Isolation 4-20mA current output circuit and relay protection system

Abstract

The invention discloses an isolation 4-20mA current output circuit and a relay protection system, and relates to the technical field of relay protection, and the isolation 4-20mA current output circuit is characterized in that the input end of an isolation circuit is connected with a control signal and a power supply, the output end of the isolation circuit is connected with the input end of a digital-to-analog conversion circuit, and the output end of the digital-to-analog conversion circuit is connected with the input end of a signal conditioning circuit; the output end of the signal conditioning circuit is connected with the input end of the EMC protection circuit; the isolation circuit is used for outputting the isolated control signal and the isolated power supply to the digital-to-analog conversion circuit; the digital-to-analog conversion circuit is used for outputting target direct-current conversion analog voltage corresponding to the current of 4-20 mA; the signal conditioning circuit is used for converting the target direct current conversion analog voltage into 4-20mA output current; and the EMC protection circuit is used for carrying out electromagnetic compatibility protection on the signal conditioning circuit. According to the invention, the technical problems of complex circuit and low precision in the prior art are solved.

Description

Technical Field

[0001] The invention relates to the technical field of relay protection, in particular to an isolated 4-20mA current output circuit and a relay protection system. Background Art

[0002] In the field of relay protection, 4-20mA current output signal is a commonly used standard signal. In order to improve the anti-interference and electrical safety of the system, the signal needs to be electrically isolated. Currently, the commonly used isolation methods include photoelectric and transformer. The isolation and linearity of this isolation method are poor and are easily affected by temperature. Analog signal isolation transmission is usually a common problem in circuit design. The circuits designed are usually complex or have low precision or large temperature drift. At the same time, it is also necessary to consider the complex electromagnetic environment of the actual application and meet the relevant EMC test performance indicators. It is necessary to add effective EMC protection circuits to improve the circuit's anti-EMC performance. Summary of the invention

[0003] The purpose of the present invention is to provide an isolated 4-20mA current output circuit and a relay protection system in order to solve at least one of the above technical problems.

[0004] In a first aspect, an embodiment of the present invention provides an isolated 4-20mA current output circuit, comprising: an isolation circuit, a digital-to-analog conversion circuit, a signal conditioning circuit and an EMC protection circuit; wherein the input end of the isolation circuit is connected to a control signal and a power supply, the output end of the isolation circuit is connected to the input end of the digital-to-analog conversion circuit, the output end of the digital-to-analog conversion circuit is connected to the input end of the signal conditioning circuit, and the output end of the signal conditioning circuit is connected to the input end of the EMC protection circuit; the isolation circuit is used to output the isolated control signal and the isolated power supply to the digital-to-analog conversion circuit; the digital-to-analog conversion circuit is used to output a target DC conversion analog voltage corresponding to a 4-20mA current; the signal conditioning circuit is used to convert the target DC conversion analog voltage into a 4-20mA output current; and the EMC protection circuit is used to perform electromagnetic compatibility protection on the signal conditioning circuit.

[0005] Furthermore, the isolation circuit includes a digital isolator and an isolation power supply module; wherein the digital isolator is used to isolate and transmit control signals; and the isolation power supply module is used to isolate the power supply.

[0006] Furthermore, the isolated power supply module includes a voltage-stabilized DC / DC power supply module with an output of 24V.

[0007] Furthermore, the digital-to-analog conversion circuit includes a DAC chip, a voltage reference source and a DCDC power chip; wherein the output end of the isolation power module is connected to the input end of the DCDC power chip, the output end of the digital isolator is connected to the control end of the DAC chip, and the output end of the DCDC power chip is respectively connected to the power input end of the digital isolator, the power input end of the DAC chip and the power input end of the voltage reference source; the output end of the voltage reference source is connected to the reference voltage input end of the DAC chip, and the output end of the DAC chip is connected to the input end of the signal adjustment circuit; the DAC chip is used to output the corresponding target DC conversion analog voltage based on the isolated control signal output by the digital isolator and the reference voltage output by the voltage reference source; the DCDC power chip is used to provide power supply for the DAC chip, the digital isolator and the voltage reference source.

[0008] Further, the signal conditioning circuit includes a first operational amplifier, the input end of the first operational amplifier is connected to the output end of the digital-to-analog conversion circuit, the output end of the first operational amplifier is connected to the gate of an N-channel enhancement mode MOS tube, the source of the N-channel enhancement mode MOS tube is connected to a second resistor, the drain of the N-channel enhancement mode MOS tube is connected in series with a third resistor and a fourth resistor, the two ends of the third resistor and the fourth resistor are respectively connected to two input ends of the second operational amplifier, the positive power supply end of the first operational amplifier is connected to the connection point of the third resistor and the fourth resistor; the output end of the second operational amplifier is connected to the gate of a P-channel enhancement mode MOS tube, the source of the P-channel enhancement mode MOS tube is connected to one end of the fourth resistor, and the drain of the P-channel enhancement mode MOS tube is connected to the EMC protection circuit.

[0009] Furthermore, a first resistor is arranged between the output end of the first operational amplifier and the gate of the N-channel enhancement-mode MOS transistor, and a fifth resistor is arranged between the output end of the second operational amplifier and the gate of the P-channel enhancement-mode MOS transistor.

[0010] Furthermore, a first capacitor is arranged between the output end of the first operational amplifier and the source of the N-channel enhancement mode MOS transistor, and a second capacitor is arranged between the output end of the second operational amplifier and the source of the P-channel enhancement mode MOS transistor.

[0011] Furthermore, the EMC protection circuit is connected to the signal output loop of the isolated 4-20mA current output circuit; the EMC protection circuit includes: a first varistor and a first safety capacitor connected between the positive end and the ground end of the signal output loop, a second varistor and a second safety capacitor connected between the negative end and the ground end of the signal output loop, a third varistor and a TVS tube connected between the positive end and the negative end of the signal output loop, a first magnetic bead and a second magnetic bead connected in series on the positive end and the negative end of the signal output loop, respectively, and a diode connected in series on the positive end of the signal output loop.

[0012] Furthermore, the EMC protection circuit also includes a third capacitor connected across the positive end and the negative end of the signal output loop.

[0013] In a second aspect, an embodiment of the present invention further provides a relay protection system, including the isolated 4-20mA current output circuit provided by an embodiment of the present invention.

[0014] The present invention provides an isolated 4-20mA current output circuit and a relay protection system, wherein the electrical isolation of a control signal and a power supply is realized through an isolation circuit and a digital-to-analog conversion circuit, the voltage output by the digital-to-analog conversion circuit is converted into a 4-20mA output current through a signal conditioning circuit, and the current output circuit is protected against electromagnetic compatibility through an EMC protection circuit. The present invention can realize the isolated output of 4-20mA current, has high precision and good temperature drift coefficient, can be applied to complex environments, can meet various EMC experimental indicators, realize a simple, reliable and easy-to-promote circuit, and alleviate the technical problems of complex circuits and low precision in the prior art. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] In order to more clearly illustrate the implementation methods of the present application or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0016] Figure 1 A schematic diagram of the overall architecture of an isolated 4-20mA current output circuit provided by an embodiment of the present invention;

[0017] Figure 2 A circuit diagram of an isolated 4-20mA current output circuit provided in an embodiment of the present invention.

[0018] In the figure: 10, isolation circuit, 11, digital isolator, 12, isolation power supply module, 20, digital-to-analog conversion circuit, 21, DAC chip, 22, voltage reference source, 23, DCDC power supply chip, 30, signal conditioning circuit, 31, first operational amplifier, 32, N-channel enhancement MOS tube, 33, second resistor, 34, third resistor, 35, fourth resistor, 36, second operational amplifier, 37, P-channel enhancement MOS tube, 38, first resistor, 39, fifth resistor, 310, first capacitor, 311, second capacitor, 40, EMC protection circuit, 41, first varistor, 42, first safety capacitor, 43, second varistor, 44, second safety capacitor, 45, third varistor, 46, TVS tube, 47, first magnetic bead, 48, second magnetic bead, 49, diode, 410, third capacitor. DETAILED DESCRIPTION

[0019] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0020] Figure 1 1 is a schematic diagram of the overall architecture of an isolated 4-20mA current output circuit provided according to an embodiment of the present invention. Figure 1 As shown, it includes: an isolation circuit 10, a digital-to-analog conversion circuit 20, a signal conditioning circuit 30 and an EMC protection circuit 40; wherein, the input end of the isolation circuit 10 is connected to the control signal and the power supply, the output end of the isolation circuit 10 is connected to the input end of the digital-to-analog conversion circuit 20, the output end of the digital-to-analog conversion circuit 20 is connected to the input end of the signal conditioning circuit 30, and the output end of the signal conditioning circuit 30 is connected to the input end of the EMC protection circuit 40.

[0021] Specifically, the isolation circuit 10 is used to output the isolated control signal and the isolated power supply to the digital-to-analog conversion circuit 20;

[0022] A digital-to-analog conversion circuit 20 is used to output a target DC conversion analog voltage corresponding to a 4-20 mA current;

[0023] A signal conditioning circuit 30, for converting the target DC conversion analog voltage into a 4-20mA output current;

[0024] The EMC protection circuit 40 is used to perform electromagnetic compatibility protection on the signal conditioning circuit 30 .

[0025] Figure 21 is a circuit diagram of an isolated 4-20mA current output circuit provided according to an embodiment of the present invention. Figure 2 As shown, the isolation circuit 10 includes a digital isolator 11 and an isolation power module 12. In some optional implementations provided by the embodiments of the present invention, the model of the digital isolator 11 is BL7141WH, and the model of the isolation power module 12 is F0524S-2WR3.

[0026] Among them, the digital isolator 11 is used to isolate and transmit control signals; optionally, the control signals include SPI signals, IIC signals, etc. Usually, digital isolation devices can withstand an isolation voltage of up to 5000V and have high common-mode immunity. In a complex environment with high voltage and high interference, they can complete signal isolation transmission and isolate digital interference to subsequent analog circuits. For example, Figure 2 The VIA, VIB, and VIC pins of the digital isolator 11 shown are connected to the SPI signal, and then the isolated SPI signal is output through the VOA, VOB, and VOC pins.

[0027] The isolated power supply module 12 is used to isolate the power supply. Preferably, the isolated power supply module 12 includes a voltage-stabilized DC / DC power supply module with an output of 24V, and the output isolated 24V voltage ISO24V is used to power the DCDC power supply chip, the operational amplifier and the output loop.

[0028] Specifically, Figure 2 As shown, the digital-to-analog conversion circuit 20 includes a DAC chip 21, a voltage reference source 22, and a DCDC power chip 23. In some optional implementations provided in the embodiments of the present invention, the model of the DAC chip 21 includes a D / A converter SGM5351, the model of the power reference source 22 includes SGM4027-4.096, and the model of the DCDC power chip 23 includes a DC / DC converter SGM61412.

[0029] Among them, the output end (VOUT+ pin) of the isolated power supply module 12 is connected to the input end (Vin pin) of the DCDC power supply chip 23, the output end (VOA pin, VOB pin and VOC pin) of the digital isolator 11 is connected to the control end (nSYNC pin, SCLK pin and DIN / OUT pin) of the DAC chip 21, and the output end (Vout pin) of the DCDC power supply chip 23 is respectively connected to the power input end (VDD2 pin) of the digital isolator 11, the power input end (VCC pin) of the DAC chip 21 and the power input end (IN pin) of the voltage reference source 22; the output end (OUT pin) of the voltage reference source 22 is connected to the reference voltage input end (VREF pin) of the DAC chip 21, and the output end (VOUT pin and VFB pin) of the DAC chip 21 is connected to the input end of the signal adjustment circuit 30.

[0030] Specifically, the DAC chip 21 is used to output a corresponding target DC conversion analog voltage based on the isolated control signal output by the digital isolator 11 and the reference voltage output by the voltage reference source 22;

[0031] The DCDC power chip 23 is used to provide power supply to the DAC chip 21 , the digital isolator 11 and the voltage reference source 22 .

[0032] Specifically, Figure 2 As shown, the signal conditioning circuit 30 includes a first operational amplifier 31, the input end of the first operational amplifier 31 is connected to the output end of the digital-to-analog conversion circuit 20 (i.e., the VOUT pin and the VFB pin of the DAC chip 21), the output end of the first operational amplifier 31 is connected to the gate of the N-channel enhancement mode MOS transistor 32, the source of the N-channel enhancement mode MOS transistor 32 is connected to the second resistor 33, the drain of the N-channel enhancement mode MOS transistor 32 is connected in series with the third resistor 34 and the fourth resistor 35, the two ends of the third resistor 34 and the fourth resistor 35 are respectively connected to the two input ends of the second operational amplifier 36, the positive power supply end of the first operational amplifier 31 is connected to the connection point of the third resistor 34 and the fourth resistor 35; the output end of the second operational amplifier 36 is connected to the gate of the P-channel enhancement mode MOS transistor 37, the source of the P-channel enhancement mode MOS transistor 37 is connected to one end of the fourth resistor 35, and the drain of the P-channel enhancement mode MOS transistor 37 is connected to the EMC protection circuit 40.

[0033] Specifically, Figure 2 As shown, the source of the N-channel enhancement mode MOS transistor 32 is also connected to the input end of the first operational amplifier 31, and the other end of the second resistor 33 is grounded.

[0034] In the embodiment of the present invention, the first operational amplifier 31 adopts an input-output rail-to-rail operational amplifier to increase the range of the output current, adjusts the voltage output by the DAC chip 21 to the second resistor 33, and then combines the N-channel enhancement MOS transistor 32 and the P-channel enhancement MOS transistor 37 to adjust the voltage on the third resistor 34 and the fourth resistor 35 to be equivalent to the voltage output by the DAC chip 21. At this time, the output current is only related to the output voltage of the DAC chip 21 and the fourth resistor 35. The output current of the circuit is equivalent to a constant current source, and the output current is:

[0035] I=VOUT

[0036] R4

[0037] Wherein, VOUT is the output voltage of the DAC chip 21, R4 is the resistance value of the fourth resistor 35, and I is the output current value of the signal conditioning circuit. It can be seen that the accuracy of the current is only related to the output accuracy of the DAC chip 21 and the accuracy of the fourth resistor 35.

[0038] Preferably, the embodiment of the present invention adopts a 16-bit DAC chip and a 0.1% precision low temperature drift resistor as the fourth resistor 35. At this time, the output current accuracy can reach 0.1%, and it has a good temperature drift coefficient. The output current flows out from the isolated power module 12. The power of the isolated power module 12 is usually a 2W isolated DCDC module with a wide load range, and can connect a load with a maximum resistance of 1K.

[0039] A first resistor 38 is further provided between the output end of the first operational amplifier 31 and the gate of the N-channel enhancement mode MOS transistor 32 , and a fifth resistor 39 is further provided between the output end of the second operational amplifier 36 and the gate of the P-channel enhancement mode MOS transistor 37 .

[0040] Specifically, the first resistor 38 and the fifth resistor 39 are used to protect the N-channel enhancement mode MOS transistor 32 and the P-channel enhancement mode MOS transistor 37 respectively.

[0041] Specifically, Figure 2 As shown, a first capacitor 310 is further arranged between the output end of the first operational amplifier 31 and the source of the N-channel enhancement mode MOS transistor 32 , and a second capacitor 311 is further arranged between the output end of the second operational amplifier 36 and the source of the P-channel enhancement mode MOS transistor 37 .

[0042] Specifically, the first capacitor 310 and the second capacitor 311 mainly function to prevent circuit oscillation and enable the operational amplifier to operate in a steady state.

[0043] Specifically, Figure 2 As shown, the EMC protection circuit 40 is connected to the signal output loop of the isolated 4-20mA current output circuit; the EMC protection circuit 40 includes: a first varistor 41 and a first safety capacitor 42 connected between the positive end (OA+) and the ground end of the signal output loop, a second varistor 43 and a second safety capacitor 44 connected between the negative end (OA-) and the ground end of the signal output loop, a third varistor 45 and a TVS tube 46 connected between the positive and negative ends of the signal output loop, a first magnetic bead 47 and a second magnetic bead 48 connected in series to the positive and negative ends of the signal output loop, respectively, and a diode 49 connected in series to the positive end of the signal output loop.

[0044] In the embodiment of the present invention, the first varistor 41, the second varistor 43, the first safety capacitor 42 and the second safety capacitor 44 are connected between the positive terminals OA+ and OA- of the signal output loop and the ground terminal to suppress common-mode interference in the circuit;

[0045] A third varistor 45 is used to suppress differential mode interference in the circuit;

[0046] The first magnetic bead 47 and the second magnetic bead 48 are used to suppress interference of high-frequency conduction signals in the circuit;

[0047] The diode 49 and the TVS tube 46 mainly play the role of resisting surge voltage.

[0048] like Figure 2 As shown, the EMC protection circuit 40 further includes a third capacitor 410 connected across the positive terminal and the negative terminal of the signal output loop, for absorbing the tip or burr interference in the part.

[0049] In the embodiment of the present invention, according to the actual application scenario, the EMC protection circuit 40 composed of devices with suitable parameters (for example, 1000V and 47V varistors are used for voltage clamping; lead beads with a center frequency of 100MHz and an impedance of 100 ohms are used for suppressing high-frequency interference; 1N4007 diodes are used for suppressing surge signals; TVS diodes using SMBJ28A are used to protect the post-op amp conditioning circuit) can greatly improve the circuit's anti-electromagnetic interference ability, and also improve the circuit's EMC performance, thereby ensuring the reliable operation of the circuit.

[0050] From the above description, it can be seen that the present invention provides an isolated 4-20mA current output circuit, which realizes electrical isolation of control signals and power supply through isolation circuit and digital-to-analog conversion circuit, converts the voltage output by the digital-to-analog conversion circuit into 4-20mA output current through signal conditioning circuit, and performs electromagnetic compatibility protection on the current output circuit through EMC protection circuit. The present invention can realize isolated output of 4-20mA current, has high precision and good temperature drift coefficient, can be applied to complex environments, and can meet various EMC experimental indicators at the same time, realizes that the circuit is simple, reliable and easy to promote, and alleviates the technical problems of complex circuit and low precision in the prior art.

[0051] The present invention also provides a relay protection system, comprising the isolated 4-20mA current output circuit provided in an embodiment of the present invention.

[0052] It will be apparent to those skilled in the art that the invention is not limited to the details of the exemplary embodiments described above and that the invention can be implemented in other specific forms without departing from the spirit or essential features of the invention. Therefore, the embodiments should be considered exemplary and non-limiting in all respects, and the scope of the invention is defined by the appended claims rather than the foregoing description, and it is intended that all variations falling within the meaning and scope of the equivalent elements of the claims be included in the invention. Any reference numeral in a claim should not be considered as limiting the claim to which it relates.

[0053] In addition, it should be understood that although the present specification is described according to implementation modes, not every implementation mode contains only one independent technical solution. This description of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment may also be appropriately combined to form other implementation modes that can be understood by those skilled in the art.

Claims

1. An isolated 4-20mA current output circuit, characterized in that: include: An isolation circuit, a digital-to-analog conversion circuit, a signal conditioning circuit and an EMC protection circuit; wherein the input end of the isolation circuit is connected to a control signal and a power supply, the output end of the isolation circuit is connected to the input end of the digital-to-analog conversion circuit, the output end of the digital-to-analog conversion circuit is connected to the input end of the signal conditioning circuit, and the output end of the signal conditioning circuit is connected to the input end of the EMC protection circuit; The isolation circuit is used to output the isolated control signal and the isolated power supply to the digital-to-analog conversion circuit; The digital-to-analog conversion circuit is used to output a target DC conversion analog voltage corresponding to a 4-20 mA current; The signal conditioning circuit is used to convert the target DC conversion analog voltage into a 4-20mA output current; The EMC protection circuit is used to perform electromagnetic compatibility protection on the signal conditioning circuit.

2. The isolated 4-20mA current output circuit according to claim 1, characterized in that: The isolation circuit includes a digital isolator and an isolation power supply module; wherein, The digital isolator is used to isolate and transmit control signals; The isolated power supply module is used to isolate the power supply.

3. The isolated 4-20mA current output circuit according to claim 2, characterized in that: The isolated power supply module includes a voltage-stabilized DC / DC power supply module with an output of 24V.

4. The isolated 4-20mA current output circuit according to claim 2, characterized in that: The digital-to-analog conversion circuit includes a DAC chip, a voltage reference source and a DCDC power chip; wherein the output end of the isolation power module is connected to the input end of the DCDC power chip, the output end of the digital isolator is connected to the control end of the DAC chip, and the output end of the DCDC power chip is respectively connected to the power input end of the digital isolator, the power input end of the DAC chip and the power input end of the voltage reference source; the output end of the voltage reference source is connected to the reference voltage input end of the DAC chip, and the output end of the DAC chip is connected to the input end of the signal adjustment circuit; The DAC chip is used to output a corresponding target DC conversion analog voltage based on the isolated control signal output by the digital isolator and the reference voltage output by the voltage reference source; The DCDC power chip is used to provide power supply for the DAC chip, the digital isolator and the voltage reference source.

5. The isolated 4-20mA current output circuit according to claim 1, characterized in that: The signal conditioning circuit includes a first operational amplifier, wherein the input end of the first operational amplifier is connected to the output end of the digital-to-analog conversion circuit, the output end of the first operational amplifier is connected to the gate of an N-channel enhancement mode MOS transistor, the source of the N-channel enhancement mode MOS transistor is connected to a second resistor, the drain of the N-channel enhancement mode MOS transistor is connected in series with a third resistor and a fourth resistor, the two ends of the third resistor and the fourth resistor are respectively connected to two input ends of the second operational amplifier, the positive power supply end of the first operational amplifier is connected to the connection point of the third resistor and the fourth resistor; the output end of the second operational amplifier is connected to the gate of a P-channel enhancement mode MOS transistor, the source of the P-channel enhancement mode MOS transistor is connected to one end of the fourth resistor, and the drain of the P-channel enhancement mode MOS transistor is connected to the EMC protection circuit.

6. The isolated 4-20mA current output circuit according to claim 5, characterized in that: A first resistor is further arranged between the output end of the first operational amplifier and the gate of the N-channel enhancement-mode MOS transistor, and a fifth resistor is further arranged between the output end of the second operational amplifier and the gate of the P-channel enhancement-mode MOS transistor.

7. The isolated 4-20mA current output circuit according to claim 5, characterized in that: A first capacitor is further arranged between the output end of the first operational amplifier and the source of the N-channel enhancement mode MOS transistor, and a second capacitor is further arranged between the output end of the second operational amplifier and the source of the P-channel enhancement mode MOS transistor.

8. The isolated 4-20mA current output circuit according to claim 1, characterized in that: The EMC protection circuit is connected to the signal output loop of the isolated 4-20mA current output circuit; the EMC protection circuit includes: a first varistor and a first safety capacitor connected between the positive end and the ground end of the signal output loop, a second varistor and a second safety capacitor connected between the negative end and the ground end of the signal output loop, a third varistor and a TVS tube connected between the positive end and the negative end of the signal output loop, a first magnetic bead and a second magnetic bead connected in series on the positive end and the negative end of the signal output loop respectively, and a diode connected in series on the positive end of the signal output loop.

9. The isolated 4-20mA current output circuit according to claim 8, characterized in that: The EMC protection circuit further includes a third capacitor connected across the positive end and the negative end of the signal output loop.

10. A relay protection system, characterized in that: The invention comprises the isolated 4-20mA current output circuit as described in any one of claims 1 to 9.

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