between Vca shown in the equivalent circuit of FIG. 7 and the voltage between the terminals of capacitor C1 in the circuit of FIG. 8, the voltage between the terminals of capacitor C1 as seen in FIG. shall be described.

From the above equation, it can be seen that the phase difference φ between the voltage waveform of the voltage Vca1 between the terminals of the capacitor C1 and the output voltage of the collectors of the transistors Q1 and Q2 in FIG. 8 is affected by the oscillation frequency. At this time, the phase difference φ is influenced according to the value of the oscillation frequency f (conditions (a), (b), and (c) below) as roughly shown in the following equation [Equation 6]. However, it is assumed that the oscillation frequency f is within a frequency range at which the impedance between the opposing electrodes P4 and P3 of the piezoelectric transformer PZT1 becomes inductive.

[Oscillation frequency for different conditions (a) to (c)]
FIG. 9 is a diagram showing the relationship between the output waveform of the oscillator OSC and the voltage waveform of the inter-terminal voltage Vca1 for each condition (A) to (C). In addition, in FIG. 9, the inter-terminal voltage Vca1 is shown as a voltage waveform during the charging time of the capacitor C3. Therefore, the voltage waveform during the discharge time of capacitor C3 has a polarity opposite to that in FIG. Conditions (a) to (c) will be explained separately below.

As shown in [condition (b)] shown in the center of FIG. 9, when the oscillation frequency f matches the target frequency (f=1/2π√(La×Ca)), the output waveform of the oscillator OSC The phase difference φ between the terminal voltage Vca1 and the terminal voltage Vca1 is -90°, and the terminal voltage Vca1 becomes 0V at the midpoint of the charging time. In this case, the average value of the voltage waveform of the inter-terminal voltage Vca1 within the charging time is 0, and the charging current Ir12 fed back via the resistor R12 in the circuit of FIG. 8 also becomes 0A. Therefore, the oscillation frequency f of the circuit constituting the multivibrator in Fig. 8 is determined purely by the charging and discharging time of the capacitor C2 by the current Ir13 via the resistor R13, and oscillation continues at the target frequency assuming there is no error. do.

Next, as shown in [condition (a)] shown on the left side of FIG. 9, if the oscillation frequency f is lower than the target frequency (f<1/2π√(La×Ca)), the output of the oscillator OSC The phase difference φ of the inter-terminal voltage Vca1 with respect to the waveform is larger than -90° but smaller than 0°. In this case, since the average value of the voltage waveform of Vca1 within the charging time is positive, the current Ir12 that returns via the resistor R12 in the circuit of FIG. 8 also becomes positive, and this feedback current Ir12 charges the capacitor C3. Work in the direction. This means that current Ir12 in addition to current Ir13 via resistor R13 charges capacitor C3, shortening its charging time. Therefore, the oscillation frequency in this case is controlled to be higher than the frequency determined only by the resistor R13 and the capacitor C3. As a result, the oscillation frequency f, which was a low frequency, approaches the target frequency (1/2π√(La×Ca)).

On the other hand, in the case of [condition (c)] shown on the right side of FIG. 9, the following is true. In this case, the oscillation frequency f is higher than the target frequency (1/2π√(La×Ca)<f), and the phase difference φ between the terminal voltage Vca1 and the output waveform of the oscillator OSC is smaller than −90°. In this case, since the average value of the voltage waveform of Vca1 within the charging time is negative, the current Ir12 that feeds back via the resistor R12 in the circuit of FIG. 8 also becomes negative, and this feedback current Ir12 reduces the charge of the capacitor C3. Since it acts on the polarity of discharging, it has the effect of extending the charging time of the capacitor C3 by the current Ir13. Therefore, the oscillation frequency in this case is controlled to be lower than the frequency determined only by the resistor R13 and the capacitor C3. As a result, the oscillation frequency f, which was a high frequency, approaches the target frequency (1/2π√(La×Ca)).

In this way, the oscillator 110 detects the error with the target frequency (the resonant frequency between the piezoelectric transformer PZT1 and the capacitance formed by the capacitors C1 and C2) as a phase difference, converts the phase difference into a voltage, and calculates the net charge. The oscillation frequency f can be controlled to be near the target frequency by increasing or decreasing the discharge current.

According to the above embodiment, the following advantages can be obtained.
(1) Even for objects for which a relatively large current is expected as the line current IL, a minute current can be detected with a simple circuit configuration. In other words, when trying to detect a minute current on a line that normally carries a large current, it is necessary to place a current detection resistor on that line and an isolation amplifier to transmit the voltage between its terminals in an isolated state to the detection side. However, the current detection device 100 of this embodiment does not require an insulation amplifier and can detect minute currents with a simple circuit configuration.
(2) In addition, even though the detected side and the detection side are sufficiently insulated by the insulation distance between the primary and secondary sides of the piezoelectric transformer PZT1, power is not supplied to the circuit network 130 that is the circuit configuration of the detected side. No supply is required. In this regard, if the circuit configuration uses an isolated amplifier as described above, a separate circuit (for example, a DC-DC converter) is required to supply drive power to the isolated amplifier, making the device configuration complicated. There is a problem.
(3) Similarly, since the detected side and the detection side are insulated by the piezoelectric transformer PZT1, there is no need to take measures against the influence of noise on the detector 120.
(4) The current detection device 100 realizes a novel circuit configuration as a whole by applying a material called a "piezoelectric transformer" to "detection of minute current." Furthermore, since the reliability of the "piezoelectric transformer" itself has already been established, there is no need to consider safety or reliability when putting the current detection device into practical use, and it has high industrial applicability.

The present invention is not limited to the embodiments described above, and can be implemented in various modifications.
The current detection device 100 can be implemented not only with the configuration shown in each figure but also with a configuration that can perform similar functions.

Further, in the embodiment, the shape and size, impedance frequency characteristics, and resonance characteristics of the piezoelectric transformer PZT1 are given as examples, and those having different shapes, sizes, and characteristics may be adopted.

The various values shown for the operation of the oscillator and detector are merely examples, and it goes without saying that circuit elements and the like will be selected and adjusted in accordance with various different conditions during implementation. In the embodiment, a minute current that is below the threshold is allowed and a minute current that exceeds the threshold is not allowed, but it is also possible to allow a minute current that is below the threshold and not allow a minute current that is above the threshold. It is arbitrary whether the upper limit or lower limit of the allowable range is set.

The structures illustrated and illustrated in each embodiment are merely preferred examples, and it goes without saying that the present invention can be suitably implemented even if various elements are added to the basic structure or some parts are replaced. do not have.

100 Current detection device 110 Oscillator 120 Detector PZT1 Piezoelectric transformer

Claims (4)

a piezoelectric transformer polarized on a primary side and a secondary side;
an oscillator that drives one side of the piezoelectric transformer in a region near a resonance frequency where the input impedance is inductive;
For a diode bridge having a configuration in which two circuits each having two diodes connected in series so as to conduct the current to be detected in the forward direction are connected in parallel between the input terminals of the current to be detected, A resistor is connected in parallel with the two circuits, and a terminal circuit that terminates the other side of the piezoelectric transformer is connected in parallel between the two diodes of each circuit that serve as an impedance output pair of the diode bridge. a circuit network having a circuit configuration;
A current detection device comprising: a detector that outputs a detection signal according to a conduction state of a current to be detected on one side of the piezoelectric transformer with the circuit network connected to the other side of the piezoelectric transformer. The current detection device according to claim 1,
The circuit network is
changing the value of a terminal equivalent resistance appearing on the other side of the piezoelectric transformer in accordance with a change in the conduction state of the detected current;
The detector is
A current detection device that outputs the detection signal based on a change in a resonance state that appears on one side of the piezoelectric transformer in response to a change in a terminal equivalent resistance that appears on the other side of the piezoelectric transformer. The current detection device according to claim 2,
The circuit network is
A resistor having a value determined from the relationship between the cut-off characteristic of the diode bridge and a threshold value defining an upper or lower limit of the current to be detected is connected to the diode bridge,
A current detection device characterized in that a resistor having a matching impedance value of the piezoelectric transformer is connected to the termination circuit. The current detection device according to any one of claims 1 to 3,
The piezoelectric transformer is
It has electrode pairs on the primary side and the secondary side, and is driven by the oscillator through the electrode pair on one side and connected to the circuit network through the electrode pair on the other side, so that the oscillator and the detector A current detection device characterized in that the current detection device is insulated from the current to be detected.

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