Circuit for stabilizing current at transmitting end of optocoupler

Abstract

The utility model relates to the technical field of optocoupler circuits, in particular to a circuit for stabilizing current at a transmitting end of an optocoupler, which comprises the optocoupler, the transmitting end of the optocoupler is connected with a current stabilizing circuit, and the current stabilizing circuit comprises an MOS (Metal Oxide Semiconductor) tube, a divider resistor and a current limiting resistor; the source electrode of the MOS tube is connected with the positive electrode of an input signal, and the positive electrode of the input signal is connected with the positive electrode of the optical coupler transmitting end through a current-limiting resistor. The drain electrode of the MOS tube is connected with the negative electrode of the transmitting end of the optocoupler and then is connected with a current-limiting resistor; the grid electrode of the MOS tube is connected between the two divider resistors, and the two divider resistors are connected to the positive electrode and the negative electrode of an input signal; when the voltage of the input signal exceeds a certain voltage value, the MOS tube is conducted and shunted, so that the current at the transmitting end of the optocoupler is stabilized within a certain range. According to the utility model, the active stabilization of the current at the transmitting end of the optocoupler is realized through the MOS tube and the resistor, the current fluctuation is effectively inhibited in a wide input voltage range by combining theoretical derivation and parameter optimization, and the reliability and the service life of the system are improved.

Description

A circuit for stabilizing the current at the emitter of an optocoupler Technical Field

[0001] This utility model relates to the field of optocoupler circuit technology, and in particular to a circuit for stabilizing the current at the emitter end of an optocoupler. Background Technology

[0002] An optocoupler, also known as an optocoupler or optoisolator, is a semiconductor device that transmits electrical signals using light as a medium. It is widely used in switching power supplies, industrial control, and digital communications, and is commonly used for signal isolation and transmission. An optocoupler converts an electrical signal into a light signal through a light-emitting diode (LED) at the transmitting end, and then the light signal is converted back into an electrical signal by a photosensitive device (such as a phototransistor) at the receiving end. The magnitude of the current at the transmitting end directly determines the luminous intensity of the LED.

[0003] Unstable transmitter current in an optocoupler circuit can lead to the following problems: The transmitter current increases with the applied voltage. When the input signal voltage to the transmitter experiences noise or sudden fluctuations, the transmitter current will also experience noise or sudden fluctuations, causing the luminous intensity to fluctuate accordingly. This results in signal distortion or noise at the receiver, especially noticeable in high-speed or high-precision signal transmission. Sudden current fluctuations can also cause electromagnetic interference (EMC) problems. When the input signal voltage is too high or spikes, the transmitter current may exceed its rated value or experience current spikes. Excessive current or current spikes can accelerate LED aging and even cause permanent damage.

[0004] The current transfer ratio (FTL), the ratio of the output current (receiver) to the input current (emitter) of an optocoupler, is a core parameter of the optocoupler. The luminous efficiency of an LED varies with temperature, causing the FTL to drift. Over long-term use, the luminous efficiency of an LED may decrease; if the emitter current is unstable, this will exacerbate the attenuation of the FTL, affecting the system's lifespan. Therefore, measures to stabilize the emitter current in the optocoupler circuit are necessary to ensure stable signal transmission. Maintaining the FTL avoids the risk of overcurrent in the optocoupler components, extends its lifespan, enhances system noise suppression, and improves electromagnetic compatibility. This is significant in maintaining the performance of the optocoupler and the reliability of the system. Summary of the Invention

[0005] The purpose of this invention is to provide a circuit for stabilizing the current at the emitter of an optocoupler. This circuit is used to stabilize the current at the emitter (LED side) of the optocoupler, avoid current instability caused by input voltage fluctuations, thereby protecting the optocoupler components, improving signal transmission reliability, and extending device life.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] A circuit for stabilizing the current at the emitter of an optocoupler includes an optocoupler. The emitter of the optocoupler is connected to a current-stabilizing circuit, which includes a MOSFET, voltage divider resistors, and a current-limiting resistor. The source of the MOSFET is connected to the positive terminal of the input signal, which is connected to the positive terminal of the optocoupler's emitter via a current-limiting resistor. The drain of the MOSFET is connected to the negative terminal of the optocoupler's emitter, and then connected to another current-limiting resistor. The gate of the MOSFET is connected between two voltage divider resistors, which are connected to the positive and negative terminals of the input signal. When the voltage of the input signal exceeds a certain value, the MOSFET conducts and shunts the current, stabilizing the current at the optocoupler's emitter within a certain range.

[0008] Furthermore, the positive terminal of the optocoupler output is connected to the power supply of the detection system through an output bias resistor, and the negative terminal of the optocoupler output is connected to the power supply or signal ground of the detection system.

[0009] Furthermore, the MOS transistor is a P-channel enhancement-mode MOS.

[0010] The current at the optocoupler's emitter increases with the input signal voltage. When the input signal voltage exceeds a certain level, the MOSFET turns on, and a portion of the current flows through the MOSFET. This current creates a voltage drop across the second resistor, offsetting the voltage applied to the first current-limiting resistor and the optocoupler's emitter due to the increased input signal voltage. The optocoupler's emitter current remains constant or changes only slightly. As the input signal voltage continues to rise, the gate-drain voltage difference (Vgs) increases, the MOSFET's on-resistance decreases, and more current is shunted from the optocoupler's input to the MOSFET, resulting in a larger voltage drop across the second resistor. This further stabilizes the voltage across the first current-limiting resistor and the optocoupler's emitter, thus keeping the optocoupler's emitter current within a certain range. This protects the optocoupler components, ensures circuit performance, and improves system reliability.

[0011] This invention utilizes dynamic current shunting of MOSFETs and resistor feedback control to achieve high stability of the optocoupler transmitter current over a wide input voltage range. It also features overcurrent protection, noise suppression, and CTR stability, making it suitable for industrial, communication, and power systems with high requirements for signal isolation and reliability. Attached Figure Description

[0012] Figure 1 is a schematic diagram of the circuit structure of this utility model.

[0013] Figure 2 is a schematic diagram of the circuit simulation structure of this utility model.

[0014] In the diagram, the following are the markings: U1, MOSFET; G, gate; S, source; D, drain; U2, optocoupler; R1, first voltage divider resistor; R2, second voltage divider resistor; R3, first current limiting resistor; R4, second current limiting resistor; R5, output bias resistor. Detailed Implementation

[0015] The technical solutions of the present utility model will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present utility model. Obviously, the described embodiments are only some embodiments of the present utility model, and not all embodiments.

[0016] This embodiment discloses a circuit for stabilizing the current at the emitter of an optocoupler, which is used to stabilize the current at the emitter of the optocoupler (LED side) and avoid current instability caused by input voltage fluctuations.

[0017] As shown in Figure 1, the circuit includes a MOSFET U1, an optocoupler U2, a first voltage divider resistor R1, a second voltage divider resistor R2, a first current-limiting resistor R3, a second current-limiting resistor R4, and an output bias resistor R5. The first voltage divider resistor R1 and the second voltage divider resistor R2 are connected in series and then in parallel between the positive input signal terminal IN+ and the negative input signal terminal IN-. The voltage sampled from the voltage divider is then connected to the gate G of the MOSFET U1. The source (S) of the MOSFET U1 is connected to the positive input signal terminal IN+, and the drain (D) is connected to the second current-limiting resistor R4. The positive input terminal of the optocoupler U2 is connected to the first current-limiting resistor R3, and the other end of the first current-limiting resistor R3 is connected to the positive input signal terminal IN+. The negative input terminal of the optocoupler U2 is connected to the second current-limiting resistor R4, and the other end of the second current-limiting resistor R4 is connected to the input signal IN-. The positive output terminal OUT of the optocoupler U2 is connected to the output bias resistor R5, which is connected to the system power supply positive terminal (5V). The negative output terminal of the optocoupler U2 is connected to the system power supply ground (GND).

[0018] The circuit provided in this embodiment was simulated. The input signal Vin of the optocoupler is in the range of 18V~60V, and the emission current at the input terminal of the optocoupler is the manufacturer's recommended value, which is about 8~10mA.

[0019] The input signal Vin is divided by voltage divider resistors R1 and R2 and then connected to the gate G of MOSFET U1. The difference between the gate voltage Vg and the source voltage Vs is the gate-source voltage as shown in Equation 1. .

[0020] This example uses a P-channel MOSFET. The gate threshold voltage VGS(th) of the MOSFET is approximately -3.5V. The gate threshold voltage VGS(th) of the P-channel MOSFET is always negative.

[0021] When the input signal Vin is below 18V, the gate-source voltage Vgs is negative, and the absolute value of Vgs (|Vgs|) is less than the absolute value of the MOSFET gate threshold voltage (|VGS(th)|) in this example. The MOSFET is in the cutoff region, and the drain current is approximately zero. The emitter current at the optocoupler input increases with the input voltage; this emitter current is the LED current. VF is the on-state voltage drop of the LED at the input of the optocoupler, which is typically 1~2V. It is an inherent parameter of the optocoupler element and is approximately a fixed value. In this example, the optocoupler VF is approximately 1.2V.

[0022] As the input signal Vin voltage increases, the gate-source voltage |Vgs| will also increase. When it reaches the gate threshold voltage |VGS(th)| of the MOSFET U1, a drain current I appears in the MOSFET. D LED current By rationally designing the circuit parameters, the MOSFET can be made to operate in the amplification region. At this time, the drain current ID is approximately proportional to │Vgs-VGS(th)│. The relationship between ID and │Vgs-VGS(th)│ is ID=K│Vgs-VGS(th)│, where K is determined by the MOSFET's component parameters and is approximately constant in the amplification region. When the absolute value of the gate-source voltage │Vgs│ exceeds the MOSFET's gate threshold voltage │VGS(th)│, the drain current ID increases proportionally to the voltage difference between │Vgs│ and │VGS(th)│.

[0023] It is important to note that the MOSFET only turns on when the input voltage Vin is above 18V. Let Vin' represent the increment of the input signal after 18V, at which point the input signal voltage is: Vin = Vin' + 18. The corresponding relationship between the optocoupler input current IF after the MOSFET turns on and the increment voltage Vin' after the input signal voltage exceeds 18V is as follows:

[0024] ;

[0025] Choose appropriate values ​​for resistors R1, R2, and R4 to satisfy: R1 + R2 = R1 × R4 × K; the first term in the above formula... The product is 0, meaning the optocoupler input current IF is independent of the increment Vin' after the input signal voltage exceeds 18V; once the input voltage reaches 18V, even if the input voltage continues to increase, the optocoupler transmitter current remains unchanged. At this point, Because K, VGS(th), V F As an approximate constant, the values ​​of resistors R3 and R4 should be reasonably selected so that IF is the current value recommended by the optocoupler, such as 8mA. Therefore, during the design, the value of R3 should be adjusted first to make IF within the current value recommended by the optocoupler.

[0026] The normal operating voltage range of the optocoupler's input signal Vin is 18V~60V. Within this voltage range, the optocoupler's emitter current needs to be stable, ideally constant. Below 18V, stabilizing the optocoupler's emitter current is not necessary. When the input voltage is 18V, the MOSFET's Vgs reaches its threshold voltage VGS(th), causing the MOSFET to turn on, shunting and stabilizing the optocoupler's emitter current. Optocoupler input emitter current IF is the LED diode's emission current when the input signal voltage is 18V. The input signal is 18V. Although the MOSFET's Vgs has reached the MOSFET's gate threshold voltage VGS(th), the MOSFET's drain current ID starts to rise from zero, but ID is still zero.

[0027] In summary, the stable expected value IF of the optocoupler transmitter current is set as the optocoupler input current when the input signal voltage Vin is 18V. IF is set to the optocoupler manufacturer's recommended 8mA, and the total resistance values ​​of R3 and R4 are obtained (VF uses parameters provided by the optocoupler manufacturer). The parameters of all components R1, R2, R3, and R4 are calculated using electronic design software.

[0028] The circuit simulation for this example is shown in Figure 2. When the input signal Vin is below the operating voltage of 18V, the MOSFET is in the cutoff region, and the emitter current at the optocoupler input terminal increases linearly with the increase of the input voltage. When the input signal Vin reaches 18V, the emitter current IF of the optocoupler is approximately 8mA. When Vin rises above the operating voltage of 18V, it reaches the turn-on voltage VGS(th) of the MOSFET U1 in this example, and the MOSFET exhibits a drain current ID. The emitter current at the optocoupler input terminal increases slowly with the increase of the input voltage. When the input voltage rises to 24.5V, the emitter current at the optocoupler input terminal reaches its peak, approximately 9.4mA. Subsequently, as the input signal voltage increases, the emitter current at the optocoupler input terminal decreases. When the input signal voltage rises to 45V, the emitter current at the optocoupler input terminal drops to its lowest point, approximately 7.6mA. Subsequently, the emitter current at the optocoupler input terminal increases slowly again with the increase of the input voltage. When the input signal reaches the upper limit of the operating voltage of 60V, the emitter current at the optocoupler input terminal returns to approximately 8mA.

[0029] According to simulation results, within the input signal voltage range of 18~60V, the emitter current at the optocoupler input terminal is 7.6mA~9.4mA, achieving the function of maintaining a stable emitter current at the optocoupler input terminal. The reason why the emitter current at the optocoupler input terminal does not remain constant at 8mA within the 18~60V input signal voltage range is that the coefficient K in Equation 4 is not a strictly constant value, but is related to the gate-source voltage VGS and drain-source voltage VDS of the MOS transistor, and is only an approximate constant.

[0030] The above description is only a preferred embodiment of the present utility model, but the protection scope of the present utility model is not limited thereto. Any equivalent substitutions or changes made by those skilled in the art within the technical scope disclosed in the present utility model, based on the technical solution and the inventive concept of the present utility model, should be included within the protection scope of the present utility model.

Claims

1. A circuit for stabilizing the current at the transmitter of an optocoupler, comprising an optocoupler, characterized in that, The emitter of the optocoupler is connected to a current stabilizing circuit, which includes a MOSFET, voltage divider resistors, and a current-limiting resistor. The source of the MOSFET is connected to the positive terminal of the input signal, and the positive terminal of the input signal is connected to the positive terminal of the optocoupler emitter through a current-limiting resistor. The drain of the MOSFET is connected to the negative terminal of the optocoupler emitter, and then connected to another current-limiting resistor. The gate of the MOSFET is connected between two voltage divider resistors, which are connected to the positive and negative terminals of the input signal. When the voltage of the input signal exceeds a certain value, the MOSFET turns on to shunt current and stabilize the current at the optocoupler emitter.

2. The circuit for stabilizing the current at the transmitter of an optocoupler according to claim 1, characterized in that: The positive terminal of the optocoupler output is connected to the power supply of the detection system through an output bias resistor, and the negative terminal of the optocoupler output is connected to the power supply or signal ground of the detection system.

3. The circuit for stabilizing the current at the emitter of an optocoupler according to claim 1, characterized in that: The MOS transistor is a P-channel enhancement-mode MOS.

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