Current input normally closed type optical isolation bidirectional solid relay

Abstract

The application discloses a current input normally closed optical isolation bidirectional solid relay, which comprises an input circuit, an isolation driving circuit and an output switch circuit. The application can realize reliable driving only by inputting current reaching the driving threshold of a light emitting diode, has strong adaptability to input voltage fluctuation, does not need additional current limiting or voltage stabilizing circuits, and simplifies external interface design. When the input polarity is reversed, the current path is automatically blocked, internal devices are prevented from being damaged, and the safety and reliability of the product are improved. The depletion mode field effect transistor is used as the output switch device, the normally closed function is realized without external bias power supply by using the natural conduction characteristic of the zero gate voltage, and the output is automatically kept on when the input is powered off, which is suitable for fault safety protection occasions.

Description

Technical Field

[0001] This invention relates to the field of solid-state relay technology, and specifically to a current-input normally closed optically isolated bidirectional solid-state relay. Background Technology

[0002] Solid-state relays can be classified into two types based on the switching state of their output terminals under normal conditions: normally open and normally closed. A normally open solid-state relay is open at the output terminal when no control signal is applied to the input terminal; while a normally closed solid-state relay is closed at the output terminal when no control signal is applied to the input terminal. Normally closed solid-state relays play an irreplaceable role in applications such as fault protection, backup power switching, and signal bypassing. Existing technologies mainly implement normally closed solid-state relays in the following ways: Normally closed functionality is achieved using enhancement-mode MOSFETs (EMFs) in conjunction with complex bias circuits and boost converters. For example, EP4145702A1 discloses a normally closed solid-state relay constructed using normally open elements. This relay combines an enhancement-mode MOSFET with a boost converter, utilizing the parasitic resistance voltage drop of the semiconductor switch during conduction as the input power supply for the boost converter. The resulting boosted output is fed back to the gate of the MOSFET to maintain its on-state. While this solution achieves normally closed functionality, it requires an additional boost converter and continuous drive energy to maintain the on-state, resulting in a complex circuit structure, high cost, and high power consumption. Furthermore, the inductor in the boost converter introduces additional electromagnetic interference.

[0003] Gallium nitride (GaN) power devices are used to implement bidirectional solid-state relay functionality. For example, CN218416166U discloses a bidirectional solid-state relay based on a gallium nitride transistor. By using bidirectional, symmetrically conducting gallium nitride power transistors as switching elements, the on-resistance and power consumption are significantly reduced. However, this technical solution requires a complex bidirectional rectifier module, switching power supply unit, and voltage detection module to work together, resulting in an extremely complex circuit structure. Furthermore, it still requires an external DC power supply, which is not conducive to miniaturization and low-cost applications.

[0004] Existing normally closed solid-state relay technology has the following shortcomings: (1) Schemes using enhanced devices require complex bias or boost circuits to maintain the normally closed state, resulting in complex circuits and high costs; (2) Schemes using voltage-type inputs have high requirements for input voltage stability, poor anti-interference capabilities, and lack input reverse protection; (3) Most existing schemes cannot simultaneously achieve the integration of normally closed, bidirectional conduction, optical isolation, and current-type input; (4) The gate protection mechanism is imperfect, and the gate of the depletion-type field-effect transistor is easily damaged by overvoltage during switching. Therefore, there is an urgent need for a normally closed optically isolated solid-state relay with a simple circuit, low cost, current-type input, and bidirectional conduction output. Summary of the Invention

[0005] The purpose of this invention is to address the shortcomings of existing technologies by providing a current-input normally closed optically isolated bidirectional solid-state relay, which aims to solve the technical problems of existing normally closed solid-state relays, such as complex circuitry, high cost, poor input voltage adaptability, and insufficient gate protection.

[0006] The technical solution of the present invention: A normally closed-circuit optically isolated bidirectional solid-state relay with current input includes an input circuit, an isolation drive circuit, and an output switching circuit. The input circuit includes a light-emitting diode (LED) D1, a switching diode D2, and a Zener diode D3. The anode of LED D1 is connected to the positive input terminal of the solid-state relay, the cathode of LED D1 is connected to the anode of the switching diode D2, the cathode of the switching diode D2 is connected to the cathode of the Zener diode D3, and the anode of the Zener diode D3 is connected to the negative input terminal of the solid-state relay. The working principle and beneficial effects of the above technical solution are as follows: LED D1, switching diode D2, and Zener diode D3 form a series input circuit. When a positive current is applied between the positive and negative input terminals, the current flows sequentially through LED D1, switching diode D2, and Zener diode D3. LED D1 emits infrared light under current drive. Since this invention adopts a current-type input method, it can emit light as long as the input current reaches the driving threshold of LED D1. Reliable drive can be guaranteed even when the input current varies within a large range. Therefore, compared with the existing voltage-type input scheme, this invention has a strong adaptability to voltage fluctuations of the input power supply and can achieve a wide range of input without additional current limiting or voltage regulation circuits. At the same time, switching diode D2 utilizes its unidirectional conductivity to achieve a series input circuit when the positive and negative input terminals are connected. When a reverse voltage is accidentally applied, the switching diode D2 is in the reverse cutoff state, blocking the reverse current path and effectively preventing the reverse current from damaging the LED D1, thus realizing automatic polarity protection at the input terminal. The Zener diode D3 utilizes its reverse breakdown characteristic to clamp the voltage between the positive and negative input terminals to the sum of its regulated voltage value, the forward voltage drop of the switching diode D2, and the forward voltage drop of the LED D1. When the input voltage exceeds this threshold, the Zener diode D3 breaks down and conducts, making the input circuit conduct and emit light. When the input voltage is lower than this threshold, the Zener diode D3 returns to cutoff, and the input circuit is disconnected. This precisely adjusts the input turn-on voltage threshold and input turn-off voltage threshold of the solid-state relay, realizing a switching action with hysteresis characteristics and avoiding relay bounce and malfunction caused by input voltage fluctuations near the threshold.

[0007] The isolated driving circuit includes the light-emitting diode D1 and the photovoltaic cell E1. The light-emitting diode D1 is optically coupled to the photovoltaic cell E1. The positive terminal of the photovoltaic cell E1 is connected to the common source node of the output switching circuit, and the negative terminal of the photovoltaic cell E1 is connected to the common gate node of the output switching circuit. The working principle and beneficial effects of the above technical solution are as follows: LED D1 and photovoltaic cell E1 form an optically isolated driving unit through optical coupling, realizing complete electrical isolation between the input circuit and the output switching circuit. The isolation voltage can reach several kilovolts, effectively protecting the low-voltage control side from interference and impact from the high-voltage load side. When LED D1 is energized and emits light, photovoltaic cell E1 receives light energy and converts it into electrical energy, generating a photogenerated voltage across photovoltaic cell E1. This photogenerated voltage is applied between the common source node and the common gate node as the gate-source driving voltage of the depletion-mode field-effect transistor. Since photovoltaic cell E1 itself has current-limiting characteristics, its output current is limited to a safe range, preventing overcurrent impact on the gate of the undriven field-effect transistor. At the same time, the optical isolation method eliminates electromagnetic coupling interference, giving the solid-state relay extremely low electromagnetic emission and extremely high anti-interference capability.

[0008] The output switching circuit includes a depletion-mode field-effect transistor (DMFET) Q1, a depletion-mode field-effect transistor (DMFET) Q2, and a gate protection circuit. The source of the DMFET Q1 and the source of the DMFET Q2 are connected to form the common source node. The gate of the DMFET Q1 and the gate of the DMFET Q2 are connected to form the common gate node. The drain of the DMFET Q1 and the drain of the DMFET Q2 serve as the output terminals of the solid-state relay. The working principle and beneficial effects of the above technical solution are as follows: Depletion-mode field-effect transistors Q1 and Q2 are connected in a common-source, common-gate configuration. The sources of the two transistors are connected to form a common source node, and their gates are connected to form a common gate node. The drains serve as the two ports for bidirectional output. Since the depletion-mode field-effect transistors are in a naturally conducting state when the gate-source voltage is zero, when no input current is applied to the input circuit, the photovoltaic cell E1 does not generate a photogenerated voltage, and the voltage between the common gate node and the common source node is zero. Therefore, both depletion-mode field-effect transistors Q1 and Q2 are in a conducting state, and the two output terminals of the solid-state relay are bidirectionally connected, achieving a normally closed function. When the input circuit... When an input current is applied to generate a negative gate-source drive voltage in photovoltaic cell E1, this negative voltage causes the gate-source voltages of depletion-type field-effect transistors Q1 and Q2 to fall below their turn-off thresholds, causing both field-effect transistors to turn off simultaneously, thus disconnecting the output terminals of the solid-state relays. The two depletion-type field-effect transistors are connected in series back-to-back. Regardless of the direction of current flow, the body diode of one field-effect transistor is always reverse-biased to block the current, while the other is in a forward-conducting state. Bidirectional current can only be blocked when both field-effect transistors are turned off simultaneously. Conversely, bidirectional current flow is allowed as long as both field-effect transistors are turned on simultaneously. This achieves a true bidirectional switching function, suitable for AC and DC bidirectional load control applications.

[0009] The gate protection circuit includes a gate resistor R1 and a gate Zener diode D4. The gate resistor R1 is connected between the common source node and the common gate node; the gate Zener diode D4 is connected between the common source node and the common gate node. The working principle and beneficial effects of the above technical solution are as follows: The gate resistor R1 is connected in parallel between the common source node and the common gate node. After the driving voltage of the photovoltaic cell E1 is removed, it provides a discharge path for the gate charge. By reasonably selecting the resistance value of the gate resistor R1, the gate discharge speed can be controlled, thereby adjusting the turn-off time of the solid-state relay. This avoids voltage spikes caused by excessively fast gate discharge and switching delays caused by excessively slow discharge. The gate Zener diode D4 is connected in parallel between the gate and the source. It uses its reverse breakdown characteristic to clamp the gate-source voltage within a safe range. When the photogenerated voltage output by the photovoltaic cell E1 or the external surge voltage exceeds the maximum rated gate-source voltage of the depletion-type field-effect transistors Q1 and Q2, the gate Zener diode D4 breaks down and conducts, limiting the gate-source voltage to its regulated value. This effectively prevents gate overvoltage damage and improves the reliability and surge resistance of the solid-state relay.

[0010] Furthermore, the light-emitting diode D1 and the photovoltaic cell E1 are optically coupled to each other to form an optically isolated driving unit. Several optically isolated driving units are configured within the solid-state relay. The working principle and beneficial effects of the above technical solution are as follows: by setting multiple sets of optically isolated driving units, the light-emitting diodes in each set can be connected in series or parallel to receive input current, and the outputs of each photovoltaic cell can be superimposed to provide a larger driving voltage and current, thereby meeting the driving requirements of high-current loads or high gate threshold field-effect transistors. Simultaneously, multiple sets of optically isolated driving units improve the redundancy and reliability of the drive.

[0011] The beneficial effects of this invention are as follows: By combining a light-emitting diode, a switching diode, and a Zener diode in series, reliable driving can be achieved as long as the input current reaches the driving threshold of the light-emitting diode. It has strong adaptability to input voltage fluctuations, eliminates the need for additional current limiting or voltage regulation circuits, and simplifies external interface design. When the input polarity is reversed, the current path is automatically blocked to prevent damage to internal components, thereby improving the safety and reliability of the product. The Zener diode enables precise adjustment of the input turn-on and turn-off voltages and generates hysteresis characteristics, avoiding relay jitter and malfunction when the input voltage fluctuates near the threshold. Using depletion-mode MOSFETs as output switching devices, leveraging their zero-gate-voltage natural conduction characteristic, they can achieve normally closed function without external bias power supply. The output automatically remains on when the input is de-energized, making them suitable for fault-safe protection applications. The two depletion-mode MOSFETs are connected in a common-source, common-gate configuration, achieving true bidirectional switching function. The output can withstand bidirectional voltage and current, making them suitable for AC and DC load control. Attached Figure Description

[0012] Figure 1 This is a schematic diagram of the principle of the present invention.

[0013] Figure 2 This is the circuit schematic diagram of the present invention.

[0014] Figure 3 This is a schematic diagram of Embodiment 1 of the present invention.

[0015] Figure 4 This is a schematic diagram of Embodiment 2 of the present invention.

[0016] Figure 5 This is a schematic diagram of Embodiment 3 of the present invention.

[0017] Figure 6 This is a schematic diagram of Embodiment 4 of the present invention.

[0018] Figure 7 This is a schematic diagram of Embodiment 5 of the present invention. Detailed Implementation

[0019] The specific embodiments of the present invention will be further described in detail below with reference to the accompanying drawings. The following examples are used to illustrate the present invention, but are not intended to limit the scope of the present invention.

[0020] Example 1 like Figure 1 and Figure 2 As shown, this embodiment provides a current-input normally closed optically isolated bidirectional solid-state relay, including an input circuit, an isolation drive circuit, and an output switching circuit.

[0021] The input circuit includes an LED D1, a switching diode D2, and a Zener diode D3. The anode of LED D1 is connected to the positive input terminal IN+ of the solid-state relay, the cathode of LED D1 is connected to the anode of switching diode D2, the cathode of switching diode D2 is connected to the cathode of Zener diode D3, and the anode of Zener diode D3 is connected to the negative input terminal IN- of the solid-state relay. Switching diode D2 prevents damage to LED D1 when the input voltage is reverse-biased. When a reverse voltage is applied between IN+ and IN-, switching diode D2 is reverse-biased and cuts off the current path. Zener diode D3 adjusts the input turn-on and turn-off voltage thresholds of the solid-state relay; the Zener voltage Vz of Zener diode D3 determines the input action voltage threshold.

[0022] The isolated driving circuit consists of a light-emitting diode (LED) D1 and a photovoltaic cell E1. LED D1 and photovoltaic cell E1 are optically coupled to form an optically isolated driving unit. The positive terminal of photovoltaic cell E1 is connected to the common source node S, and the negative terminal of photovoltaic cell E1 is connected to the common gate node G. When current flows through LED D1 in the input circuit, D1 emits infrared light to illuminate photovoltaic cell E1, generating a photogenerated voltage Vpv. This voltage is applied between the common source node and the common gate node, i.e., VGS = -Vpv (negative voltage).

[0023] The output switching circuit includes a depletion-mode field-effect transistor (DMFET) Q1, a depletion-mode field-effect transistor (DMFET) Q2, and a gate protection circuit. The sources of DMFETs Q1 and Q2 are connected to form a common source node S, and their gates are connected to form a common gate node G. The drain of DMFET Q1 serves as the first output terminal OUT1 of the solid-state relay, and the drain of DMFET Q2 serves as the second output terminal OUT2 of the solid-state relay.

[0024] The gate protection circuit includes a gate resistor R1 and a gate Zener diode D4. The gate resistor R1 is connected between the common source node S and the common gate node G, providing a discharge path for the gate charge after the photovoltaic cell E1 stops generating electricity, thus controlling the gate discharge rate. The gate Zener diode D4 is connected between the common source node S and the common gate node G, clamping the gate-source voltage within a safe range to prevent gate overvoltage damage.

[0025] The working principle of this embodiment is as follows: When no current is applied between the input terminals IN+ and IN- of the solid-state relay, the light-emitting diode D1 does not emit light, the photovoltaic cell E1 does not generate a photovoltage, and the voltage between the common gate node G and the common source node S is zero. Since the depletion-mode field-effect transistors Q1 and Q2 are in a naturally conducting state at zero gate voltage, the output terminals OUT1 and OUT2 are bidirectionally conducting, and the solid-state relay is in a normally closed state.

[0026] When a positive current is applied between the input terminals IN+ and IN-, the current flows through the LED D1, causing it to emit light. The light emitted by LED D1 illuminates the photovoltaic cell E1, generating a negative photovoltage. This negative voltage is applied between the common gate node G and the common source node S, causing the gate-source voltages of the depletion-mode MOSFETs Q1 and Q2 to become negative. When this negative voltage is lower than the turn-off threshold voltage VGS(OFF) of the depletion-mode MOSFETs Q1 and Q2, both MOSFETs turn off simultaneously, disconnecting the output terminals OUT1 and OUT2, and the solid-state relay is in the off state. When the input current is removed, the photovoltaic cell E1 stops generating electricity, the gate charge is slowly discharged through the gate resistor R1, the gate-source voltage gradually returns to zero, the depletion-mode MOSFETs Q1 and Q2 turn on again, and the solid-state relay returns to its normally closed state.

[0027] Example 2 like Figure 3 As shown, this embodiment provides an application circuit for a current-input normally closed optically isolated bidirectional solid-state relay, which is used in DC load control applications.

[0028] The application circuit in this embodiment includes: a solid-state relay SSR1, an input control voltage source Vin, an input current-limiting resistor Rin, a DC output power supply Vdc, and a load resistor RL. The positive terminal of the input control voltage source Vin is connected to the positive input terminal IN+ of the solid-state relay SSR1 through the input current-limiting resistor Rin, and the negative terminal of the input control voltage source Vin is connected to the negative input terminal IN- of the solid-state relay SSR1. The positive terminal of the DC output power supply Vdc is connected to the first output terminal OUT1 of the solid-state relay SSR1, and the negative terminal of the DC output power supply Vdc is connected to the second output terminal OUT2 of the solid-state relay SSR1 through the load resistor RL.

[0029] The working principle of this embodiment is as follows: When the input control voltage source Vin is not applied, the solid-state relay SSR1 is in the normally closed state, and the depletion-type field-effect transistors Q1 and Q2 are turned on. The DC output power supply Vdc forms a conducting path through the load resistor RL and the solid-state relay SSR1, and current flows through the load resistor RL, so the load is in the working state. At this time, the current path is: positive terminal of DC output power supply Vdc → OUT1 of solid-state relay SSR1 → depletion-type field-effect transistor Q1 → common source node → depletion-type field-effect transistor Q2 → OUT2 of solid-state relay SSR1 → load resistor RL → negative terminal of DC output power supply Vdc.

[0030] When the input control voltage source Vin applies a voltage, the input current flows through the LED D1, causing it to light up. The photovoltaic cell E1 generates a driving voltage that turns off the depletion-mode MOSFETs Q1 and Q2. The output terminals OUT1 and OUT2 of the solid-state relay SSR1 are disconnected, and no current flows through the load resistor RL, thus stopping the load from operating. The input current-limiting resistor Rin is used to convert the input voltage into an appropriate input current; its resistance value is determined based on the input voltage range and the driving current requirements of the LED D1.

[0031] Example 3 like Figure 4 As shown, this embodiment provides another DC load control application circuit, with a load connection method different from that in Embodiment 2.

[0032] In the application circuit of this embodiment, the positive terminal of the DC output power supply Vdc is connected to the first output terminal OUT1 of the solid-state relay SSR1 through the load resistor RL, and the negative terminal of the DC output power supply Vdc is connected to the second output terminal OUT2 of the solid-state relay SSR1. That is, the load resistor RL is connected between the positive terminal of the DC output power supply Vdc and the OUT1 terminal of the solid-state relay SSR1.

[0033] The working principle of this embodiment is as follows: When the input control voltage source Vin is not applied, the solid-state relay SSR1 is in the normally closed state, and the DC output power supply Vdc forms a conducting path through the load resistor RL and the solid-state relay SSR1, with current flowing through the load resistor RL. At this time, the current path is: positive terminal of DC output power supply Vdc → load resistor RL → OUT1 of solid-state relay SSR1 → depletion-mode MOSFET Q1 → common source node → depletion-mode MOSFET Q2 → OUT2 of solid-state relay SSR1 → negative terminal of DC output power supply Vdc.

[0034] When the input control voltage source Vin applies a voltage, the solid-state relay SSR1 turns off, no current flows through the load resistor RL, and the load stops working. The difference between this embodiment and Embodiment 2 is the different load connection positions, but both can achieve the normally closed control function of a DC load.

[0035] Example 4 like Figure 5 As shown, this embodiment provides another DC load control application circuit, with the current flow direction reversed from that in Embodiment 2.

[0036] In the application circuit of this embodiment, the positive terminal of the DC output power supply Vdc is connected to the second output terminal OUT2 of the solid-state relay SSR1, and the negative terminal of the DC output power supply Vdc is connected to the first output terminal OUT1 of the solid-state relay SSR1 through the load resistor RL. That is, current flows in from the OUT2 terminal of the solid-state relay SSR1 and flows out from the OUT1 terminal.

[0037] The working principle of this embodiment is as follows: When the input control voltage source Vin is not applied, the solid-state relay SSR1 is in a normally closed state, and the DC output power supply Vdc forms a conducting path through the solid-state relay SSR1 and the load resistor RL. At this time, the current path is: positive terminal of DC output power supply Vdc → OUT2 of solid-state relay SSR1 → depletion-mode MOSFET Q2 → common source node → depletion-mode MOSFET Q1 → OUT1 of solid-state relay SSR1 → load resistor RL → negative terminal of DC output power supply Vdc. Compared with embodiment two, the current flow direction is reversed, but because this invention adopts a bidirectional switching structure, normal conduction can still be achieved.

[0038] When the input control voltage source Vin applies a voltage, the solid-state relay SSR1 turns off, and the load stops working. This embodiment demonstrates that the solid-state relay of the present invention has true bidirectional conduction capability, unrestricted by the direction of current.

[0039] Example 5 like Figure 6 As shown, this embodiment provides an AC load control application circuit, demonstrating the application capability of the solid-state relay of the present invention in AC applications.

[0040] The application circuit in this embodiment includes: a solid-state relay SSR1, an input control voltage source Vin, an input current-limiting resistor Rin, an AC output power supply Vac, and a load resistor RL. The positive terminal of the input control voltage source Vin is connected to the positive input terminal IN+ of the solid-state relay SSR1 through the input current-limiting resistor Rin, and the negative terminal of the input control voltage source Vin is connected to the negative input terminal IN- of the solid-state relay SSR1. One end of the AC output power supply Vac is connected to one end of the load resistor RL, and the other end of the AC output power supply Vac is connected to the second output terminal OUT2 of the solid-state relay SSR1. The other end of the load resistor RL is connected to the first output terminal OUT1 of the solid-state relay SSR1.

[0041] The working principle of this embodiment is as follows: When the input control voltage source Vin is not applied, the solid-state relay SSR1 is in the normally closed state, and the depletion-type field-effect transistors Q1 and Q2 are turned on. During the positive half-cycle of AC, the current flows from the AC output power supply Vac → load resistor RL → OUT1 of solid-state relay SSR1 → depletion-type field-effect transistor Q1 → common source node → depletion-type field-effect transistor Q2 → OUT2 of solid-state relay SSR1 → AC output power supply Vac; during the negative half-cycle of AC, the current direction is reversed, from the AC output power supply Vac → OUT2 of solid-state relay SSR1 → depletion-type field-effect transistor Q2 → common source node → depletion-type field-effect transistor Q1 → OUT1 of solid-state relay SSR1 → load resistor RL → AC output power supply Vac. Since the two depletion-type field-effect transistors are connected back-to-back, a conduction path can be formed regardless of the direction of the AC current, and AC current flows through the load resistor RL, so the load is in the working state.

[0042] When the input control voltage source Vin applies a voltage, the solid-state relay SSR1 turns off, and the depletion-mode MOSFETs Q1 and Q2 simultaneously turn off, blocking the bidirectional current path. No AC current flows through the load resistor RL, and the load stops working. This embodiment demonstrates that the solid-state relay of the present invention is applicable not only to DC load control but also to AC load control, exhibiting wide applicability.

Claims

1. A normally closed, current-input, optically isolated, bidirectional solid-state relay, characterized in that, Includes input circuitry, isolation drive circuitry, and output switching circuitry; The input circuit includes a light-emitting diode D1, a switching diode D2, and a Zener diode D3. The anode of the light-emitting diode D1 is connected to the positive input terminal of the solid-state relay, the cathode of the light-emitting diode D1 is connected to the anode of the switching diode D2, the cathode of the switching diode D2 is connected to the cathode of the Zener diode D3, and the anode of the Zener diode D3 is connected to the negative input terminal of the solid-state relay. The isolation driving circuit includes the light-emitting diode D1 and the photovoltaic cell E1. The light-emitting diode D1 is optically coupled to the photovoltaic cell E1. The positive terminal of the photovoltaic cell E1 is connected to the common source node of the output switching circuit, and the negative terminal of the photovoltaic cell E1 is connected to the common gate node of the output switching circuit. The output switching circuit includes a depletion-mode field-effect transistor (MOSFET) Q1, a depletion-mode field-effect transistor (MOSFET) Q2, and a gate protection circuit. The source of the MOSFET Q1 and the source of the MOSFET Q2 are connected to form the common source node. The gate of the MOSFET Q1 and the gate of the MOSFET Q2 are connected to form the common gate node. The drain of the MOSFET Q1 and the drain of the MOSFET Q2 serve as the output terminal and output terminal of the solid-state relay, respectively. The gate protection circuit includes a gate resistor R1 and a gate Zener diode D4. The gate resistor R1 is connected between the common source node and the common gate node; the gate Zener diode D4 is connected between the common source node and the common gate node.

2. The normally closed optically isolated bidirectional solid-state relay with current input according to claim 1, characterized in that, The light-emitting diode D1 and the photovoltaic cell E1 are optically coupled to each other to form an optically isolated driving unit. Several optically isolated driving units are set in the solid-state relay.

3. The normally closed optically isolated bidirectional solid-state relay with current input according to claim 1, characterized in that, The switching diode D2 is used to block the current path when a reverse voltage is applied between the positive and negative input terminals of the solid-state relay, thereby protecting the light-emitting diode D1.

4. The normally closed optically isolated bidirectional solid-state relay with current input according to claim 1, characterized in that, The Zener diode D3 is used to adjust the input turn-on voltage threshold and / or input turn-off voltage threshold of the solid-state relay.

5. The normally closed optically isolated bidirectional solid-state relay with current input according to claim 1, characterized in that, The depletion-mode field-effect transistors Q1 and Q2 constitute a normally closed bidirectional switch. When no input current is applied to the input circuit, the gate-source voltages of Q1 and Q2 are zero, and both are in the on state, enabling bidirectional conduction between the output terminals. When an input current is applied to the input circuit to generate a driving voltage for the photovoltaic cell E1, the gate-source voltages of Q1 and Q2 are negatively biased, falling below their turn-off threshold, thereby turning off the path between the output terminals.

6. The current-input normally closed optically isolated bidirectional solid-state relay according to claim 1, characterized in that, The gate protection circuit controls the gate discharge rate after the driving voltage of the photovoltaic cell E1 is removed through the gate resistor R1, and limits the gate voltage of the depletion field-effect transistors Q1 and Q2 through the gate Zener diode D4 to prevent gate overvoltage damage.

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