JP2004012246A - Insulation resistance measuring apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an insulation resistance measuring apparatus for acquiring a constant current at all times even when a voltage of a measuring power source is changed and making noise of a constant current circuit low. <P>SOLUTION: A bias supply 9 includes a photoelectromotive circuit 10 (to be concrete, a combination of an infrared LED 27 and a photodiode array element 28, what is called a photovoltaic circuit). A DC current source 23 is connected to the primary side of the photoelectromotive circuit 10, and a resistor 14 is serially connected to the DC current source 23 for input protection of the photoelectromotive circuit 10. In addition, a contact 15 is serially connected to the DC current source 23 on the primary side of the photoelectromotive circuit 10. When the contact 15 is turned on, the infrared LED 27 emits light, and the light is received by the photodiode array element 28 to generate a voltage on the secondary side. The voltage on the secondary side is supplied for the gates (G) of EFT 51 and FET 52. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an insulation resistance measuring device for measuring an insulation resistance of a device under test.
[0002]
[Prior art]
Generally, when measuring the insulation resistance of a device under test such as a capacitor using an insulation resistance measurement device, if the device under test is a defective product with a short circuit, the measurement circuit of the device may be destroyed. is there. Therefore, conventionally, an insulation resistance measuring device provided with a current limiting circuit in which a plurality of limiting resistors are connected in parallel has been used. With this insulation resistance measuring device, it is possible to prevent the measurement circuit from being destroyed even if the device under test is short-circuited.
[0003]
By the way, since the measurement of the insulation resistance of a capacitor also serves as a withstand voltage test, the applied measurement voltage differs depending on the type of capacitor. However, in the case of the conventional insulation resistance measuring device, since a current limiting circuit using a resistor is used, if the voltage of the measuring power supply is changed, the charging current will change. Therefore, in order to keep the charging current constant, it is necessary to prepare and switch several types of limiting resistors in accordance with the applied measured voltage value, and this has a problem that the circuit becomes complicated and the switching takes time.
[0004]
In addition, since the charging of the capacitor of the device under test becomes an exp curve based on the time constant of the CR, it takes a long time to charge the battery and the measurement efficiency is poor.
[0005]
In order to solve such a problem, one of the inventors of the present invention has proposed an insulation resistance measuring device disclosed in Japanese Patent Application Laid-Open No. 4-131770. That is, this insulation resistance measuring device includes a constant current circuit combining an FET and a bias power supply (DC / DC converter). With this insulation resistance measuring device, the constant current flowing through the device under test is made constant by utilizing the constant current characteristics of the FET, so that a constant current can always be obtained even when the voltage of the measurement power supply is changed. It is not required and does not require the trouble of switching. Further, since the FET is used, the charging time can be reduced, and the measurement efficiency is good.
[0006]
[Problems to be solved by the invention]
However, in the case of the insulation resistance measuring device disclosed in Japanese Patent Application Laid-Open No. 4-131770, a DC / DC converter is used for the constant current circuit. Although the DC / DC converter is electrically insulated on the primary side and the secondary side, oscillation noise is generated because the secondary side voltage is generated using oscillation. Therefore, the oscillation noise of the DC / DC converter is a factor that deteriorates the measurement accuracy.
[0007]
Therefore, an object of the present invention is to provide an insulation resistance measuring device that can always obtain a constant current even when the voltage of a measurement power supply is changed and can realize low noise of a constant current circuit.
[0008]
Means and action for solving the problem
In order to achieve the above object, an insulation resistance measuring device according to the present invention includes a measurement power supply for applying a measurement voltage to a device under test, a constant current circuit for stabilizing a current supplied from the measurement power supply, and a constant current circuit. An I / V converter connected to an output terminal for converting a leakage current of the device under test into a voltage, and measuring an insulation resistance of the device under test based on an output voltage of the I / V converter. The constant current circuit includes an FET, a bias power supply for applying a DC voltage between the gate and the source of the FET, a constant current negative feedback resistor connected in series to the source of the FET, and an output voltage of the bias power supply for the FET. A gate-source voltage setting resistor to be applied between the gate and the source of the FET by dividing the voltage, and that the bias power supply of the FET includes one of a photovoltaic circuit and a charge pump circuit. Special To.
[0009]
Further, the insulation resistance measuring apparatus according to the present invention includes a measurement power supply for applying one of the positive and negative measurement voltages to the device under test, a switch for selectively switching to one of the positive and negative measurement voltages, and a power supply from the measurement power supply. A constant current circuit for stabilizing the measured current, and an I / V converter connected to the output terminal of the constant current circuit for converting a leakage current of the device under test into a voltage. The output voltage of the I / V converter A constant current circuit comprising: two FETs each having one of source side and drain side connected in series; and a gate-source of each FET. A bias power supply for applying a direct-current voltage between them, a constant current negative feedback resistor connected in series to the source of each FET, and a voltage between the gate and source of each FET by dividing the output voltage of the bias power supply of the FET. It applied to the gate - a resistance for the source voltage setting, bias power FET, characterized in that it contains one of the circuits of the photovoltaic circuit and the charge pump circuit.
[0010]
With the above configuration, the bias power supply for the FET includes a photovoltaic circuit and a charge pump circuit that hardly generate noise, so that a low-noise constant current circuit can be obtained.
[0011]
In addition, the insulation resistance measuring device according to the present invention includes a capacitor having a capacitance larger than a parasitic capacitance generated between the gate and the source of the FET, connected between the gate and the source of the FET and connected to the source of the FET. Connected via a current negative feedback resistor. Thus, the capacitor connected between the constant current negative feedback resistor and the gate of the FET functions as a rush current preventing capacitor.
[0012]
Also, by providing a contact on the primary side of the photovoltaic circuit and controlling ON / OFF of the application of the measurement voltage at the contact, contacts with small contact withstand voltage, switching capacity and contact capacity can be used regardless of the applied measurement voltage. it can.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, an embodiment of an insulation resistance measuring device according to the present invention will be described with reference to the accompanying drawings. Each embodiment describes an insulation resistance measuring device for a capacitor, but is not necessarily limited to a capacitor.
[0014]
[First Embodiment, FIG. 1]
As shown in FIG. 1, the insulation resistance measuring device 1 includes a DC measurement power supply 2, an earth terminal 3, a switch 4, a constant current circuit 5 for making the current supplied from the DC measurement power supply 2 constant, and a constant current circuit 5. An OP amplifier 6 which is an I / V converter for converting a leakage current of the DUT 31 connected to the output terminal B of the current circuit 5 into a voltage, and an A / D converter for digitizing an output voltage of the OP amplifier 6 7 is provided.
[0015]
The DC measurement power supply 2 has a positive voltage source 21 and a negative voltage source 22. One ends of these voltage sources 21 and 22 are grounded, and the other ends are connected to an input terminal A of a constant current circuit 5 via a switch 4. The ground terminal 3 is connected to the input terminal A of the constant current circuit 5 via the switch 4. Therefore, the switching device 4 can selectively switch the positive voltage source 21, the negative voltage source 22, and the ground terminal 3.
[0016]
The output terminal B of the constant current circuit 5 is connected to one end of a capacitor 31 which is a device under test, and the other end of the capacitor 31 is connected to a negative input of the OP amplifier 6 via a contact C. The positive input of the OP amplifier 6 is grounded. The output terminal of the OP amplifier 6 is connected to the A / D converter 7. In addition, in the insulation resistance measuring device 1, the input protection circuit of the OP amplifier 6 and the like are omitted to simplify the circuit. 25 is a feedback resistor.
[0017]
Next, a description will be given of a method of measuring the insulation resistance of the capacitor 31, which is the device under test, using the insulation resistance measurement device 1. First, the output voltages of the positive and negative voltage sources 21 and 22 are set according to the type of the capacitor 31.
[0018]
Next, the switch 4 is switched to the positive voltage source 21. As a result, the current supplied from the positive voltage source 21 is made constant by the constant current circuit 5, and the constant current is charged in the capacitor 31. After the charging, the leakage current of the capacitor 31 is subjected to I / V conversion by the OP amplifier 6, the output voltage digitized by the A / D converter 7 is analyzed, and the insulation resistance of the capacitor 31 is calculated in terms of voltage.
[0019]
When measuring the reverse voltage, it is necessary to discharge the positive charging voltage of the capacitor 31 once, so the switch 4 is switched to the ground terminal 3 and then to the negative voltage source 22. Thereafter, the insulation resistance measurement using a negative voltage is performed in the same manner as the insulation resistance measurement using a positive voltage described above. Thus, it is possible to reliably measure the leakage current both when the positive voltage is applied and when the negative voltage is applied.
[0020]
Next, the constant current circuit 5 used in the insulation resistance measuring device 1 will be described. The constant current circuit 5 includes two FETs (field effect transistors) 51 and 52, a bias power supply 9 for applying a DC voltage between the gate (G) and the source (S) of each FET 51 and FET 52, and a bias power supply 9. A constant voltage diode 11 for stabilizing an output voltage, a variable resistor 12 for setting a voltage between a gate (G) and a source (S) of each of the FETs 51 and 52, and an inrush generated immediately after the start of application of a measurement voltage. It comprises a capacitor 13 for preventing a current, and constant current negative feedback resistors 16 and 17 connected in series to the sources (S) of the FETs 51 and 52.
[0021]
The FET 51 is connected in forward and reverse series so as to face the FET 52. That is, the drain (D) side of the FET 51 is connected to the input terminal A, and the source (S) side is connected to the source (S) side of the FET 52. The drain (D) side of the FET 52 is connected to the output terminal B. The FETs 51 and 52 are enhancement-type MOS-FETs, have excellent withstand voltage, and are suitable for measuring the withstand voltage of the capacitor 31 to which several hundred volts are applied. Further, the FET 51 and the FET 52 can shorten the charging time of the capacitor 31 and have good measurement efficiency.
[0022]
Between the source (S) and the drain (D) of each of the FETs 51 and 52, protection diodes 18 and 19 that allow a flow from the source (S) to the drain (D) are connected in parallel.
[0023]
The constant voltage diode 11 is connected between the gate (G) and the source (S) of each of the FETs 51 and 52 in parallel. The variable resistor 12 is also connected between the gate (G) and the source (S) of each of the FETs 51 and 52 in parallel. The variable resistor 12 is applied between the gate (G) and the source (S) of each of the FETs 51 and 52 according to the increase and decrease of the current flowing between the drain (D) and the source (S) of each of the FETs 51 and 52. It functions as a gate-source voltage setting resistor for adjusting the voltage.
[0024]
The bias power supply 9 includes a photovoltaic circuit 10 (specifically, a combination of an infrared LED 27 and a photodiode array element 28, a so-called photovol).
[0025]
A DC power supply 23 is connected to the primary side of the photovoltaic circuit 10, and a resistor 14 is connected in series with the DC power supply 23 for input protection of the photovoltaic circuit 10. The contact 15 is connected to the primary side of the photovoltaic circuit 10 in series with the DC power supply 23. When the contact 15 is turned on, the infrared LED 27 emits light, and this light is received by the photodiode array element 28 to generate a secondary voltage. This secondary voltage is supplied to the gates (G) of the FETs 51 and 52. When the output voltage of the photovoltaic circuit 10 is stable, it is not necessary to use the constant voltage diode 11.
[0026]
In the constant current circuit 5 having such a configuration, when measuring the insulation resistance of the capacitor 31 which is the device under test by the positive voltage, or when discharging after applying the negative voltage, the current flows from the input terminal A to the output terminal B. Flows to At this time, a current flows through the protection diode 19 on the FET 52 side, and the constant current operation is performed by the FET 51.
[0027]
This will be described more specifically. The value of the current flowing between the drain (D) and the source (S) of the FET 51 is determined by the potential difference between the gate (G) and the source (S) of the FET 51. The voltage output from the bias power supply 9 is applied to the gate (G) of the FET 51. This voltage is stabilized by a constant voltage diode 11 and divided by a variable resistor 12. When the current flowing from the input terminal A to the output terminal B increases, that is, when the current flowing between the drain (D) and the source (S) of the FET 51 increases, both ends of the constant current negative feedback resistor 16 , The voltage applied between the gate (G) and the source (S) of the FET 51 decreases, and the current flowing between the drain (D) and the source (S) of the FET 51 (the current flowing from the input terminal A to the output terminal B) ).
[0028]
Conversely, when the current flowing from the input terminal A to the output terminal B decreases, the potential difference between both ends of the constant current negative feedback resistor 16 decreases, and the voltage applied between the gate (G) and the source (S) of the FET 51. And the current flowing between the drain (D) and the source (S) of the FET 51 (the current flowing from the input terminal A to the output terminal B) is increased. As described above, even if the voltage of the positive voltage source 21 is changed, a constant current can always flow through the capacitor 31 by utilizing the constant current characteristics of the FET 51.
[0029]
In addition, when the insulation resistance due to the negative voltage of the capacitor 31 is measured, or when the discharge is performed after the application of the positive voltage, the current flows from the output terminal B to the input terminal A. At this time, a current flows through the protection diode 18 on the FET 51 side, and the constant current operation is performed by the FET 52. In this way, by connecting two FETs in series, the current at the time of applying a positive and negative voltage and the current at the time of discharging can be made constant.
[0030]
In general, a parasitic capacitance (hereinafter referred to as an FET parasitic capacitance) exists between the gate (G) and the source (S) of the FET. Therefore, the follow-up property of the increase / decrease operation of the potential difference between the gate (G) and the source (S) with respect to the increase / decrease of the current value flowing between the drain (D) -source (S) of the FET (hereinafter, the follow-up property of the constant current operation and ) Is determined by the charge / discharge time of the FET parasitic capacitance. The charge / discharge time is determined by the time constant CR of the combined resistance value R of the variable resistor 12 and the constant current negative feedback resistor 16 (or 17) and the capacitance value C of the FET parasitic capacitance.
[0031]
When the driving current of the bias power supply 9 for supplying a voltage applied between the gate (G) and the source (S) of the FET is sufficiently large, the resistance value of the variable resistor 12 can be reduced, and Good follow-up of current operation. However, the photovoltaic circuit 10 used in the first embodiment has a small driving current and needs to increase the resistance value of the variable resistor 12. For this reason, the followability of the constant current operation is poor. In particular, immediately after the start of the application of the measurement voltage (when the current value changes suddenly), an inrush current of about several μs to several ms (current larger than the set current value) is generated. Flows.
[0032]
The capacitor 13 prevents this inrush current, and is connected between the FET parasitic capacitance and the constant current negative feedback resistor 16 (or 17). By connecting the capacitor 13, the charge / discharge of the FET parasitic capacitance when the current flowing between the drain (D) and the source (S) of the FET changes abruptly is performed by discharging / charging the capacitor 13 instantaneously. Therefore, the followability of the constant current operation is improved, and occurrence of an inrush current is prevented. In this case, the followability of the constant current operation is determined by the FET parasitic capacitance and the constant current negative feedback resistor 16 (or 17), and does not depend on the resistance value of the variable resistor 12. Therefore, regardless of the magnitude of the drive current of the photovoltaic circuit 10, inrush current can be prevented, and damage to the DUT 31 and the insulation resistance measuring device 1 due to overcurrent can be prevented.
[0033]
The applied measurement voltage varies depending on the type of the DUT 31. A switch for performing positive / negative switching of the measurement voltage, switching between charge / discharge, and ON / OFF of the application of the measurement voltage has a large contact withstand voltage, switching capacity, and contact capacity. It is necessary to use something, which increases the cost. In the first embodiment, the voltage output from the bias power supply 9 is applied to each of the FET 51 and the FET 52. By turning on / off the primary contact 15 of the bias power supply 9, the DUT 31 is measured. ON / OFF control of the application of the measurement voltage to the power supply is performed. Therefore, it can be realized with the contact 15 having a small contact withstand voltage, switching capacity, and contact capacity. Further, by switching the switch 4 when the application of the measurement voltage is OFF, an inexpensive switch 4 having a small contact withstand voltage and a small switching capacity can be used regardless of the magnitude of the applied measurement voltage.
[0034]
In the insulation resistance measuring device 1 having the above configuration, the bias power supply 9 of the FET 51 and the FET 52 includes the photovoltaic circuit 10. Therefore, the primary side and the secondary side of the photovoltaic circuit 10 are electrically insulated, and the secondary side voltage is generated by using the photoelectric effect, so that the power supply has low noise. As a result, low noise of the constant current circuit 5 can be realized, and accurate insulation resistance measurement can be realized.
[0035]
[Second embodiment, FIG. 2]
As shown in FIG. 2, the constant current circuit 5A of the insulation resistance measuring apparatus 1A includes a bias power supply 9a, 9b for each FET 51, 52, constant voltage diodes 11a, 11b, variable resistors 12a, 12b, and an inrush current. It comprises prevention capacitors 13a and 13b and constant current negative feedback resistors 16 and 17. The drains (D) of the FETs 51 and 52 are connected in series.
[0036]
The bias power supplies 9a and 9b include photovoltaic circuits 10a and 10b, respectively. A common DC power supply 23 is connected to the primary side of the photovoltaic circuits 10a and 10b, and input protection resistors 14a and 14b are connected in series to the DC power supply 23. A contact 15 is connected to the primary side of the photovoltaic circuits 10a and 10b in series with the DC power supply 23.
[0037]
The insulation resistance measuring device 1A having the above configuration has the same operation and effect as the device 1 of the first embodiment. Further, even if there is an individual difference between the two FETs 51 and 52, the voltage value applied to the gate (G) is set by setting various electric constants of the variable resistors 12a and 12b and the bias power supplies 9a and 9b. Can be set independently.
[0038]
[Third embodiment, FIG. 3]
The insulation resistance measuring apparatus 1B shown in FIG. 3 is used when it is not necessary to measure the insulation resistance with both positive and negative voltages and when it is not necessary to discharge the voltage charged in the DUT 31. Since the insulation resistance measuring device 1B requires only one FET 51 and the constant current negative feedback resistor 16, the number of components is small and the cost is low.
[0039]
[Fourth embodiment, FIG. 4]
An insulation resistance measuring apparatus 1C shown in FIG. 4 uses a bias power supply 40 including a charge pump circuit for a constant current circuit 5C. The DC power supply 23 and the input protection resistor 14 are connected to the primary side of the bias power supply 40. The first-stage capacitor 41 is connected in parallel to the DC power supply through first-stage switches S1a and S1b, and is connected in parallel to the second-stage capacitor 42 through second-stage switches S2a and S2b.
[0040]
The method of generating the secondary voltage by the bias power supply 40 is as follows. First, the first-stage switches S1a and S1b are turned on, the second-stage switches S2a and S2b are turned off, and the first-stage capacitor 41 is charged by the DC power supply 23. Next, the first-stage switches S1a and S1b are turned off, the second-stage switches S2a and S2b are turned on, and the second-stage capacitor 42 is charged by the electric charge charged in the first-stage capacitor 41. Further, the above-described charging operation is repeated. Thus, the secondary voltage generated in the second-stage capacitor 42 is supplied to the gates (G) of the FETs 51 and 52.
[0041]
In the insulation resistance measuring apparatus 1C having the above configuration, the bias power supply 40 for the FETs 51 and 52 includes a charge pump circuit. Therefore, the primary side and the secondary side of the charge pump circuit are electrically insulated by the first-stage switches S1a and S1b, and the secondary-side voltage is charged to the charged second-stage capacitor 42 (so-called battery). ), The power supply has low noise. As a result, low noise of the constant current circuit 5C can be realized, and accurate insulation resistance measurement can be realized.
[0042]
[Other Embodiments]
The insulation resistance measuring device according to the present invention is not limited to the above-described embodiment, but can be variously modified within the scope of the gist. For example, in the above-described embodiment, the variable resistor is used as the gate-source voltage setting resistor for adjusting the voltage applied between the gate (G) and the source (S) of the FET. Adjustment may be unnecessary.
[0043]
【The invention's effect】
As apparent from the above description, according to the present invention, the bias power supply for the FET includes the photovoltaic circuit and the charge pump circuit that hardly generate noise, so that a low-noise constant current circuit can be obtained.
[Brief description of the drawings]
FIG. 1 is an electric circuit diagram showing a first embodiment of an insulation resistance measuring device according to the present invention.
FIG. 2 is an electric circuit diagram showing a second embodiment of the insulation resistance measuring device according to the present invention.
FIG. 3 is an electric circuit diagram showing a third embodiment of the insulation resistance measuring device according to the present invention.
FIG. 4 is an electric circuit diagram showing a fourth embodiment of the insulation resistance measuring device according to the present invention.
[Explanation of symbols]
1, 1A, 1B, 1C Insulation resistance measurement device 2 DC measurement power supply 4 Switchers 5, 5A, 5B, 5C Constant current circuit 6 OP amplifier (I / V converter)
9, 9a, 9b, 40 ... bias power supplies 10, 10a, 10b ... photovoltaic circuits 12, 12a, 12b ... gate-source voltage setting resistors 13, 13a, 13b ... inrush current prevention capacitors 15 ... contacts 16, 17 ... Constant current negative feedback resistors 51, 52 ... FET

Claims (4)

A measurement power supply for applying a measurement voltage to the device under test,
A constant current circuit for stabilizing a current supplied from the measurement power supply,
An I / V converter connected to an output terminal of the constant current circuit and configured to convert a leakage current of the device under test into a voltage;
An insulation resistance measuring device for measuring an insulation resistance of an object to be measured based on an output voltage of the I / V converter,
The constant current circuit includes: an FET;
A bias power supply for applying a DC voltage between the gate and the source of the FET;
A constant current negative feedback resistor connected in series to the source of the FET;
A gate-source voltage setting resistor that divides an output voltage of a bias power supply of the FET and applies the divided voltage between a gate and a source of the FET;
The bias power supply of the FET includes one of a photovoltaic circuit and a charge pump circuit,
An insulation resistance measuring device characterized by the above-mentioned. A measurement power supply for applying one of positive and negative measurement voltages to the device under test,
A switch for selectively switching to one of the positive and negative measurement voltages,
A constant current circuit for stabilizing a current supplied from the measurement power supply,
An I / V converter connected to an output terminal of the constant current circuit and configured to convert a leakage current of the device under test into a voltage;
An insulation resistance measuring device for measuring an insulation resistance of an object to be measured based on an output voltage of the I / V converter,
The constant current circuit includes two FETs in which either the source side or the drain side is connected in series,
A bias power supply for applying a DC voltage between the gate and the source of each of the FETs,
A constant current negative feedback resistor connected in series to the source of each of the FETs;
A gate-source voltage setting resistor that divides the output voltage of the bias power supply of the FET and applies the divided voltage between the gate and the source of each FET;
The bias power supply of the FET includes one of a photovoltaic circuit and a charge pump circuit,
An insulation resistance measuring device characterized by the above-mentioned. A capacitor having a capacitance larger than a parasitic capacitance generated between the gate and the source of the FET is connected between the gate and the source of the FET via a constant current negative feedback resistor connected to the source of the FET. The insulation resistance measuring device according to claim 1 or 2, wherein 4. The insulation resistance measuring device according to claim 1, wherein a contact is provided on a primary side of the photovoltaic circuit, and ON / OFF control of the application of the measurement voltage is performed at the contact.

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