Optocoupler, also known as optocoupler or optoisolator, is a semiconductor device that transmits electrical signals through optical signals. Its core function is to achieve electrical isolation between input and output circuits, while also completing signal transmission. It is widely used in industrial control, communication equipment, medical equipment, and other fields.
1、 Definition and core functions
Optocouplers are composed of light-emitting diodes (LEDs) and phototransistors (or photoresponsive thyristors, photoresponsive integrated circuits, etc.), packaged in an opaque housing. When an electrical signal is applied to the input terminal, the LED emits light, and the photosensitive device receives the light signal and converts it into an electrical signal output, thereby achieving electrical isolation between the input and output circuits. This isolation can effectively prevent the transmission of interference signals such as high voltage, noise, and ground current between circuits, and protect low-voltage circuits from damage.
2、 Classification and Application Scenarios
Transistor output optocoupler: The output terminal is a photosensitive transistor, suitable for transmitting switch signals, such as PLC input/output modules, home appliance control boards, etc.
Controllable silicon output optocoupler: The output terminal is a bidirectional controllable silicon, which can directly drive AC loads, such as motor speed regulation, lighting control, etc.
IGBT output optocoupler: Combining optocoupler with IGBT (Insulated Gate Bipolar Transistor), suitable for high voltage and high current drive, such as inverters, electric vehicle charging stations, etc.
High speed optocoupler: with a response time of less than 1 microsecond, used for digital signal transmission, such as communication interfaces, high-speed AD/DA conversion, etc.
3、 Key parameters and technical indicators
Input current (IF): The current required to make the LED light up normally, with a typical value of 5-20mA.
Output current (IC): The maximum current that a photosensitive transistor can output determines the load driving capability.
Isolation voltage (VISO): The maximum voltage that the input and output terminals can withstand, and industrial grade optocouplers can reach over 5000Vrms.
Response time (tPHL/tPLH): The delay time of an optical signal from input to output, which can be as low as 50ns for high-speed optocouplers.
Current Transfer Ratio (CTR): The ratio of output current to input current, reflecting signal amplification capability, typically ranging from 50% to 600%.
Working temperature range: Industrial grade optocouplers support -40 ℃ to 125 ℃, meeting the requirements of harsh environments.
4、 Working principle and technical implementation
The working principle of optocouplers is based on the photoelectric effect:
Signal input: When a voltage is applied to the input terminal, the LED emits light, and the light intensity is proportional to the input current.
Optical signal transmission: Light is transmitted to photosensitive devices through transparent media such as air or transparent resin.
Electrical signal output: After receiving the light signal, the photosensitive transistor conducts, and the output current is proportional to the light intensity, completing the electrical optical electrical conversion.
5、 Development Trends and Challenges
High speed: With the development of 5G and data centers, the response time of optocouplers needs to be shortened to nanoseconds.
Miniaturization: Adopting SOP, QFN and other packaging, the size is reduced to below 3mm × 2mm to meet the needs of portable devices.
Integration: Integrating optocouplers with driver circuits and protection circuits to form a modular solution.
High reliability: Passed AEC-Q101 automotive certification, meeting high reliability requirements in automotive electronics, aerospace, and other fields.
Optocouplers, as the "security guards" of electronic systems, achieve secure signal transmission and circuit protection through optoelectronic isolation technology. With the continuous advancement of technology, optocouplers will play a more important role in high-speed communication, new energy, intelligent manufacturing and other fields.