Classic Review: Application of Linear Optocoupler in Current Sampling

1 Introduction

In modern electrical measurement and control, low-voltage devices are often needed to measure and control analog quantities such as high voltage and high current. If there is no electrical isolation between analog and digital quantities, high voltage and high current can easily be connected to low-voltage devices and burn them out. The linear optocoupler HCNR200 can achieve good isolation between analog and digital quantities, with an isolation voltage peak of 8000V; the output follows the input change, with a linearity of 0.01%.

2 HCNR200/201 Introduction

The principle of HCNR200 linear optocoupler is shown in Figure 1. It consists of light-emitting diode D1, feedback photodiode D2, and output photodiode D3. When D1 passes the driving current If, it emits infrared light (servo light flux). The light is irradiated on D2 and D3 respectively, and the feedback photodiode absorbs part of the light flux of D2, thereby generating a control current I1 (I1=0.005If). This current is used to adjust If to compensate for the nonlinearity of D1. The output current I2 generated by the output photodiode D3 is linearly proportional to the servo light flux emitted by D1. Let the servo current gain K1=I1/If, the forward gain K2=I2/If, then the transmission gain K3=K2/K1=I2/I1, and the typical value of K3 is 1.

3 Current Detection Circuit

3.1 Design of current detection circuit in photoconductive mode

The detection current circuit of HCNR200 working in the photoconductive mode is shown in Figure 2. The signal is positive input and positive output. In the isolation circuit, R1 adjusts the input bias current of the primary operational amplifier, and C1 plays a feedback role. At the same time, it filters out the burr signal in the circuit to prevent the aluminum gallium arsenide light-emitting diode (LED) in HCNR200 from being accidentally impacted. However, as the frequency increases, the impedance will become smaller, the primary current of HCNR200 will increase, and the gain will increase accordingly. Therefore, the introduction of C1 has a certain effect on the gain of the channel at high frequencies. Although reducing the value of C1 can expand the bandwidth, it will affect the gain of the primary operational amplifier. At the same time, the large burr signal output by the primary operational amplifier is not easy to be filtered out. R3 can control the luminous intensity of the LED and play a certain role in controlling the gain of the channel.

3.2 Current detection circuit design in photovoltage mode

The detection current circuit of HCNR200 working in the photovoltage mode is shown in Figure 3. The signal is positive input and positive output. The functions of R1, R2, R3, and C1 are basically the same as those in the photoconductivity mode. Amplifier A1 adjusts the current If. When the input voltage Vin increases, I1 increases, and at the same time, the voltage at the "+" input terminal of amplifier A1 increases, causing the current If to increase. Due to the connection between D1 and D2, I1 will pull the "+" input terminal voltage back to 0V, forming a negative feedback. If the input current of amplifier A1 is very small, then the current flowing through R1 is Vin/R1=I1. Obviously, there is a linear proportional relationship between I1 and Vin. I1 changes steadily and linearly, and If also changes steadily and linearly. Because D3 is illuminated by D1, I2 also changes steadily and linearly. Amplifier A2 and resistor R2 convert I2 into a voltage VOut=I2×R2.

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4. Operational Amplifier Selection

HCNR200/201 is a current-driven device, and the working current of its LED is 1mA~20mA. Therefore, the driving current of operational amplifier A1 must also reach 20mA. The output stage of operational amplifiers that can achieve this output current capability is generally bipolar, so it is more appropriate to choose a bipolar operational amplifier. At the same time, according to the input voltage range, the operational amplifier is also required to have corresponding common-mode input and output capabilities. This design circuit uses the HA17324 integrated operational amplifier powered by a single power supply, and its output current can reach 40mA.

5. Resistor Selection

The selection of resistors for photoconductive mode is discussed below.

The equivalent circuit of the driver stage composed of A1 is shown in Figure 4. In the figure, Rf is the equivalent feedback resistor. This equivalent circuit is a typical non-inverting amplifier, so U+=U-, and U+=Vin, so Vin=U-.

It is obvious from Figure 2 that

Where VD1 is the forward voltage drop of D1.

As can be seen from Figure 4,

So substitute formula (3) into formula (4):

Due to the discreteness of device parameters, I1 is approximately equal to 0.005If, K3=I2/I1≈1, so R1, R2, and R3 still need to be adjusted near the estimated values ​​in order to obtain the best linearity.

After adjustment, the best linearity is 220Ω.

6 Conclusion

The analog signal isolation circuit composed of linear optical couplers has good linearity and simple circuit, which effectively solves the problem of electrical isolation between analog signals and single-chip microcomputer application systems. If the drive stage and buffer stage use combined operational amplifiers, the linearity can be improved.

HCNR200 can be widely used in analog signal isolation occasions that require good stability, linearity and bandwidth. Two HCNR200s can work in bipolar input/bipolar output mode; at the same time, it can also work in AC/DC circuits, converter isolation, thermocouple isolation, 4mA~20mA analog current loop transmission/reception and other modes. It can be widely used in digital communication, voltage and current detection, switching power supply, measurement and testing of industrial process control and other aspects.

This device is used for motor current measurement. The current feedback is accurate and reliable, and plays a role in realizing current closed-loop control.