A constant current opto-isolator circuit based on depletion mode MOSFET

Abstract

The present application relates to the technical field of optical coupling, and particularly relates to a constant current optical coupling isolation circuit based on a depletion mode MOSFET; through the cooperation of a reverse protection module, a self-bias constant current module and a response enhancement module, super-wide voltage input, low power consumption and high-speed dynamic response are realized in the circuit, to a certain extent, the inherent technical contradiction that the traditional resistance current limiting scheme cannot simultaneously consider reliable conduction and low power consumption under wide voltage, and the slow response of high CTR optical coupling is solved, the structure is extremely simple and the reliability is high, a self-bias constant current source based on a depletion mode MOSFET is adopted, only one resistor is needed to realize relatively accurate constant current output in a super-wide input range of 9.6V to 400V, replacing the current limiting resistor with huge power consumption under high voltage or the active constant current source with complex circuit in the traditional scheme, and the power consumption under high voltage application is reduced; the technical problem that the existing resistance current limiting scheme is limited in widening the input voltage range of the optical coupling is solved.

Description

Technical Field

[0001] This invention relates to the field of optocoupler technology, and more particularly to a constant current optocoupler isolation circuit based on a depletion-type MOSFET. Background Technology

[0002] In the fields of industrial control and power conversion, optocouplers are core components for achieving electrical isolation and suppressing electromagnetic interference. Traditional optocoupler input circuits typically employ a single resistor current limiting scheme, which is only suitable for fixed or narrow range input voltages.

[0003] As industrial equipment becomes increasingly integrated, the input voltage range often needs to be significantly expanded, such as from 9.6V to 400V. In such ultra-wide voltage scenarios, traditional resistor-based current limiting solutions require increasing the resistance value of the current limiting resistor to adapt to high voltage (e.g., 400V). However, this increased resistance may result in insufficient current flowing through the optocoupler LED at low voltage (e.g., 9.6V), making it difficult for the LED to conduct reliably. On the other hand, to ensure the optocoupler LED conducts at low voltage, the resistance value must be very small. This would lead to excessive current flowing through the LED at high voltage (400V), resulting in power consumption in the range of several watts, which could easily burn out the device. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides a constant current optocoupler isolation circuit based on a depletion-mode MOSFET, which solves the technical problem that existing resistor current limiting schemes have limited effectiveness in widening the input voltage range of optocouplers.

[0005] To solve the above-mentioned technical problems, the present invention provides the following technical solution: a constant current optocoupler isolation circuit based on a depletion-mode MOSFET, comprising an optocoupler U2, wherein the optocoupler U2 includes an IR chip as an input terminal and an IP chip as an output terminal, and further comprising: A reverse protection module is installed at the input terminal of the IR chip to prevent reverse power connection; The self-biased constant current module, located between the input terminal of optocoupler U2 and the output terminal of the reverse protection module, is used to limit the current flowing into optocoupler U2 to a preset current range. The response enhancement module is used to improve the turn-on speed of optocoupler U2 when the input voltage of the self-biased constant current module changes.

[0006] Preferably, the self-biased constant current module includes a depletion-type MOSFET Q1, the drain of which is connected to the output of the reverse protection module, and a resistor R1 is connected between the gate and source of the depletion-type MOSFET Q1. The output of the resistor R1 is connected to the input of the IR chip.

[0007] Preferably, the reverse protection module is a Schottky diode D1.

[0008] Preferably, the response enhancement module is a capacitor C1 connected in parallel across the resistor R1.

[0009] Preferably, the capacitance of capacitor C1 is between 1nF and 100nF.

[0010] Preferably, the resistance value of the resistor R1 is set such that the input current value is within a preset current range of 0.5mA to 2mA.

[0011] Preferably, the depletion-type MOSFET Q1 has a withstand voltage of not less than 400V, and the CTR range of the optocoupler U2 is 600% to 7000%.

[0012] Preferably, a capacitor C2 is connected between the ground terminal of the IR chip and the ground terminal of the IP chip to improve the common-mode transient suppression capability.

[0013] Preferably, the capacitance of capacitor C2 is between 10pF and 100pF.

[0014] By employing the above technical solution, the present invention provides a constant current optocoupler isolation circuit based on a depletion-mode MOSFET, which has at least the following beneficial effects: 1. This invention achieves ultra-wide voltage input, low power consumption and high-speed dynamic response in the circuit through the coordinated operation of the reverse protection module, the self-biased constant current module and the response enhancement module. It solves to a certain extent the inherent technical contradictions of traditional resistor current limiting schemes, which cannot achieve reliable conduction and low power consumption under wide voltage, and the slow response of high CTR optocouplers. The structure is extremely simple and highly reliable.

[0015] 2. This invention uses a self-biased constant current source based on depletion-mode MOSFETs. It can achieve relatively accurate constant current output in an ultra-wide input range of 9.6V to 400V with only one resistor. It replaces the current-limiting resistor with huge power consumption under high voltage or the active constant current source with complex circuitry in traditional solutions, thereby reducing power consumption in high voltage applications.

[0016] 3. This invention provides a transient high-current drive for optocoupler U2 by connecting a capacitor C1 with a specific capacitance range in parallel across resistor R1, which significantly accelerates its switching process, effectively compensates for the slow response speed of high CTR optocouplers, enables the circuit to reliably transmit high-speed pulse signals, and broadens the application scenarios. Attached Figure Description

[0017] The accompanying drawings, which are included to provide a further understanding of this application and form part of this application, illustrate exemplary embodiments and are used to explain this application, but do not constitute an undue limitation of this application. In the drawings: Figure 1 This is a circuit diagram of the constant current optocoupler isolation circuit for the depletion-type MOSFET of the present invention; Figure 2 This is a circuit schematic diagram of an optocoupler in the prior art; Figure 3 A comparison chart of the current stability results for each scheme; Figure 4 This is a comparison chart of the total power consumption of the circuits for each scheme.

[0018] In the diagram: 1. Reverse protection module; 2. Self-biased constant current module; 3. Response enhancement module. Detailed Implementation

[0019] To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. This will allow for a full understanding of how the present application uses technical means to solve technical problems and achieve technical effects, and to facilitate its implementation.

[0020] To address the limited effectiveness of existing resistor-based current limiting schemes in widening the input voltage range of optocouplers, this invention provides a constant current optocoupler isolation circuit based on a depletion-mode MOSFET, such as... Figure 1 As shown, a depletion-mode MOSFET is used to stabilize the current of the input optocoupler U2 within a smaller range, thereby further widening the range of the optocoupler's input voltage. The optocoupler U2 includes an IR chip as the input terminal and an IP chip as the output terminal, specifically as follows... Figure 2 The diagram shown is a schematic of the current optocoupler principle. Based on the structure of optocoupler U2, improvements have been made, including the following components: 1. A reverse protection module 1, installed at the input terminal of the IR chip, is used to prevent reverse power connection. This module addresses the complex wiring environment in industrial settings. If the input power (Vin) is accidentally reverse-connected, this module immediately blocks the reverse current, preventing high voltage from directly impacting and damaging downstream semiconductor devices. Without this module, the reverse voltage would cause the LED (i.e., the IR chip) of the optocoupler U2 to break down in reverse, and may also cause the body diode of the depletion-type MOSFET Q1 to conduct, leading to overcurrent burnout and permanent circuit failure. Traditional reverse protection can use a common PN junction diode (such as 1N4007), but this embodiment uses a Schottky diode D1, such as model SS14, mainly because it has a lower forward voltage drop. Schottky diode D1 is a device that utilizes the metal-semiconductor junction principle, and its forward voltage drop is... The forward voltage drop is 0.3V to 0.5V, far lower than the 0.7V to 1.0V forward voltage drop of ordinary silicon diodes. In the scenario of this invention, the current flowing through the diode D1 is constant (e.g., 1mA). Although the absolute power consumption difference is not large, the lower forward voltage drop is significant in battery-powered applications or those with extremely high efficiency requirements. This means less wasted power consumption and lower device temperature rise, which directly improves the circuit's energy efficiency and long-term reliability. Furthermore, the Schottky diode D1 is a majority carrier conducting device with virtually no minority carrier storage effect, resulting in an extremely short, usually negligible, reverse recovery time. This is crucial in scenarios requiring rapid signal transmission. If a diode with a slow recovery time were used, its own switching delay would become a new bottleneck limiting the overall circuit response speed when high-speed pulse signals are input. Choosing the Schottky diode D1 ensures that the reverse protection module 1 itself will not negatively impact the performance of the response enhancement module 3.

[0021] 2. Traditional resistor-based current limiting schemes struggle to balance low-voltage turn-on and high-voltage power consumption under ultra-wide voltage ranges (e.g., 9.6V-400V). Therefore, to ensure a relatively constant current to the input optocoupler U2, a self-biased constant current module 2 is placed between the input of optocoupler U2 and the output of reverse protection module 1. This module limits the current flowing into optocoupler U2 to a preset current range. Regardless of the input voltage fluctuations between 9.6V and 400V, this module can limit the current flowing to the subsequent optocoupler U2 to a preset, extremely small constant range. The specific circuit structure of the self-biased constant current module 2 is described in detail below. The self-biased constant current module 2 includes a depletion-type MOSFET Q1. The drain of the depletion-type MOSFET Q1 is connected to the output of reverse protection module 1. A resistor R1 is connected between the gate and source of the depletion-type MOSFET Q1. The output of resistor R1 is connected to the input of the IR chip. The reason for choosing a depletion-type MOSFET Q1 instead of an enhancement-type MOSFET or a transistor is that the depletion-type MOSFET has a lower gate-source voltage... A conductive channel exists at that time, such as Figure 1 As shown, by directly connecting its gate G and source S through a resistor R1, the simplest self-biased circuit is formed. The drain D of the depletion-type MOSFET Q1 is connected to the output terminal of the front-end reverse protection module 1 to withstand the changing input voltage. The current flowing through the depletion-type MOSFET Q1 and the resistor R1 (i.e., the input current of the optocoupler U2) is... A voltage drop will occur across resistor R1. When the circuit is powered on and current begins to flow, the source current will generate a voltage drop across R1. This makes the source potential (S) higher than the gate potential (G), thus generating an effective negative gate-source voltage. For depletion-type MOSFETs, when As the current changes from 0 to negative, the conductive channel narrows, and the drain current... (This is equal to) The input voltage will decrease, which is a negative feedback process. If the increase in input voltage leads to... If there is an increasing trend, then A more negative channel resistance automatically increases, thereby suppressing... The increase eventually stabilizes it around a specific value, which is the stable current value. The inherent parameters of the depletion-mode MOSFET Q1 (i.e., the pinch-off voltage) The source resistance R1 and the source resistance R1 together determine the relationship, which approximately satisfies the following: Therefore, the resistance of resistor R1 is determined. It is then basically determined that, in relation to the input voltage It is unaffected by a wide range of variations, thus ensuring that the current flowing through the optocoupler LED (i.e., the IR chip) remains relatively constant across the full range of input voltages from 9.6V to 400V.

[0022] To ensure that the preset current range can guarantee the operation of LED U2, the resistance value of resistor R1 is set to keep the input current value within the preset current range of 0.5mA to 2mA.

[0023] The drain-source breakdown voltage of the depletion-type MOSFET Q1 must be higher than the maximum possible input voltage, with sufficient margin to cope with voltage surges and spikes in industrial environments. Therefore, the drain-source breakdown voltage of the depletion-type MOSFET Q1 needs to meet a minimum standard of not less than 400V. For example, depletion-type MOSFETs Q1 with models DN2540 and IXTH20N60X2 meet the above requirements. To drive the same load in a relatively fixed range (i.e., 0.5–2 mA), a higher CTR requires more power. The lower the setting, the better. For example, an optocoupler with a CTR of 20% might require 10mA. To achieve a 2mA output, the power consumption reaches 4W with a 400V input voltage, while an optocoupler with a CTR of 600% only requires about 0.33mA. The same 2mA output can be obtained, with a power consumption of only about 0.13W at 400V. The high CTR directly and proportionally reduces the static power consumption of the system, and the total power consumption of the circuit is less than 0.4W. Traditionally, it is believed that the high CTR optocoupler U2 has a larger junction capacitance and slower response speed due to the use of a larger photodetector area. However, this embodiment compensates for this disadvantage by introducing a response enhancement module 3. The lower limit of the CTR range (600%) ensures that even at the lowest sensitivity, it can still achieve a power consumption of 0.5mA or more. It can also generate a sufficiently strong output signal, so that the CTR range of optocoupler U2 can be set to 600% to 7000%.

[0024] 3. Response Enhancement Module 3 is used to improve the turn-on speed of optocoupler U2 when the input voltage of self-biased constant current module 2 changes rapidly. This module works in conjunction with self-biased constant current module 2. To achieve high sensitivity, the high CTR optocoupler U2 has a large photodetector area, resulting in a large junction capacitance and a slow response speed (especially rise / fall time). The function of Response Enhancement Module 3 is to temporarily provide an additional, low-impedance transient current path when the input voltage changes rapidly (i.e., at the signal edge), thereby providing a rapid charging and discharging current for the parasitic capacitance inside optocoupler U2, significantly improving the overall circuit's response speed to rapidly changing signals.

[0025] The resistance value of resistor R1 in the self-biased constant current module 2 is crucial for setting the constant current value. However, while stabilizing the DC current, this resistor also limits the rate of change of the current. When a rapidly rising voltage edge is input, due to the presence of resistor R1, the current of the optocoupler's LED (i.e., IR chip) cannot change abruptly, but can only rise slowly according to an exponential law. This greatly restricts the turn-on speed of optocoupler U2. The high CTR optocoupler, due to its large junction capacitance, exacerbates this problem because it requires more charge to establish the potential for conduction. Therefore, the response enhancement module 3 uses capacitor C1 connected in parallel across resistor R1. For slowly changing DC or low-frequency signals, the capacitive reactance of capacitor C1 is extremely large, equivalent to an open circuit. In this case, the constant current value is entirely determined by resistor R1. However, for instantaneous changes in input voltage, the capacitive reactance of capacitor C1 becomes extremely small, thus providing an almost unobstructed bypass path for the input current. This results in a current much larger than the steady-state current arriving at the signal edge. The transient pulse current can directly drive the optocoupler LED, enabling it to conduct rapidly. Therefore, the introduction of capacitor C1, without changing the constant current value of the input optocoupler U2, specifically optimizes the transient response characteristics of the circuit, effectively overcoming the response delay caused by the combined characteristics of resistor R1 and the high CTR optocoupler U2, enabling the circuit of this invention to reliably transmit digital switching signals with narrower pulse widths and higher frequencies.

[0026] The function of capacitor C1 is to provide a low-impedance path for the transition of the input signal. Its capacitance value C1 and the resistance value R1 together determine the time constant of the acceleration network. ( If the capacitance value C1 is too small, such as 100pF, the amount of charge that capacitor C1 can store and release is limited. The transient current pulse provided is insufficient to quickly overcome the influence of the junction capacitance of the LED and detector inside the optocoupler, and the acceleration effect will become negligible, failing to effectively improve the response speed. If the capacitance value C1 is too large, such as 1μF, its time constant will become very large, which will lead to two problems: First, when the input signal is a continuous high-frequency pulse, capacitor C1 cannot be fully charged / discharged between adjacent pulses, and its acceleration effect will decay or even disappear. Second, an excessively large capacitor will significantly prolong the falling edge of the output signal, resulting in waveform distortion. In extreme cases, it may prevent the circuit from correctly responding to switching signals with narrow pulse widths. Therefore, the capacitance value of capacitor C1 is set from 1nF to 100nF. Numerous experiments have shown that when the capacitance value C1 is about 10nF, it can provide a significant acceleration effect and has the least impact on the edge characteristics of the output waveform (especially the fall time), achieving relatively better overall performance.

[0027] To determine the validity of the aforementioned range and the optimal capacitance value, this embodiment performed steady-state constant current characteristic verification and dynamic response characteristic verification. The steady-state constant current characteristic verification aimed to verify the circuit's constant current characteristics and static power consumption within an input range of 9.6V to 400V, with and without capacitor C1. A programmable DC power supply was used to provide the input voltage. Tests were conducted at key voltage levels of 9.6V, 24V, 110V, 220V, and 400V, using a high-precision digital multimeter to measure the current flowing through the optocoupler LED. And calculate the circuit input power. Table 1 below shows some of the data. Experimental data indicates that, in the range of 1nF to 100nF, the presence of C1 affects the steady-state DC current. The impact is minimal (the change is far less than 5%), and the circuit can maintain a stable constant current value and extremely low power consumption (about 0.4W at 400V) across the entire voltage range.

[0028] Table 1. Steady-state constant current characteristic test data The dynamic response characteristic verification aims to validate the actual acceleration effect of capacitors C1 with different capacitance values ​​on the optocoupler switching speed under high-voltage switching signals. To simulate relatively harsh industrial conditions, a high-voltage power amplifier was used to generate a square wave signal switching between 10V and 300V, with a frequency of 1kHz and a rise / fall time of <100ns. A high-voltage differential probe was used to measure the input voltage, a current probe was used to measure the current passing through the LED in optocoupler U2, and an oscilloscope was used to measure the output voltage of the optocoupler. As shown in Table 2, the experimental data clearly demonstrates that within the range of 1nF to 100nF, capacitor C1 can generate significant transient current spikes, thereby shortening the optocoupler's output response time from several microseconds to sub-microseconds. A capacitance value of around 10nF achieves the best balance between acceleration and signal overshoot / tailing; a capacitance value less than 1nF results in insufficient acceleration, while a value greater than 100nF may lead to slowed recovery or waveform distortion due to overcharging.

[0029] Table 2 Dynamic Response Test Data 4. In applications such as industrial motor drives and power converters, high-frequency, high-amplitude common-mode noise may exist between the ground potentials at both ends of the isolation circuit. This high-speed common-mode noise can couple through the parasitic capacitance between the input and output terminals of the optocoupler, causing glitches or false triggers in the output signal. In other words, the common-mode noise is misinterpreted as a differential-mode signal. Therefore, a capacitor C2 is connected between the ground terminals of the IR chip and the IP chip to improve common-mode transient rejection. Capacitor C2 provides a pre-defined, low-impedance, clean return path for high-frequency common-mode noise. Most of the common-mode noise current will preferentially return through capacitor C2, rather than through the optocoupler's internal parasitic capacitance and subsequent sensitive detection circuitry, thus significantly reducing the noise amplitude coupled to the output terminal. When the capacitance value is less than 10pF, its impedance at high frequencies is still relatively high, which is insufficient to effectively shunt noise. The improvement effect on CMTI (common-mode transient rejection ratio) is limited. Its upper limit is set based on international safety standards. The leakage current generated by a 100pF capacitor C2 at 250VAC / 50Hz is about 7.9μA, which is usually still within the safety limit. If the capacitance value exceeds 100pF, the leakage current is very likely to exceed the standard, making it difficult for the product to pass safety certification.

[0030] In addition, such as Figure 3 and Figure 4 As shown, this embodiment is also compared with several traditional solutions (such as transistor current limiting and original resistor current limiting). This solution is solution 3. The current stability and total power consumption of the circuit under different solutions are compared. It can be found that the solution in this embodiment performs better in terms of current stability and power consumption, and the cost is relatively low.

[0031] Those skilled in the art will understand that all or part of the steps in the methods of the above embodiments can be implemented by a program instructing related hardware. Therefore, this application can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Moreover, this application can take the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0032] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on its differences from other embodiments. Similar or identical parts between embodiments can be referred to interchangeably. Since the above embodiments are substantially similar to the method embodiments, their descriptions are relatively simple; relevant parts can be referred to the descriptions of the method embodiments.

[0033] The above embodiments provide a detailed description of the present invention. Specific examples have been used to illustrate the principles and implementation methods of the present invention. The descriptions of the above embodiments are only for the purpose of helping to understand the method and core ideas of the present invention. At the same time, for those skilled in the art, there will be changes in the specific implementation methods and application scope based on the ideas of the present invention. Therefore, the content of this specification should not be construed as a limitation of the present invention.

Claims

1. A constant current optocoupler isolation circuit based on a depletion-mode MOSFET, comprising an optocoupler U2, wherein the optocoupler U2 includes an IR chip as an input terminal and an IP chip as an output terminal, characterized in that, Also includes: A reverse protection module (1) is installed at the input terminal of the IR chip to prevent reverse power connection. The self-biased constant current module (2) is set between the input terminal of optocoupler U2 and the output terminal of reverse protection module (1) to limit the current flowing into optocoupler U2 to a preset current range. The response enhancement module (3) is used to improve the turn-on speed of the optocoupler U2 when the input voltage of the self-biased constant current module (2) changes.

2. The constant current optocoupler isolation circuit according to claim 1, characterized in that, The self-biased constant current module (2) includes a depletion-type MOSFET Q1. The drain of the depletion-type MOSFET Q1 is connected to the output terminal of the reverse protection module (1). A resistor R1 is connected between the gate and source of the depletion-type MOSFET Q1. The output terminal of the resistor R1 is connected to the input terminal of the IR chip.

3. The constant current optocoupler isolation circuit according to claim 1, characterized in that, The reverse protection module (1) is a Schottky diode D1.

4. The constant current optocoupler isolation circuit according to claim 2, characterized in that, The response enhancement module (3) is a capacitor C1 connected in parallel across the resistor R1.

5. The constant current optocoupler isolation circuit according to claim 4, characterized in that, The capacitance of capacitor C1 is between 1nF and 100nF.

6. The constant current optocoupler isolation circuit according to claim 2, characterized in that, The resistance value of the resistor R1 is set to ensure that the input current value is within a preset current range of 0.5mA to 2mA.

7. The constant current optocoupler isolation circuit according to claim 2, characterized in that, The depletion-type MOSFET Q1 has a withstand voltage of not less than 400V, and the CTR range of the optocoupler U2 is 600% to 7000%.

8. The constant current optocoupler isolation circuit according to claim 1, characterized in that, A capacitor C2 is connected between the ground terminal of the IR chip and the ground terminal of the IP chip to improve the common-mode transient suppression capability.

9. The constant current optocoupler isolation circuit according to claim 8, characterized in that, The capacitance of capacitor C2 is between 10pF and 100pF.

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