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 high voltage,

High current and other analog quantities. If there is no electrical isolation between analog quantities and digital quantities, then high voltage,

Strong current can easily flow into low-voltage devices and burn them out. Linear optocoupler HCNR200 can better

Realize the isolation between analog and digital quantities, the peak isolation voltage reaches 8000V; the output follows the input

change, the linearity reaches 0.01%.

2 Introduction to HCNR200/201
    The principle of HCNR200 linear optocoupler is shown in Figure 1. It consists of light-emitting diode D1, feedback light

It is composed of electric diode D2 and output photodiode D3. When D1 passes the driving current If _, it emits infrared

Light (servo luminous flux). The light is illuminated on D2 and D3 respectively, and the feedback photodiode absorbs the D2 light pass.

part of the amount, thus generating a control current I ₁ (I ₁ =0.005I _f ). This current is used to adjust If _to compensate

Compensate for the nonlinearity of D1. The output current I produced by the output photodiode D3 _and the servo light emitted by D1

The amount is linearly proportional. Let the servo current gain K ₁ =I ₁ /I _f and the forward gain K ₂ =I ₂ /I _f , then the transmission

Gain K ₃ =K ₂ /K ₁ =I ₂ /I ₁ , the typical value of K ₃ is 1.

3 Current detection circuit
3.1 Current detection circuit design in photoconductivity mode The
    detection current circuit of HCNR200 working in photoconductivity mode is shown in Figure 2. The signal is positive

Polar input, positive polarity output. In the isolation circuit, R ₁ adjusts the input bias of the primary operational amplifier

The size of the current, C ₁ plays a feedback role, and at the same time filters out glitch signals in the circuit to avoid HCNR200

Aluminum gallium arsenide light-emitting diodes (LEDs) have been hit unexpectedly. 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 C _{1 will}

The gain of the channel at high frequencies has a certain impact. Although reducing the value of C ₁ can expand the bandwidth, however,

It will affect the gain of the primary operational amplifier, and at the same time, there will be a large glitch in the output of the primary operational amplifier.

The signal is not easily filtered out. R ₃ can control the luminous intensity of the LED and play a role in controlling the channel gain.

certain effect.

3.2 Design of current detection circuit in photovoltage mode
    The detection current circuit of HCNR200 working in photovoltage mode is shown in Figure 3. The signal is

Positive polarity input, positive polarity output. The functions of R ₁ , R ₂ , R ₃ and C ₁ are the same as those in photoconductive mode.

The function is basically the same. Amplifier A1 regulates current _If . When the input voltage V _in increases, I ₁ increases,

At the same time, the voltage at the "+" input terminal of amplifier A1 increases, causing the current If _to increase. Since D1 and D2

Due to the connection between them, I ₁ will pull the "+" input terminal voltage back to 0V, forming negative feedback. if put

The input current of amplifier A1 is very small, so the current flowing through R ₁ is V _in /R ₁ =I ₁ . Obviously,

There is a linear proportional relationship between I ₁ and V _in . I ₁ changes stably and linearly, and I _f also changes stably and linearly.

Because D3 is illuminated by D1, I ₂ also changes steadily and linearly. Amplifier A2 and resistor R ₂ turn I ₂

Formation voltage V _Out =I ₂ ×R ₂ .

4 Selection of operational amplifier
    HCNR200/201 is a current drive device, and its LED operating current is 1mA ~ 20mA.

Therefore, the driving current of operational amplifier A1 must also reach 20mA to achieve this output current.

The output stage of operational amplifiers with high performance is generally bipolar. Therefore, it is more convenient to choose a bipolar operational amplifier.

suitable. At the same time, depending on the input voltage range, the operational amplifier is also required to have corresponding common mode input and

output capability. This design circuit uses the HA17324 integrated operational amplifier powered by a single power supply.

The output current can reach 40mA.

5 Resistor Selection
    The selection of resistors in photoconductive mode is discussed below.
    The equivalent circuit of the driver stage composed of A1 is shown in Figure 4. In the figure, R _f is the equivalent feedback resistor. Should

The equivalent circuit is a typical non-inverting amplifier, so U ₊ =U _– , and U ₊ =V _in , so V _in =U _– .

    It is obvious from Figure 2 that 
  
In the formula, VD1 is the forward voltage drop of D1.
As can be seen from Figure 4,  
 
Therefore, substitute equation (3) into equation (4)  
   
    Due to the discrete nature of device parameters, I ₁ is approximately equal to 0.005I _f , K ₃ =I ₂ /I ₁ ≈1, so,

R ₁ , R ₂ , and R ₃ 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 optocoupler has good linearity and simple circuit.

Effectively solves the problem of electrical isolation between analog signals and microcontroller application systems. If the driver level,

The buffer stage uses a combined operational amplifier to improve linearity.
    HCNR200 can be widely used in analog signals that require good stability, linearity and bandwidth.

Number of isolation occasions. Using two HCNR200 chips, it can work in bipolar input/bipolar output mode;

At the same time, it can also work in AC and DC circuits, converter isolation, thermocouple isolation,

4mA～20mA analog current loop transmit/receive and other modes, can be widely used in digital communications,

Voltage and current detection, switching power supply , measurement and testing industrial process control, etc.
    When this device is used for motor current measurement, the current feedback is accurate and reliable, and the current closed loop is realized.

played a role in control.

references

[1] Wuhan Liyuan Electronics Co., Ltd. HCNR200 Precision Linear Optocoupler Data Manual [Z], 1999.
[2] Tong Shibai . Analog Electronic Technology [M]. Beijing: Higher Education Press , 2001