An optical coupler (OC) is also known as an opto-isolator or optocoupler, referred to as an optocoupler. It is a device that transmits electrical signals by means of light. Generally, an illuminator (infrared light emitting diode (LED)) and a light receiver (photosensitive semiconductor tube) are packaged in the same package. When the input terminal energizes the signal, the illuminator emits light, and after receiving the light, the photoreceptor generates a photocurrent, which flows out from the output end, thereby realizing the "electric-optical-electrical" conversion. A common optocoupler can only transmit digital (switching) signals and is not suitable for transmitting analog signals. Linear optocouplers, which have been introduced in recent years, are capable of transmitting continuously varying analog or analog current signals, broadening their application areas.
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1 Type and performance characteristics of optocoupler
1.1 Types of Optocouplers  Optocouplers are available in a variety of packages, such as dual in-line, tubular, and optical fiber. The classification and internal circuit of the optocoupler are shown in Figure 1. The figure shows the models of eight typical products: (a) general purpose (without base lead); (b) general purpose (with base lead); (c) Darlington type; (d) high speed type; Optical integrated circuit; (f) optical fiber type; (g) photosensitive thyristor type; (h) photosensitive field effect tube type.
1.2 Performance characteristics of optical coupler ä¸»è¦ The main advantage of optical coupler is that it transmits signals in one direction, and the input and output terminals are completely electrically isolated, with strong anti-interference ability, long service life and high transmission efficiency. It is widely used in level shifting, signal isolation, interstage isolation, switching circuits, long-distance signal transmission, pulse amplification, solid state relays (SSR), instrumentation, communication equipment and computer interfaces. In the monolithic switching power supply, the optocoupler feedback circuit can be constructed by using a linear optical coupler, and the duty ratio is changed by adjusting the current of the control terminal to achieve the purpose of precision voltage regulation.
技术 The technical parameters of the optocoupler are LED forward voltage drop VF, forward current IF, current transfer ratio CTR, insulation resistance between input stage and output stage, and collector-emitter reverse breakdown voltage V ( BR) CEO, collector-emitter saturation voltage drop VCE(sat). In addition, parameters such as rise time, fall time, delay time, and storage time need to be considered when transmitting digital signals. 
The current transfer ratio is an important parameter of the optocoupler and is usually expressed by the DC current transfer ratio. When the output voltage remains constant, it is equal to the percentage of the DC output current IC to the DC input current IF. Its formula is:
An optocoupler using a phototransistor, the CTR range is mostly 20% to 300% (such as 4N35), while the PC817 is 80% to 160%, and the Darlington optocoupler (such as 4N30) can reach 100%. ~5000%. This indicates that the same output current is required, which requires a smaller input current. Therefore, the CTR parameters have some similarities to the hFE of the transistor. Typical CTR-IF characteristics of linear optocouplers and common optocouplers are shown by dashed and solid lines in Figure 2, respectively.
It can be seen from Fig. 2 that the CTR-IF characteristic curve of the ordinary optical coupler is nonlinear, and the nonlinear distortion is particularly serious when the IF is small, so it is not suitable for transmitting an analog signal. The CTR-IF characteristic curve of the linear optical coupler has good linearity, especially when transmitting small signals, its AC current transmission ratio (ΔCTR=ΔIC/ΔIF) is very close to the DC current transmission ratio CTR value. Therefore, it is suitable for transmitting analog voltage or current signals, enabling a linear relationship between output and input. This is an important feature.
2 Product classification and selection principles of linear optocouplers
2.1 Product Classification of Linear Optocouplers 典型 Typical products and main parameters of linear optocouplers are shown in Table 1. These optocouplers use phototransistors as receiving tubes.
2.2 Selection principle of linear optocoupler åž‹å· When designing optocoupler feedback switching power supply, the model and parameters of linear optocoupler must be correctly selected. The selection principle is as follows:
å…许 1 The optocoupler's current transfer ratio (CTR) is allowed to range from 50% to 200%. This is because when CTR<50%, the LED in the optocoupler needs a large operating current (IF> 5.0mA) to properly control the duty cycle of the monolithic switching power supply IC, which will increase the work of the optocoupler. Consumption. If CTR>200%, when the circuit is started or when the load is abrupt, it is possible to falsely trigger the single-chip switching power supply, which affects the normal output.
 2 It is recommended to use a linear optocoupler, which is characterized by a linear adjustment of the CTR value within a certain range.
 3 4N×× series (such as 4N25, 4N26, 4N35) optical couplers produced by Isocom and Motorola, USA, are widely used in China. In view of the switching characteristics of such optocouplers, the linearity is poor, and it is suitable for transmitting digital signals (high and low levels), so it is not recommended for use in switching power supplies.
3 Linear Optocoupler Application Example The multi-output power converter circuit is shown in Figure 3. The input voltage is a quasi-square wave voltage of 36V to 90V. The three outputs are: UO1=+5V(2A), UO2=+15V (0.17A), UO3=-15V(0.17A). Now UO1 is set as the main output, its voltage regulation rate is SV=±0.4%; UO2 and UO3 are auxiliary outputs, and the total power efficiency can reach 75%~80%. A TOP104Y three-terminal monolithic switching power supply integrated circuit is used in the circuit. After the main output winding voltage is rectified and filtered by VD2, C2, L1 and C3, a voltage of +5V is obtained. VD2 uses MBR735 type 35V/7.5A Schottky diode. The two auxiliary output windings and the output circuit are completely symmetrical. Because the ±15V output current is small, the rectifiers VD4 and VD5 use UF4002 type 100V/1A ultra-fast recovery diodes. The optocoupler feedback precision switching power supply is composed of a linear optocoupler CNY17-2 and an adjustable precision shunt regulator TL431C, which can precisely adjust the +5V voltage. The feedback winding voltage is rectified and filtered by VD3 and C4 to obtain a 12V feedback voltage. The drain clamp protection circuit consisting of P6KE120 transient voltage suppressor and UF4002 ultra-fast recovery diode can absorb the spike voltage formed by the high-frequency transformer leakage inductance and protect the power MOSFET inside the chip from damage.
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