Principle of Linear Photocoupler and Circuit Design

1. Introduction of linear optocoupler

Optical isolation is a very common form of signal isolation. Common optical coupling devices and their peripheral circuits are composed. Because of the simplicity of the optical coupling circuit, it is often used in digital isolation circuit or data transmission circuit, such as 20mA current loop of UART protocol. For analog signals, the application of optocoupler in analog signal isolation is limited because of poor input and output linearity and large temperature variation.

Transformer isolation is the most common choice for high frequency AC analog signals, but not for branch signals. Some manufacturers offer isolation amplifiers as solutions to analog signal isolation, such as ADI AD202, which can provide 0.025% linearity in frequencies ranging from DC to a few Ks, but this isolation device internally performs voltage-frequency conversion, isolates the resulting AC signal by transformer, and then performs frequency-voltage conversion to achieve isolation effect. The integrated isolation amplifier is not suitable for large-scale applications because of its complex internal circuit, large size and high cost.

A better choice for analog signal isolation is to use linear optocouplers. The isolation principle of linear optocoupler is not different from that of ordinary optocoupler, except that the single receiving mode of ordinary optocoupler is slightly changed, and an optical receiving circuit for feedback is added. Thus, although both optical acceptance circuits are non-linear, the non-linear characteristics of the two optical acceptance circuits are the same, so that the non-linear of the direct path can be offset by the non-linear of the feedback path, thus achieving the purpose of linear isolation.

Linear optical couplers in the market have several chips to choose from, such as Agilent's HCNR200/201, TIL300 of TI subsidiary TOAS, LOC 111 of CLARE, etc. HCNR200/201 is introduced here as an example

2. Chip Introduction and Principle Explanation

The internal diagram of HCNR200/201 is shown below

Among them, 1 and 2 pins are used as input of isolation signal, 3 and 4 pins are used for feedback, and 5 and 6 pins are used for output. Currents between pins 1 and 2 are counted as IF, and between pins 3 and 4 and between pins 5 and 6 are counted as IPD 1 and IPD2, respectively. The input signal undergoes voltage-current conversion, and the voltage changes are reflected in the current IF. IPD1 and IPD2 are basically linear with IF, and the linear coefficients are recorded as K1 and K2, respectively.

K1 and K2 are generally small (HCNR200 is 0.50%) and vary widely with temperature (HCNR200 ranges from 0.25% to 0.75%), but the chip design makes K1 and K2 equal. It can be seen later that in a reasonable peripheral circuit design, the ratio K3 of the two really affects the output/input ratio, which is used by linear photocouple to achieve satisfactory linearity.

The internal structure of HCNR200 and HCNR201 are identical, the difference is in some indicators. HCNR201 provides higher linearity than HCNR200.

Some indicators for isolation with HCNR200/201 are as follows:

* Linearity: HCNR200:0.25%, HCNR201:0.05%;

*Linear coefficient K3:HCNR200:15%, HCNR201:5%;

* Temperature coefficient: -65ppm/oC;

*Isolation voltage: 141414V;

*Signal bandwidth: DC to greater than 1MHz.

It can be seen from the above that, like ordinary photocouples, linear photocouples really isolate the current. To truly isolate the voltage, additional auxiliary circuits such as Operational Amplifiers need to be added at the output and output. The typical circuit of HCNR200/201 is analyzed below, and the feedback and current-voltage, voltage-current conversion in the circuit are deduced and explained.

3. Typical circuit analysis

The Manual of Agilent's HCNR200/201 gives several useful circuits, one of which is more typical as shown in the following figure:

Figure 2

Set the input voltage to Vin and the output voltage to Vout. The two current transfer coefficients guaranteed by the photocoupler are K1 and K2. Obviously, the relationship between and depends on the relationship between and.

Look at the circuit of the front stage amplifier as shown in the following figure:

Set the voltage at the negative end of the op-amp and the voltage at the output end of the op-amp to satisfy the following relationship when the operation is unsaturated:

Vo=Voo-GVi (1)

Among them, the output voltage is when the input difference mode of the operational amplifier is 0, and G is the increase of the operational amplifier, which is generally larger.

Ignoring the input current at the negative end of the amplifier, the current through R1 can be regarded as IP1, according to Ohm's Law of R1:

The current through the R3 ends is IF, according to Ohm's Law:

Among them, the voltage of the two legs of the photocoupler is treated as a constant, considering that the voltage () of the LED is basically unchanged when it is turned on.

Depending on the characteristics of the photocouple, i.e.

K1=IP1/IF(4)

By substituting the expression for sum into the upper expression, you can get:


The above formula is deformed to obtain:

Replace the expression with (3):

Considering that G is particularly large, the following approximations can be made:

In this way, the relationship between output and input voltage is as follows:

It can be seen that in the above circuit, the output is proportional to the input, and the scale factor is only determined by K3 and R1, R2. In general, R1=R2 is selected to isolate without amplification.

4. Assisted circuit and parameter determination

The above deductions assume that all circuits operate within the linear range. To do this, a reasonable selection of operational amplifiers and determination of resistance values are required.

4.1 Operating Amplifier Selection

Operating amplifiers can be powered by either a single power supply or a positive or negative power supply. The example given above is powered by a single power supply. In order to make the input range from 0 to VCC, the operational amplifier needs to work in full swing. In addition, the operational speed and swing rate of the operational amplifier will not affect the performance of the entire circuit. TI's LMV321 single-transport amplifier circuit can meet the above requirements and can be used as the peripheral circuit of HCNR200/201.

4.2 Determination of resistance value

The selection of resistance needs to take into account the linear range of the operational amplifier and the working current IFmax of the linear photocouple. When K1 is known, IFmax also determines the value of IPD1, IPD1max, so that the range of Vo can be as small as 0, so that, because

Considering that IFmax is large enough to facilitate the transfer of energy, this generally takes

In addition, since the operational amplifier working in deep negative feedback satisfies the virtual-short characteristic, the limitation of IPD1 is considered.

In this way,

The determination of R2 can be determined by the desired magnification, e.g. if no method is required, just R2=R1.

In addition, due to the high frequency noise produced by the photocouple, a low-pass filter is usually formed by shunting capacitors at R2. The specific capacitance value is determined by the input frequency and the noise frequency.

4.3 Parameter determination example

Assuming that the Vcc = 5V is determined, the input is between 0 and 4V, and the output is equal to the input, the LMV321 op-amp chip and the above circuit are used. The process of determining the parameters is given below.

*Determine the recommended device operating 25mA in the manual for IFmax:HCNR200/201;

*Determine R3:R3=5V/25mA=200;

*Determine R1:;

*Determine R2:R2=R1=32K.

5. Summary

This paper gives a brief introduction of linear optocoupler, considerations and reference designs in circuit design and parameter selection, and gives the corresponding deduction and explanation of circuit design method for the reference of general electronic engineers.