An optical coupler is also known as an optoelectronic isolator, optocoupler or 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 is powered, the light emitting diode emits light, and the phototransistor generates light current after receiving the light, and flows out from the output end, thereby realizing "electrical-optical-electrical" conversion. A typical application circuit is shown in Figure 1-1. The main advantages of optocoupler are: signal one-way transmission, the input end and the output end fully realize the complete electrical isolation between the front end and the load, the output signal has no influence on the input end, reduce the circuit interference, simplify the circuit design, work stably, no touch Point, long service life and high transmission efficiency. Optocouplers are new devices developed in the 1970s and are now widely used in electrical insulation, level shifting, interstage coupling, drive circuits, switching circuits, choppers, multivibrators, signal isolation, interstage isolation, Pulse amplifier circuit, digital instrumentation, long-distance signal transmission, pulse amplification, solid state relay (SSR), instrumentation, communication equipment and computer interface. 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. 2 optocoupler typical circuit The optocoupler models commonly used for feedback are TLP521, PC817, etc. Here, the TLP521 is taken as an example to introduce the characteristics of such an optocoupler. Figure 2-1 shows the internal structure of the optocoupler and the pin diagram. The primary side of the TLP521 is equivalent to a light-emitting diode. The larger the primary current If, the stronger the light intensity, and the larger the current Ic of the secondary transistor. The ratio of the secondary triode current Ic to the primary diode current If is called the current amplification factor of the optocoupler, which varies with temperature and is greatly affected by temperature. The optocoupler used for feedback uses the "primary current change will cause the secondary side current to change" to achieve feedback. Therefore, in the case of severe changes in ambient temperature, since the temperature drift of the amplification factor is relatively large, it should be achieved without optocoupler. Feedback. In addition, the use of such optocouplers must pay attention to the design of peripheral parameters, so that it works in a relatively wide linear band, otherwise the circuit is too sensitive to operating parameters, which is not conducive to the stable operation of the circuit. Feedback is usually selected in conjunction with TLP521. At this time, the working principle of TL431 is equivalent to a voltage error amplifier with internal reference of 2.5 V (the output voltage is compared with the error amplification, and then the sampling voltage is controlled by the photoelectric coupling to control the pulse width duty ratio to achieve the purpose of stabilizing the voltage) Therefore, between the 1st and 3rd feet, the compensation network should be connected. The TL431 is a controllable precision regulated source produced by Texas Instruments, as shown in Figure 2-3. Its output voltage can be arbitrarily set to any value from 2.5V to 36V with two resistors. The device's typical dynamic impedance is 0.2Ω, which is used in many applications to replace Zener diodes, such as digital voltmeters, op amp circuits, adjustable voltage supplies, switching power supplies, and more. Figure 2-2 shows the TL431 pinout and connection diagram. The common optocoupler feedback is the first connection. Vo is the output voltage and Vd is the supply voltage of the chip. The com signal is connected to the error amplifier output pin of the chip. Note that the ground on the left is the output voltage ground, and the ground on the right is the power supply voltage of the chip. The two are isolated by optocouplers. The operation of the connection shown in Figure 2-3 is as follows: When the output voltage rises, the voltage of pin 1 of the TL431 (corresponding to the inverting input of the voltage error amplifier) ​​rises, and the 3 pin (corresponds to the output pin of the voltage error amplifier) The voltage drops, the primary current If of the optocoupler TLP521 increases, the output current Ic of the other end of the optocoupler increases, the voltage drop across the resistor R4 increases, the com pin voltage drops, the duty cycle decreases, and the output voltage decreases. Small; conversely, when the output voltage is reduced, the adjustment process is similar. In the form of higher potential than the inverting terminal, one of the characteristics of the op amp is used. When the output current of the op amp is too large (beyond the op amp current output capability), the output voltage of the op amp will decrease, and the output current will be larger. The more you drop. Therefore, in the circuit using this connection method, the two input pins of the error amplifier of the PWM (Pulse Width Modulation) chip must be connected to a fixed potential, and the potential of the same direction terminal must be higher than the potential of the opposite terminal, so that The error amplifier initial output voltage is high. The working principle of the connection shown in Figure 2-3 is: When the output voltage rises, the primary current If increases, and the output current Ic increases. Since Ic has exceeded the current output capability of the voltage error amplifier, the com pin voltage drops. The duty ratio is reduced and the output voltage is decreased; conversely, when the output voltage is decreased, the adjustment process is similar. A common third connection is shown in Figure 2-4. Basically similar to the first one, the difference is that there is an additional resistor R6. The function of this resistor is to inject an additional current into the TL431 to prevent the TL431 from working properly due to the injection current being too small. In fact, if the resistance value R3 is appropriately selected, the resistor R6 can be omitted. The adjustment process is basically the same as the 1 connection. A common fourth connection is shown in Figure 2-4. The connection method is similar to the second connection method. The difference is that a resistor R4 is connected between the com terminal and the photocoupler pin 4, and its function is consistent with R6 in the third connection method, and the working principle is basically the same. 2. The feedback modes 1, 3 are applicable to any duty cycle (the ratio of the on-time to the cycle), and the feedback modes 2 and 4 are more suitable for use in a case where the duty ratio is relatively small. 3, summary The optocoupler of the switching power supply is mainly isolated, providing feedback signals and switching functions. The power supply of the optocoupler in the switching power supply circuit is supplied from the secondary voltage of the high-frequency transformer. When the output voltage is lower than the voltage of the Zener diode, the signal optocoupler is turned on, and the duty ratio is increased to increase the output voltage; Turning off the optocoupler reduces the duty cycle, causing the output voltage to decrease. When the secondary load of the high-frequency transformer is overloaded or the switch circuit is faulty, there is no optocoupler power supply. The optocoupler controls the switch circuit to not start, thus protecting the switch tube from being broken down and burned. The sound quality of small speakers is also good. It does not have the same large speakers and power as HIFI speakers, and its sound quality cannot compete with large speakers due to physical limitations. But for the vast majority of users who are not golden ears, the use of small speakers with tablets and mobile phones is sufficient to meet their hearing needs. Wireless Retro speaker,Waterproof Retro speaker, Portable Vintage speaker,Rechargeable Vintage speaker Shenzhen Focras Technology Co.,Ltd , https://www.focras.com
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