CN112968436A

CN112968436A - Characteristic current generating circuit

Info

Publication number: CN112968436A
Application number: CN202110265935.2A
Authority: CN
Inventors: 冬松
Original assignee: Shenzhen Hongxu Data Technology Co ltd
Current assignee: Shenzhen Hongxu Data Technology Co ltd
Priority date: 2021-03-11
Filing date: 2021-03-11
Publication date: 2021-06-15

Abstract

The invention discloses a characteristic current generating circuit, which comprises a surge suppression circuit, an AC-to-DC circuit, a characteristic current circuit and an overcurrent protection circuit, wherein the surge suppression circuit is connected with the AC-to-DC circuit; the characteristic current circuit comprises a first characteristic current circuit and a second characteristic current circuit which are connected with each other; the surge suppression circuit is connected with the AC-to-DC circuit, the second characteristic current circuit and the overcurrent protection circuit; the AC-to-DC circuit is connected with the first characteristic current circuit, the second characteristic current circuit and the overcurrent protection circuit; the overcurrent protection circuit is connected with the first characteristic current circuit and the second characteristic current circuit. The characteristic current generating circuit disclosed by the invention is high in safety, capable of effectively avoiding power grid faults, high in circuit flexibility, small in equipment size, and capable of being embedded into an equipment terminal, and the technical problems of low safety, large size and poor flexibility in the existing product are solved.

Description

Characteristic current generating circuit

Technical Field

The invention relates to the technical field of intelligent power grid terminals with a characteristic current topology recognition function, in particular to a characteristic current generating circuit.

Background

Electric energy is an economic, practical, clean and easily controlled and converted energy form, and the application degree of the electric energy is one of the main signs of the national economic development level. The lightning, the equipment are connected or disconnected, and the accidents of electric fire and personal electric shock casualty caused by improper use of the electrical equipment or electric leakage of the distribution line occur occasionally, so that great threat is brought to the life and property safety of people. The abnormal current in the power grid is usually monitored by adopting the characteristic current, and the detected abnormal current is eliminated to ensure the safety of the power grid and ensure that equipment is not damaged. The existing characteristic current generation methods are roughly divided into two types: the first type is that a thyristor is used as a switch, a resistor is used as a load, the thyristor and the resistor are connected in series, two ends of the thyristor and the resistor are respectively connected with an N line and a phase line, and voltage (V) is applied to two ends of the resistor (R) to generate current (I) through the turn-off of the thyristor, wherein I is V/R. The scheme has lower cost and simple principle; the second type is the same as the first type in principle, only the thyristor is replaced by a relay, physical disconnection can be realized through the relay when characteristic current is not started, and the safety is high.

Based on the principles of the two existing characteristic current schemes, in the first category, due to the characteristic that the positive anode of the thyristor is conducted and the negative anode of the thyristor is closed, namely, the control signal of the thyristor can only turn on the thyristor, and the thyristor can be closed only after the alternating voltage is reversed if the thyristor is closed, so that the characteristic current generation has uncontrollable continuity, and the resistor requires large power, so that the resistor has large volume and is not beneficial to being embedded into a device terminal; and in the second type, a relay is used as a switch for generating the characteristic current, the opening and closing time of the relay is discrete, the pulse width of the output characteristic current pulse is inconsistent and is not easy to identify, and meanwhile, the relay has certain time delay for starting and closing, so that the pulse width of the output characteristic current has minimum time limit, poor flexibility and larger volume.

The magnitude of the characteristic current of the above two schemes depends on the magnitude of the voltage when the characteristic current is started, and if the voltage fluctuates or the starting point shifts to be close to the peak point of the voltage, the current will also increase, which may cause overcurrent fault to damage the equipment.

Disclosure of Invention

The invention aims to provide a characteristic current generating circuit, which solves the defects of low safety, large volume and poor flexibility of products in the prior art.

The characteristic current generating circuit of the invention is realized by the following technical scheme: the device comprises a surge suppression circuit, an AC-to-DC circuit, a characteristic current circuit and an overcurrent protection circuit; the characteristic current circuit comprises a first characteristic current circuit and a second characteristic current circuit which are connected with each other;

the surge suppression circuit is connected with the AC-to-DC circuit; the AC-DC conversion circuit is connected with the first characteristic current circuit, the second characteristic current circuit and the overcurrent protection circuit; the overcurrent protection circuit is connected with the first characteristic current circuit and the second characteristic current circuit;

the surge suppression circuit can relieve and absorb surge voltage generated by lightning or equipment connection or disconnection in the circuit;

the AC-DC conversion circuit can convert alternating current into direct current, and the direct current is output into stable power supply voltage VCC through voltage stabilization and filtering;

the first characteristic current circuit can drive the second characteristic current circuit to generate characteristic current through the optocoupler;

the overcurrent protection circuit can monitor the characteristic current and the voltage in the second characteristic current circuit, and carry out safety protection on the second characteristic current circuit access equipment.

The surge suppression circuit comprises an inductor L1, a diode D2 and a piezoresistor VAR 1;

one end of the inductor L1 is sequentially connected with an alternating current power supply and the cathode of the diode D2, and the other end of the inductor L1 is sequentially connected with the anode of the diode D2 and one end of the piezoresistor VAR 1; the other end of VAR1 is connected to ground.

As a preferable technical solution, the AC-to-DC circuit includes a diode D1, a current limiting resistor ER1, a current limiting resistor ER2, a transient diode TVS1, a zener diode D3, an electrolytic capacitor EC1, a capacitor C1, and a capacitor C2;

the diode D1, the current-limiting resistor ER1 and the current-limiting resistor ER2 are sequentially connected, the anode of the diode D1 is connected with one end of the voltage dependent resistor VAR1, and one end of the current-limiting resistor ER2 is sequentially connected with the cathode of the voltage stabilizing diode D3, the electrolytic capacitor EC1, the capacitor C1 and one polar plate of the capacitor C2; the anode of the voltage-stabilizing diode D3, the electrolytic capacitor EC1, the capacitor C1 and the other polar plate of the capacitor C2 are all connected with the ground wire; one pin of the transient diode TVS1 is connected between the current limiting resistor ER1 and the current limiting resistor ER2, and the other pin thereof is connected to the ground line.

As a preferred technical scheme, the zener diode D3 adopts a 15V zener diode;

the alternating current power supply is half-wave rectified by a diode D2 to form a direct current power supply VD1, and the output stabilized power supply voltage VCC of the direct current power supply VD1 is 15V after being stabilized by a voltage stabilizing diode D3, stored and filtered by an electrolytic capacitor EC1 and filtered by a capacitor C1 and a capacitor C2.

As a preferable technical solution, the second characteristic current circuit includes a thermistor PTC1, an N _ MOS transistor Q1, a capacitor C3, a capacitor C4, a transient diode TVS2, a resistor R7, a resistor R5, and a diode D4;

one end of the thermistor PTC1 is connected between the diode D1 and the current-limiting resistor ER1, and the other end of the thermistor PTC1 is connected with the drain electrode of the N _ MOS tube Q1; a pin of the transient diode TVS2 is connected to both the gate of the N _ MOS transistor Q1 and one end of the resistor R7; the other end of the resistor R7, the other pin of the transient diode TVS2, the source of the N _ MOS transistor Q1 and one electrode plates of the capacitor C3 and the capacitor C4 are all connected with a ground wire; the other polar plate of the capacitor C3 is connected with a power supply voltage VCC; the other polar plate of the capacitor C4 is connected between the drain of the N _ MOS transistor Q1 and the thermistor PTC 1; the anode and the cathode of the diode D4 are connected to both ends of the resistor R5, respectively, and the anode of the diode D4 is connected between the resistor R7 and the resistor R5.

Preferably, the thermistor PTC1 is a time-delay thermistor, the resistance value during the time-delay period is RPTC, and the formula for calculating the characteristic current I generated during the time-delay period is:

I＝VD1/RPTC；

VD1 is a direct current power supply converted by an AC-to-DC circuit.

Preferably, the N _ MOS transistor Q1 is an IGBT switch.

As a preferable technical solution, the first characteristic current circuit includes a driving optocoupler U1, a transistor Q2, a resistor R2, a resistor R4, a resistor R6, a resistor R9, and a control signal TX.

The overcurrent protection circuit comprises an N _ MOS transistor Q3, a voltage detector U2, a transient diode TVS3, a capacitor C5, a resistor R1, a resistor R3, a resistor R8, a resistor R10, a resistor R11, a resistor R12, a resistor R13 and a resistor R14.

As a preferred technical solution, the calculation formula of the input voltage VDD of the 1 pin of the voltage detector U2 is as follows:

VDD＝VD1×R12/(R1+R3+R8+R10+R12)；

the point difference between the input voltage VDD of the 1 pin of the voltage detector U2 and the dc power VD1 is trimmed by the capacitor C5.

The invention has the beneficial effects that:

1. the circuit safety is high, can effectively avoid the electric wire netting trouble. The combination of the inductor and the piezoresistor is adopted to effectively suppress surge impact, and a voltage monitoring system is designed to perform overcurrent protection, so that circuit equipment and circuit safety can be effectively protected;

2. the circuit has high flexibility, the IGBT is adopted as a switching device, the voltage resistance is high, the switching speed is high, the circuit can be effectively promoted to be quickly powered off under high current, and the delay effect of the existing circuit is avoided;

3. the circuit equipment is small in size and can be embedded into the equipment terminal well.

Drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, it is obvious that the drawings in the following description are only some embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to the drawings without creative efforts.

FIG. 1 is a circuit configuration diagram of a characteristic current generating circuit of the present invention;

fig. 2 is a circuit schematic of a characteristic current generating circuit of the present invention.

Detailed Description

All of the features disclosed in this specification, or all of the steps in any method or process so disclosed, may be combined in any combination, except combinations of features and/or steps that are mutually exclusive.

Any feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving equivalent or similar purposes, unless expressly stated otherwise. That is, unless expressly stated otherwise, each feature is only an example of a generic series of equivalent or similar features.

In the description of the present invention, it is to be understood that the terms "one end", "the other end", "outside", "upper", "inside", "horizontal", "coaxial", "central", "end", "length", "outer end", and the like, indicate orientations or positional relationships based on those shown in the drawings, and are used only for convenience in describing the present invention and for simplicity in description, and do not indicate or imply that the device or element being referred to must have a particular orientation, be constructed in a particular orientation, and be operated, and thus, should not be construed as limiting the present invention.

Further, in the description of the present invention, "a plurality" means at least two, e.g., two, three, etc., unless specifically limited otherwise.

The use of terms such as "upper," "above," "lower," "below," and the like in describing relative spatial positions herein is for the purpose of facilitating description to describe one element or feature's relationship to another element or feature as illustrated in the figures. The spatially relative positional terms may be intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as "below" or "beneath" other elements or features would then be oriented "above" the other elements or features. Thus, the exemplary term "below" can encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly.

In the present invention, unless otherwise explicitly specified or limited, the terms "disposed," "sleeved," "connected," "penetrating," "plugged," and the like are to be construed broadly, e.g., as a fixed connection, a detachable connection, or an integral part; can be mechanically or electrically connected; they may be directly connected or indirectly connected through intervening media, or they may be connected internally or in any other suitable relationship, unless expressly stated otherwise. The specific meanings of the above terms in the present invention can be understood by those skilled in the art according to specific situations.

As shown in fig. 1, a characteristic current generating circuit of the present invention includes a surge suppressing circuit 1, an AC-to-DC circuit 2, a characteristic current circuit, and an overcurrent protection circuit 5; the characteristic current circuit comprises a first characteristic current circuit 3 and a second characteristic current circuit 4 which are connected with each other;

the surge suppression circuit 1 is connected with the AC-to-DC circuit 2; the AC-DC conversion circuit 2 is connected with the first characteristic current circuit 3, the second characteristic current circuit 4 and the overcurrent protection circuit 5; the overcurrent protection circuit 5 is connected with the first characteristic current circuit 3 and the second characteristic current circuit 4;

the surge suppression circuit 1 can relieve and absorb surge voltage generated by lightning or equipment connection or disconnection in the circuit;

the AC-DC conversion circuit 2 can convert alternating current into direct current, and the direct current is output into stable power supply voltage VCC through voltage stabilization and filtering;

the first characteristic current circuit 3 can drive the second characteristic current circuit 4 to generate characteristic current through an optical coupler;

the overcurrent protection circuit 5 can monitor the characteristic current and the voltage in the second characteristic current circuit 4, and can perform safety protection on the access equipment of the second characteristic current circuit 4.

As shown in fig. 2:

in this embodiment, the surge suppression circuit 1 includes an inductor L1, a diode D2, and a varistor VAR 1;

In this embodiment, the AC-to-DC circuit 2 includes a diode D1, a current-limiting resistor ER1, a current-limiting resistor ER2, a transient diode TVS1, a zener diode D3, an electrolytic capacitor EC1, a capacitor C1, and a capacitor C2;

In this embodiment, the zener diode D3 is a 15V zener diode;

The output voltage of the diode D3 is 15V direct current, and the stable 15V power supply voltage VCC can be output after the output voltage is filtered by the electrolytic capacitor EC1, the capacitor C1 and the capacitor C2 and is supplied to the first characteristic current circuit 3, the second characteristic current circuit 4 and the overcurrent protection circuit 5 for use.

In this embodiment, the second characteristic current circuit 4 includes a thermistor PTC1, an N _ MOS transistor Q1, a capacitor C3, a capacitor C4, a transient diode TVS2, a resistor R7, a resistor R5, and a diode D4;

In this embodiment, the thermistor PTC1 is a time-delay thermistor, the resistance value during the time delay is RPTC, and the calculation formula of the characteristic current I occurring during the time delay is:

I＝VD1/RPTC；

the VD1 is a direct current power supply converted by the AC to DC circuit 2.

In this embodiment, the N _ MOS transistor Q1 is an IGBT switch.

In this embodiment, the first characteristic current circuit 3 includes a driving optocoupler U1, a transistor Q2, a resistor R2, a resistor R4, a resistor R6, a resistor R9, and a control signal TX.

Specifically, pins 1 and 3 of the driving optocoupler U1 are respectively connected with one ends of a resistor R2 and a resistor R4, pin 5 is connected with a cathode of a diode D4 and one end of a resistor R5, and pin 6 is connected with a power supply voltage VCC; the other ends of the resistor R2 and the resistor R4 are respectively connected with a 12V voltage and a collector of a triode Q2; one ends of the resistor R6 and the resistor R9 are both connected with a control signal TX, and the other ends of the resistor R6 and the resistor R9 are respectively connected with a base electrode and a grounding wire of the triode Q2; the emitter of transistor Q2 is connected to ground. Furthermore, the N _ MOS transistor Q1 is an IGBT, and the N _ MOS transistor Q1 needs to be driven by a driving optocoupler U1 dedicated to the IGBT, and an input end of the driving optocoupler U1 is composed of a power supply terminal (VCC is a 12V power supply), a control signal TX, and a ground terminal; when the control signal TX is at a low level, the triode Q2 is not turned on, the level of the 5 th pin of the driving optocoupler U1 is the same as the level of the 4 th pin, VGS of the N _ MOS transistor Q1 is 0, the N _ MOS transistor Q1 is in an off state, and a gap between the drain (D) and the source (S) is cut off; when the control signal TX is at a high level, the transistor Q2 is turned on, the level of the pin 5 of the driving optocoupler U1 is the same as the level of the pin 4, VGS of the N _ MOS transistor Q1 is 15V, the N _ MOS transistor Q1 is in an open state, and the drain (D) and the source (S) are turned on. Thereby driving the second characteristic current circuit to generate the characteristic current.

In this embodiment, the overcurrent protection circuit 5 includes an N _ MOS transistor Q3, a voltage detector U2, a transient diode TVS3, a capacitor C5, a resistor R1, a resistor R3, a resistor R8, a resistor R10, a resistor R11, a resistor R12, a resistor R13, and a resistor R14.

Specifically, the drain of the N _ MOS transistor Q3 is connected to the anode of the diode D4, and is connected between the resistor R5 and the resistor R7, the source thereof is connected to the ground line, and the gate thereof is connected to one end of the resistor R13; the other end of the resistor R13 is connected with one end of the resistor R11, one end of the resistor R14 and the pin 1 of the voltage detector U2; the 2 pins of the voltage detector U2 are connected between the resistors R12 and R10, and the 3 pins are connected with the grounding wire; the other ends of the resistor R11 and the resistor R14 are respectively connected with a power supply voltage VCC and a grounding wire; the resistor R1, the resistor R3, the resistor R8, the resistor R10 and the resistor R12 are connected in sequence; one ends of the resistor R1 and the resistor R12 are respectively connected with the thermistor PTC1 and the grounding wire; one polar plate of the capacitor C5 is connected to one pin of the transient diode TVS3, and is connected between the resistor R10 and the resistor R12, and the other polar plate of the capacitor C5 and the other pin of the transient diode TVS3 are both connected to the ground line.

In this embodiment, the calculation formula of the input voltage VDD at pin 1 of the voltage detector U2 is as follows:

VDD＝VD1×R12/(R1+R3+R8+R10+R12)；

further, the trigger signal of the overcurrent protection is an overvoltage protection signal, when the input voltage of pin 1 of the voltage detector U2 is greater than 5V, pin 2 is in a high-impedance state, the N _ MOS transistor Q3 is in a conducting state, the gate (G) of the N _ MOS transistor Q1 is forced to pull down VGS 0, and the N _ MOS transistor Q1(IGBT switch) is turned off to enter an overcurrent protection state; when the input voltage of a pin 1 of the voltage detector U2 is less than 5V, the level of a pin 2 is the same as that of a pin 3, the N _ MOS transistor Q3 is in a cut-off state, and the closed state of the N _ MOS transistor Q1(IGBT switch) is controlled by a driving optocoupler U1; the point difference between the input voltage VDD of the 1 pin of the voltage detector U2 and the DC power supply VD1 is subjected to fine tuning through a capacitor C5, and the effect of fine tuning the protection voltage threshold can be started; the 1 pin input voltage VDD of the voltage detector U2 can calculate the voltage threshold for entering the protection mode by equation (2). Since the N _ MOS transistor Q1(IGBT switch) has a maximum through current IDSMAX limit, the characteristic current does not necessarily occur in the time period of the voltage peak, and the IDSMAX of the N _ MOS transistor Q1(IGBT switch) needs to be selected according to actual requirements based on cost and volume considerations; meanwhile, voltage fluctuation caused by abnormal factors of a power grid is considered, characteristic current is increased directly due to rising fluctuation of the power grid voltage and may exceed IDSMAX, and the overcurrent protection circuit can solve the problem and effectively protect an IGBT switch.

In this embodiment, alternating current power supply is 220V commercial power, and after the commercial power is connected, the electric wire netting can produce instantaneous surge voltage because of thunder and lightning, or the circumstances such as access, or disconnection equipment, and supply voltage reduces suddenly, the electric current increases in the twinkling of an eye, and surge voltage will directly lead to electric wire netting short circuit, equipment to damage, consequently, this embodiment has increased surge suppression circuit 1. The surge voltage shock wave is subjected to waveform slowing after passing through the inductor L1, voltage clamping is carried out through the voltage dependent resistor VAR1 of the rear stage, surge energy is absorbed, and therefore the input end and the rear stage electronic component are protected.

In this embodiment, the 220V commercial power is half-wave rectified by a diode D1 to become a direct current VD1, and ER1 and ER2 are current limiting resistors.

In this embodiment, the thermistor PTC1 has an action delay T, the thermistor PTC1 can be used as a resistor RPTC within the time of T, since the characteristic current is not generated continuously, the thermistor PTC1 generates heat from the characteristic current, and the heat is dissipated during the interval of the characteristic current generation, so as to keep the resistance of the thermistor PTC1 substantially constant, when the drain (D) and the source (S) of the N _ MOS transistor Q1 are turned on, the voltage at both ends of the PTC1 is VD1, the IGBT belongs to a semiconductor switching device, when the on-time of the N _ MOS transistor Q1 is longer than T or is continuously turned on due to an uncontrollable fault of the IGBT, the thermistor PTC1 generates power consumption, the temperature rise resistance RPTC increases, and the characteristic current calculation formula shows that the current will decrease or approach to an off state. Thereby protecting each element in the circuit. The IGBT switch needs to use a special driving circuit, i.e., a first characteristic current circuit.

In summary, the characteristic current generating circuit provided in this embodiment uses an IGBT as a switch, and uses a PTC as a load to generate a characteristic current (millisecond pulse current signal), and the control signal is isolated by an opto-coupler. The 220V mains supply circuit effectively inhibits surge impact by adopting the combination of an inductor and a piezoresistor, and a voltage monitoring system is designed for overcurrent protection, so that the safety of the whole IGBT power grid can be effectively protected. The technical effects are as follows: the characteristic current generating circuit has high circuit safety and can effectively avoid power grid faults; the IGBT is adopted as the switch, so that the circuit has high flexibility; moreover, the circuit equipment is small in size and can be embedded into the equipment terminal well.

The above description is only an embodiment of the present invention, but the scope of the present invention is not limited thereto, and any changes or substitutions that are not thought of through the inventive work should be included in the scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope defined by the claims.

Claims

1. A characteristic current generating circuit, characterized by: the device comprises a surge suppression circuit (1), an AC-to-DC circuit (2), a characteristic current circuit and an overcurrent protection circuit (5); the characteristic current circuit comprises a first characteristic current circuit (3) and a second characteristic current circuit (4) which are connected with each other;

the surge suppression circuit (1) is connected with the AC-to-DC circuit (2); the AC-to-DC conversion circuit (2) is connected with the first characteristic current circuit (3), the second characteristic current circuit (4) and the overcurrent protection circuit (5); the overcurrent protection circuit (5) is connected with the first characteristic current circuit (3) and the second characteristic current circuit (4);

the surge suppression circuit (1) can relieve and absorb surge voltage generated by lightning or equipment connection or disconnection in the circuit;

the AC-DC conversion circuit (2) can convert alternating current into direct current, and the direct current is output into stable power supply voltage VCC through voltage stabilization and filtering;

the first characteristic current circuit (3) can drive the second characteristic current circuit (4) to generate the characteristic current through an optical coupler;

the overcurrent protection circuit (5) can monitor the characteristic current and the voltage in the second characteristic current circuit (4), and can perform safety protection on the access equipment of the second characteristic current circuit (4).

2. The characteristic current generating circuit according to claim 1, wherein: the surge suppression circuit (1) comprises an inductor L1, a diode D2 and a piezoresistor VAR 1;

one end of the inductor L1 is sequentially connected with an alternating current power supply and the cathode of the diode D2, and the other end of the inductor L1 is sequentially connected with the anode of the diode D2 and one end of the piezoresistor VAR 1; the other end of the VAR1 is connected to a ground line.

3. The characteristic current generating circuit according to claim 1 or 2, characterized in that: the AC-to-DC circuit (2) comprises a diode D1, a current limiting resistor ER1, a current limiting resistor ER2, a transient diode TVS1, a voltage stabilizing diode D3, an electrolytic capacitor EC1, a capacitor C1 and a capacitor C2;

the diode D1, the current-limiting resistor ER1 and the current-limiting resistor ER2 are sequentially connected, the anode of the diode D1 is connected with one end of a voltage dependent resistor VAR1, and one end of the current-limiting resistor ER2 is sequentially connected with the cathode of a voltage stabilizing diode D3, an electrolytic capacitor EC1, a capacitor C1 and one polar plate of a capacitor C2; the anode of the voltage-stabilizing diode D3, the electrolytic capacitor EC1, the capacitor C1 and the other plate of the capacitor C2 are all connected with the grounding wire; one pin of the transient diode TVS1 is connected between the current limiting resistor ER1 and the current limiting resistor ER2, and the other pin thereof is connected to the ground line.

4. A characteristic current generating circuit according to claim 1, 2 or 3, wherein: the voltage stabilizing diode D3 adopts a 15V voltage stabilizing tube;

the alternating current power supply forms a direct current power supply VD1 through half-wave rectification of a diode D2, and the output stabilized power supply voltage VCC of the direct current power supply VD1 is 15V after voltage stabilization of a voltage stabilizing diode D3, energy storage filtering of an electrolytic capacitor EC1 and capacitance filtering of a capacitor C1 and a capacitor C2.

5. A characteristic current generating circuit according to claim 1 or 3, characterized in that: the second characteristic current circuit (4) comprises a thermistor PTC1, an N _ MOS transistor Q1, a capacitor C3, a capacitor C4, a transient diode TVS2, a resistor R7, a resistor R5 and a diode D4;

one end of the thermistor PTC1 is connected between the diode D1 and the current-limiting resistor ER1, and the other end of the thermistor PTC1 is connected with the drain electrode of the N _ MOS transistor Q1; a pin of the transient diode TVS2 is connected to the gate of the N _ MOS transistor Q1 and one end of the resistor R7; the other end of the resistor R7, the other pin of the transient diode TVS2, the source of the N _ MOS transistor Q1 and one plates of the capacitor C3 and the capacitor C4 are all connected with the ground line; the other polar plate of the capacitor C3 is connected with the power supply voltage VCC; the other polar plate of the capacitor C4 is connected between the drain electrode of the N _ MOS transistor Q1 and the thermistor PTC 1; the anode and the cathode of the diode D4 are respectively connected to two ends of the resistor R5, and the anode of the diode D4 is connected between the resistor R7 and the resistor R5.

6. The characteristic current generating circuit according to claim 5, wherein: the thermistor PTC1 is a time-delay thermistor, the resistance value during the time delay is RPTC, and the calculation formula of the characteristic current I generated during the time delay is as follows:

I＝VD1/RPTC；

the VD1 is a direct current power supply converted by the AC-DC conversion circuit (2).

7. The characteristic current generating circuit according to claim 5, wherein: the N _ MOS pipe Q1 is an IGBT switch.

8. The characteristic current generating circuit according to claim 1, wherein: the first characteristic current circuit (3) comprises a driving optocoupler U1, a triode Q2, a resistor R2, a resistor R4, a resistor R6, a resistor R9 and a control signal TX.

9. The characteristic current generating circuit according to claim 1, wherein: the overcurrent protection circuit (5) comprises an N _ MOS transistor Q3, a voltage detector U2, a transient diode TVS3, a capacitor C5, a resistor R1, a resistor R3, a resistor R8, a resistor R10, a resistor R11, a resistor R12, a resistor R13 and a resistor R14.

10. The characteristic current generating circuit according to claim 9, wherein: the calculation formula of the input voltage VDD of the 1 pin of the voltage detector U2 is as follows:

VDD＝VD1×R12/(R1+R3+R8+R10+R12)；