Electronic components, electrical signals, and power lines are all subject to voltages generated by lightning, electrostatic discharge, electromagnetic interference, switching pulses, and power disturbances. Distant lightning can sense voltages as large as 10 kV, which is 1,000 times more than many electronic components. Circuits can also be designed to connect to high voltages, which requires safe and reliable devices to connect high and low voltage components.
One of the main functions of optocouplers is to block high voltages as well as voltage transients from affecting the rest of the circuit. In the past, this type of function would be performed by an isolation transformer, which uses inductive coupling to transmit signals from the galvanically isolated input and output. Voltages and optocouplers are the only two features that provide enhanced protection and protect both the equipment and the people who use it. These devices have only a single layer of physical isolation, but provide protection equivalent to that of Class II appliances with double isolation in the IEC electrical rating. The safety, testing and acceptance of optocoupler components are regulated by national and international standards: IEC60747-5-2, European Electrotechnical Standards 60747-5-2, UL1577 and CSAComponent Acceptance Notice#5. Manufacturer-issued specifications for optocoupler components must comply with at least one of these regulatory standards.
The optocoupler connects the input and output terminals through a beam modulated by the input signal, converting the useful input information into light, which passes through the dielectric medium to the receiver and then converts it into an electrical signal. The essence of a transformer is that it can transfer energy in both directions and has a high conversion efficiency, while optocouplers are different from transformers, which mostly only allow one-way signal conversion (although there are exceptions, refer to bidirectional optocouplers) and cannot convert power. Generally, the optocoupler does not convert energy, but can only provide a signal, and then output the energy on the output side after the signal is modulated. The optocoupler can convert DC or slow-changing signals without the need for impedance matching on the input and output sides. Transformers and optocouplers can destroy undesirable ground loops, which are common in industry and electrical equipment, and can cause high currents or noisy currents due to ground wires.
The physical configuration of the optocoupler is related to the specification of its isolation voltage. If the withstand voltage is less than a few kV, a flat (or sandwich) structure is generally used. The sensor die is mounted directly on the lead frame of its package (mostly 6-pin or 4-pin dual in-line package). The sensor grain is covered with a layer of glass or transparent plastic. LEDs emit light from above to downwards to reduce light loss, and the absorption spectrum of the sensor needs to match the output spectrum of the LED, mostly in the near-infrared range. The thinner the optical channel will be under the condition that it can withstand the breakdown voltage without destroying it, and the thinner it is, the better. For example, if the voltage is to withstand 3.75 kV for a short period of time and the voltage change rate is 1 kV/μs, the transparent polyimide layer of AvagoASSR-300 is only 0.08 mm thick. The breakdown voltage of a planar component is related to the thickness of the optical channel layer and the configuration of the bond line connecting the die and pins. The actual insulation voltage in the circuit is related to the creepage distance between the printed circuit board and the packaging surface. Safety design guidelines require a minimum distance of 25 mm/kV from bare metal conductors and a minimum distance of 8.3 mm/kV from coated conductors.
In the case of optocouplers with withstand voltages between 2.5 and 6 kV, another architecture called a silicon dome is used. In this architecture, LEDs and sensors are placed on both sides of the package, and LEDs emit light to the sensors on the sides. The LEDs, the sensors, and the space between the two are covered with a transparent silicone dome. The dome acts like a mirror, reflecting all the stray light onto the sensor, reducing the loss caused by longer optical channels. In the case of a dual-mode design, the silicon dome (inner mold) and the outer shell (outer mold) are filled with a dark dielectric material, and there is an appropriate coefficient of thermal expansion.,