09-04-2012, 10:23 AM
Opto-isolator
Opto-isolator.docx (Size: 83.81 KB / Downloads: 40)
From Wikipedia, the free encyclopedia
(Redirected from Optocoupler)
This article is about the electronic component. For the optical component, see optical isolator.
Schematic diagram of an opto-isolator showing source of light (LED) on the left, dielectric barrier in the center, and sensor (phototransistor) on the right.[note 1]
In electronics, an opto-isolator, also called an optocoupler, photocoupler, or optical isolator, is "an electronic device designed to transfer electrical signals by utilizing light waves to provide coupling with electrical isolation between its input and output".[1] The main purpose of an opto-isolator is "to prevent high voltages or rapidly changing voltages on one side of the circuit from damaging components or distorting transmissions on the other side."[2] Commercially available opto-isolators withstand input-to-output voltages up to 10 kV[3] and voltage transients with speeds up to 10 kV/μs.[4]
An opto-isolator contains a source (emitter) of light, almost always a near infrared light-emitting diode (LED), that converts electrical input signal into light, a closed optical channel (also called dielectrical channel[5]), and a photosensor, which detects incoming light and either generates electric energy directly, or modulates electric current flowing from an external power supply. The sensor can be a photoresistor, a photodiode, a phototransistor, a silicon-controlled rectifier (SCR) or a triac. Because LEDs can sense light in addition to emitting it, construction of symmetrical, bidirectional opto-isolators is possible. An optocoupled solid state relay contains a photodiode opto-isolator which drives a power switch, usually a complementary pair of MOSFET transistors. A slotted optical switch contains a source of light and a sensor, but its optical channel is open, allowing modulation of light by external objects obstructing the path of light or reflecting light into the sensor.
Photoresistor-based opto-isolators were introduced in the 1960s. They are the slowest, but also the most linear isolators and still retain a niche market in audio and music industry. Commercialization of LED technology in 1968–1970 caused a boom in optoelectronics, and by the end of the 1970s the industry developed all principal types of opto-isolators. The majority of opto-isolators on the market use bipolar silicon phototransistor sensors.[6] They attain medium data transfer speed, sufficient for applications likeelectroencephalography.[7] The fastest opto-isolators use PIN diodes in photoconductive mode and contain electronic circuitry for amplification, shaping and interfacing of the signal detected by the sensor, and can attain data transfer rates of 50 MBd.[8] Their role in computing and communications is being challenged by new integrated isolation devices based on microminiature transformers, capacitive coupling or spin valves.
Contents
[hide]
• 1 Electric isolation
• 2 Types of opto-isolators
o 2.1 Resistive opto-isolators
o 2.2 Photodiode opto-isolators
o 2.3 Phototransistor opto-isolators
o 2.4 Bidirectional opto-isolators
• 3 Alternatives
• 4 Notes
• 5 References
• 6 Sources
• 7 External links
[edit]Electric isolation
Planar (top) and silicone dome (bottom) layouts - cross-section through a standarddual in-line package. Relative sizes of LED (red) and sensor (green) are exaggerated.[note 2]
Electronic equipment and signal and power transmission lines can be subjected to voltage surges induced by lightning, electrostatic discharge, radio frequency transmissions, switching pulses (spikes) and perturbations in power supply.[9] Remote lightning strikes can induce surges up to 10 kV, one thousand times more than the voltage limits of many electronic components.[10] A circuit can also incorporate high voltages by design, in which case it needs safe, reliable means of interfacing its high-voltage components with low-voltage ones.[11]
The main function of an opto-isolator is to block such high voltages and voltage transients, so that a surge in one part of the system will not disrupt or destroy the other parts.[2] Or, according to the authors of The Art of Electronics, "in a nutshell, opto-couplers let you send digital (and sometimes analog) signals between circuits with separate grounds."[12] Historically, this function was delegated to isolation transformers, which use inductive coupling between galvanically isolated input and output sides. Transformers and opto-isolators are the only two classes of electronic devices that offer reinforced protection — they protect both the equipmentand the human user operating this equipment.[13] They contain a single physical isolation barrier, but provide protection equivalent todouble isolation.[13] Safety, testing and approval of opto-couplers are regulated by national and international standards: IEC 60747-5-2, EN (CENELEC) 60747-5-2, UL 1577, CSA Component Acceptance Notice #5, etc.[14] Opto-isolator specifications published by manufacturers always follow at least one of these regulatory frameworks.
An opto-isolator connects input and output sides with a beam of light modulated by input current. It transforms useful input signal into light, sends it across the dielectric channel, captures light on the output side and transforms it back into electric signal. Unlike transformers, which pass energy in both directions[note 3] with very low losses, opto-isolators are unidirectional (see exceptions) and they cannot transmit power.[15] Typical opto-isolators can only modulate the flow of energy already present on the output side.[15]Unlike transformers, opto-isolators can pass DC or slow-moving signals and do not require matching impedances between input and output sides.[note 4] Both transformers and opto-isolators are effective in breaking ground loops, common in industrial and stage equipment, caused by high or noisy return currents in ground wires.[16]
The physical layout of an opto-isolator depends primarily on the desired isolation voltage. Devices rated for less than a few kV have planar (or sandwich) construction.[17] The sensor die is mounted directly on the lead frame of its package (usually, a six-pin or a four-pin dual in-line package).[5] The sensor is covered with a sheet of glass or clear plastic, which is topped with the LED die.[5] The LED beam fires downward. To minimize losses of light, the useful absorption spectrum of the sensor must match the output spectrum of the LED, which almost invariably lies in the near infrared.[18] The optical channel is made as thin as possible for a desired breakdown voltage.[17] For example, to be rated for short-term voltages of 3.75 kV and transients of 1 kV/μs, the clear polyimide sheet in the Avago ASSR-300 series is only 0.08 mm thick.[19] Breakdown voltages of planar assemblies depend on the thickness of the transparent sheet[17] and the configuration of bonding wires that connect the dies with external pins.[5] Real in-circuit isolation voltage is further reduced by creepage over the PCB and the surface of the package. Safe design rules require a minimal clearance of 25 mm/kV for bare metal conductors or 8.3 mm/kV for coated conductors.[20]
Opto-isolators rated for 2.5 to 6 kV employ a different layout called silicone (sic) dome.[21] Here, the LED and sensor dies are placed on the opposite sides of the package; the LED fires into the sensor horizontally.[21] The LED, the sensor and the gap between them are encapsulated in a blob, or dome, of transparent silicone. The dome acts as areflector, retaining all stray light and reflecting it onto the surface of the sensor, minimizing losses in a relatively long optical channel.[21] In double mold designs the space between the silicone blob ("inner mold") and the outer shell ("outer mold") is filled with dark dielectric compound with a matched coefficient of thermal expansion.[22]