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Introduction of Linear Optocoupler
Optical isolation is a very common form of signal isolation. Commonly used optocoupler device and its peripheral PCB circuit composition. Due to the simplicity of the optocoupler PCB circuit, it is often used in digital isolation PCB circuits or data transmission PCB circuits, such as the 20mA current loop of the UART protocol. For the analog signal, the optocoupler has poor input and output linearity and has a large change with temperature, which limits its application in analog signal isolation.
For high frequency AC analog signals, transformer isolation is the most common choice, but it is not suitable for tributary signals. Some manufacturers provide isolation amplifiers as solutions for analog signal isolation. The AC signal is isolated by transformer, and then the frequency-voltage conversion is performed to obtain the isolation effect. The internal PCB circuit of the integrated isolation amplifier is complex, large in size, and high in cost, which is not suitable for large-scale applications.
A better choice for analog signal isolation is to use linear optocouplers. The isolation principle of linear optocoupler is no different from that of ordinary optocoupler, except that the single-shot and single-receive mode of ordinary optocoupler is slightly changed, and a light-receiving PCB circuit for feedback is added for feedback. In this way, although the two light-receiving PCB circuits are non-linear, the non-linear characteristics of the two light-receiving PCB circuits are the same. In this way, the non-linearity of the through path can be offset by the non-linearity of the feedback path, so as to achieve To achieve the purpose of linear isolation.
There are several optional chips for linear optocouplers on the market, such as Agilent’s HCNR200/201, TI’s subsidiary TOAS’s TIL300, CLARE’s LOC111 and so on. Here we take HCNR200/201 as an example to introduce
2. Chip introduction and principle explanation
The internal block diagram of HCNR200/201, among them 1, 2 is used as the input of the isolation signal, 3, 4 pins are used for feedback, 5, 6 pins are used for output. The current between pins 1 and 2 is recorded as IF, and the current between pins 3 and 4 and between pins 5 and 6 are recorded as IPD1 and IPD2, respectively. The input signal undergoes a voltage-current conversion, and the voltage change is reflected in the current IF. IPD1 and IPD2 are basically linear with IF, and the linear coefficients are denoted as K1 and K2, respectively.
K1 and K2 are generally very small (HCNR200 is 0.50%), and vary greatly with temperature (HCNR200 varies from 0.25% to 0.75%), but the chip design makes K1 and K2 equal. As you can see later, in a reasonable peripheral PCB circuit design, what really affects the output/input ratio is the ratio K3 of the two. Linear optocouplers are using this characteristic to achieve satisfactory linearity.
The internal structure of HCNR200 and HCNR201 is exactly the same, the difference lies in some indicators. Compared with HCNR200, HCNR201 provides higher linearity.
Some indicators of isolation using 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: 1414V;
* Signal bandwidth: DC to greater than 1MHz.
It can be seen from the above that, like ordinary optocouplers, linear optocouplers really isolate the current. If you want to really isolate the voltage, you need to add auxiliary PCB circuits such as operational amplifiers at the output and output. The following analyzes the typical PCB circuit of HCNR200/201, and derives and explains how to realize feedback and current-voltage and voltage-current conversion in the PCB circuit.
3. Auxiliary PCB circuit and parameter determination
The above derivation assumes that all PCB circuits work in the linear range. To do this, you need to select the op amp reasonably and determine the resistance of the resistor.
3.1 Operational amplifier selection
The op amp can be powered by a single power supply or a positive and negative power supply. The example given above is a single power supply. In order to enable the input range to be from 0 to VCC, the op amp needs to be capable of full swing operation. In addition, the operating speed and slew rate of the op amp will not affect the performance of the entire PCB circuit. TI's LMV321 single op amp PCB circuit can meet the above requirements and can be used as the peripheral PCB circuit of HCNR200/201.
3.2 Determination of resistance
The selection of resistance needs to consider the linear range of the operational amplifier and the maximum operating current IFmax of the linear optocoupler. When K1 is known, IFmax determines the maximum value of IPD1, IPD1max. In this way, since the range of Vo can be at least 0, in this way, because IFmax is considered to be beneficial to energy transmission, it is generally taken in addition, because the work is in The op amp in the deep negative feedback state satisfies the virtual short characteristic. Therefore, considering the limitation of IPD1, the determination of R2 can be determined according to the required magnification. For example, if the method is not needed, just set R2=R1.
In addition, because the optocoupler will generate some high-frequency noise, a capacitor is usually connected in parallel at R2 to form a low-pass filter. The value of the specific capacitor is determined by the input frequency and the noise frequency.
3.3 Examples of parameter determination
Assuming that Vcc=5V, the input is between 0-4V, and the output is equal to the input, using the LMV321 op amp chip and the PCB circuit above, the process of parameter determination is given below.
* Determine IFmax: about 25mA recommended in the manual of HCNR200/201;
* Determine R3: R3=5V/25mA=200;
* Determine R1:;
* Determine R2: R2=R1=32K.
4. Summary
This article gives a brief introduction to the linear optocoupler as well as the precautions and reference design in the use of PCB circuit design, parameter selection, etc., and the corresponding derivation and explanation of the PCB circuit design method for the reference of the majority of electronic engineers.
The above is an introduction to the principle of linear optocoupler and PCB circuit design. Ipcb is also provided to PCB manufacturers and PCB manufacturing technology.
