Overview of Linear Optocoupler and Nonlinear Optocoupler Models

What is a linear optocoupler? What does it do? The current transmission characteristic curve of a linear optocoupler is close to a straight line, and its performance is better for small signals, and it can perform isolation control with linear characteristics. Linear optocouplers are commonly used in switching power supplies. If nonlinear optocouplers are used, the oscillation waveform may deteriorate. In severe cases, parasitic oscillation may occur, causing the oscillation frequency of several kilohertz to be modulated by low-frequency oscillations of tens to hundreds of Hz. . The consequence is that it will interfere with the image of color TVs, color displays, VCDs, DCDs, etc. At the same time, the power supply load capacity decreases. If the optocoupler is damaged during the maintenance of switching power supplies such as color TVs and monitors, it must be replaced with a linear optocoupler.

Commonly used 4-pin linear optocouplers (non-feedback linear optocouplers) include PC817A-C, PC111, TLP521, etc.

Commonly used 6-pin linear optocouplers include LP632, TLP532, PC614, PC714, PS2031, etc.

Commonly used nonlinear optocoupler models

4N25 transistor output

4N25MC transistor output

4N26 transistor output

Common optocoupler models

4N27 transistor output

4N28 transistor output

4N29 Darlington output

4N30 Darlington output

4N31 Darlington output

4N32 Darlington output

4N33 Darlington output

4N33MC Darlington output

4N35 Darlington output

4N36 transistor output

4N37 transistor output

4N38 transistor output

4N39 thyristor output

Common high-speed optocoupler models

100Kbit/S：

6N138, 6N139, PS8703

1Mbit/S：

6N135, 6N136, CNW135, CNW136, PS8601, PS8602, PS8701, PS9613, PS9713, CNW4502, HCPL-2503, HCPL-4502, HCPL-2530 (dual), HCPL-2531 (dual)

10Mbit/S：

6N137, PS9614, PS9714, PS9611, PS9715, HCPL-2601, HCPL-2611, HCPL-2630 (dual channel), HCPL-2631 (dual channel)

The principle of linear optocoupler

The isolation principle of a linear optocoupler is no different from that of an ordinary optocoupler. It just slightly changes the single-transmit and single-receive mode of an ordinary optocoupler and adds an optical receiving circuit for feedback. In this way, although both light-receiving circuits are nonlinear, the nonlinear characteristics of the two light-receiving circuits are the same. In this way, the nonlinearity of the feedthrough path can be offset by the nonlinearity of the feedback path, thereby achieving the purpose of linear isolation.

Classification of linear optocouplers

Linear optocoupler devices are divided into two types: feedback-free type and feedback type;

1. Non-feedback linear optocoupler devices actually take certain measures in the material and production process of the device (so that the nonlinearity of the input and output characteristics of the optocoupler device is improved. However, due to the inherent characteristics of light-emitting diodes and phototransistors, The improvement is very limited. This kind of optocoupler device is mainly used in situations where the range of the linear region is not large, such as linear optocouplers such as PC816A and NEC2501H that are often used in the voltage isolation feedback circuit of switching power supplies. Since the switching power supply is in normal operation The voltage adjustment rate is not large. By appropriately selecting the parameters of the feedback circuit, the optocoupler device can be made to work in the linear region. However, since this optocoupler device only has high linearity within a limited range, it is not suitable for use in Occasions with high requirements on test accuracy and range.

2. Another type of linear optocoupler is a feedback device. Its working principle is to slightly change the single-transmit and single-receive mode of the ordinary optocoupler, and add an optical receiving circuit for feedback for feedback. In this way, although both light-receiving circuits are nonlinear, the nonlinear characteristics of the two light-receiving circuits are the same. In this way, the nonlinearity of the feedthrough path can be offset by the nonlinearity of the feedback path, thereby achieving linear isolation. the goal of. The principle of linearization of ordinary optocoupler devices introduced earlier is similar, except that it takes certain measures in the production process to make the characteristics of two optocouplers in the same device more consistent. Such devices include the TIL300A produced by Texas Instruments and now discontinued, the LOC series linear optocoupler produced by CLARE, and the HCNR200/201 linear optocoupler produced by Hewlett-Packard.

Analog signal linear isolation circuit constructed with nonlinear optocoupler

When using a nonlinear optocoupler to replace a linear optocoupler, the first question that needs to be considered is whether to use two independent single-channel optocouplers or a dual-channel optocoupler. Due to the derivation in Formula 3 above, the default K and K2 of the linear optocoupler are equal. , so that the physical characteristics of the two optocouplers we choose should be consistent. The two optocouplers packaged together have better consistent characteristics than two independent single-channel optocouplers, so a dual optocoupler is selected.

Secondly, since the signal has been isolated, the power supply of the integrated circuit before and after isolation must be isolated, otherwise complete isolation cannot be truly achieved. Of course, the isolation circuit made of nonlinear optocoupler is not as good as the linear optocoupler when laying out the printed board, because two sets of isolated power supplies are added to pins 5, 6 and 7, 8 on the side of the nonlinear optocoupler, as shown in Figure 3 ). Printed boards made of linear optocouplers can completely lay out the isolated power supply on both sides of the optoelectronic device. Based on the parameters of the linear optocoupler, we selected TPP521-2 after comparison. The isolation circuit constructed based on this optocoupler is as follows:

The sampling isolation circuit mainly consists of a dual-channel nonlinear optocoupler, two operational amplifiers and resistors and capacitors. Pin 7 of one optocoupler is used as output, and pin 5 of the other optocoupler is used as feedback. The feedback is used to compensate for the time of the light-emitting diode. The nonlinearity of the temperature characteristics ensures that the output signal generated by the phototransistor is linearly proportional to II) the luminous flux emitted by the light-emitting diode.

In the isolation circuit, IR adjusts the input bias current of the input operational amplifier. C plays a feedback role. At the same time, the glitch signals in the circuit are filtered out. Avoid unexpected impact on the light-emitting diode (U.ED). However, as the frequency increases, the impedance of the light-emitting diode will become smaller, the current will increase, and the gain will increase. Therefore, the introduction of C will have a certain impact on the gain of the channel at high frequencies. , although reducing the value of C can expand the bandwidth. However, it will affect the gain of the primary operational amplifier, and the large glitch signal output by the primary operational amplifier is not easy to be filtered. But for our current analog signal sampling frequency is not high , a capacitor of 0.47pk is enough.

During the adjustment process of the sampling circuit, the input voltage has two changing trends. When the input voltage Vin increases, the product of Vin is greater than B and the current flowing through it causes the voltage at the output terminal of the op amp to increase. Through the two light-emitting diodes, The current also increases, and the current of pin 6.5 of the phototransistor also increases, so the current fed back to 1R also increases. The final adjustment result is that the voltages at the + and - terminals of the input op amp are equal, and at the same time, the current at pin 8.7 is also increased. increases, the voltage across the sampling resistor I increases linearly.

On the contrary, when the input voltage Vim decreases, the voltage at the output terminal of the op amp decreases, and the current through the light-emitting diode also decreases. Similarly, the output voltage also decreases in proportion to the decrease in the input voltage Vin. The above derivation assumes that all circuits operate within a linear and ideal range. To achieve this, you need to make a reasonable selection of the op amp and carefully select the resistance value of the resistor:

The op amp can be powered by a single power supply or a positive and negative power supply. The example given above is an example of a single positive power supply. In order to enable the input range from 0 to Voc, the op amp needs to be able to operate at full rail. In addition, the operating speed slew rate of the op amp will not affect the performance of the entire circuit. Since the optocoupler is a current-driven device and its 11:1) working current is ImA-20mA, therefore, the driving current of the operational amplifier must also reach 20mA. The current driving capability of the operational amplifier IM 358 we selected reaches 40mA4. The above is the relevant analysis of linear optocoupler, I hope it can help you.