Optocouplers and their application
The idea of creating and using optocouplers dates back to 1955, when Loebner EE "Optoelectronic devices network" proposed a series of devices with optical and electrical connections between the elements, which made it possible to amplify and spectrally convert light signals, create devices with two stable states - bistable optocouplers, optoelectronic devices for storing and storing information, logic circuits, shift registers. The term "optocoupler" was also proposed there, formed as an acronym for the English "optical-electronic device".
The optocouplers described in this work, perfectly illustrating the principles, turned out to be unsuitable for industrial implementation, since they were based on an imperfect elementary base - inefficient and inertial powder electroluminescent capacitors (emitter) and photoresistors (receiver). The most important operational characteristics of the devices were imperfect: low-temperature and temporary stability of the parameters, insufficient resistance to mechanical stress. Therefore. at first, the optocoupler remained only an interesting scientific achievement, not finding application in technology.
Only in the mid 60-ies of the development of semiconductor light-emitting diodes and technologically advanced high-performance high-speed silicon photodetectors with p - n junctions (photodiodes and phototransistors) did the elementary base of modern optoelectronic technology begin to be created. By the beginning of the 70s, the production of optocouplers in the leading countries of the world turned into an important and rapidly developing branch of electronic technology, successfully complementing traditional microelectronics.
Optocouplers are such optoelectronic devices in which there is a radiation source and receiver (light emitter and photodetector) with one or another type of optical and electrical communication between them, structurally connected to each other.
The principle of operation of optocouplers of any kind is based on the following. In the emitter, the energy of the electric signal is converted into light, in the photodetector, on the contrary, the light signal causes an electric response.
Only optocouplers, which have direct optical communication from the emitter to the photodetector, are practically widely used and, as a rule, all types of electrical communication between these elements are excluded.
According to the degree of complexity of the structural scheme, two groups of devices are distinguished among optoelectronic devices. An optocoupler (also called "elementary optocoupler") is an optoelectronic semiconductor device consisting of emitting and photodetector elements, between which there is an optical coupling that provides electrical isolation between the input and output. An optoelectronic integrated circuit is a microcircuit consisting of one or more optocouplers and one or more matching or amplifying devices electrically connected to them.
Thus, in an electronic circuit, such a device performs the function of a communication element in which at the same time an electrical (galvanic) isolation of the input and output is carried out.
Distinctive features of optocouplers
The advantages of these devices are based on the general optoelectronic principle of using electrically neutral photons to transfer information. The main ones are as follows:
- the ability to provide perfect electrical (galvanic) isolation between the input and output; for optocouplers, there are no fundamental physical or structural limitations on achieving arbitrarily high isolation voltages and isolation resistances and arbitrarily small passage capacitance;
- the possibility of implementing non-contact optical control of electronic objects and the resulting diversity and flexibility of design solutions for control circuits;
- unidirectional distribution of information through the optical channel, the absence of the receiver's backward reaction to the emitter;
- a wide frequency bandwidth of the optocoupler, the absence of restrictions from the low frequencies (which is typical of pulse transformers); the ability to transmit via an optocoupler circuit, both a pulse signal and a constant component;
- the ability to control the output signal of the optocoupler by affecting (including non-electric) the material of the optical channel and the consequent possibility of creating a variety of sensors, as well as a variety of devices for transmitting information;
- the ability to create functional microelectronic devices with photodetectors, the characteristics of which under lighting change according to a complex given law;
- the immunity of optical communication channels to electromagnetic fields, which in the case of "long" optocouplers (with an extended optical fiber between the emitter and the receiver) determines their immunity from interference and information leakage, and also eliminates mutual interference;
- physical, structural and technological compatibility with other semiconductor and microelectronic devices.
Optocouplers also have certain disadvantages:
- significant power consumption due to the need for double energy conversion (electricity - light - electricity) and low efficiency of these transitions;
- increased sensitivity of parameters and characteristics to the effects of elevated temperature and penetrating nuclear radiation;
- more or less noticeable temporary degradation (deterioration) of parameters;
- a relatively high level of intrinsic noise, due, like the two previous drawbacks, to the physics of LEDs;
- the complexity of the implementation of feedbacks caused by electrical isolation of the input and output circuits;
- structural and technological imperfection associated with the use of hybrid non-planar technology (with the need to combine several - separate crystals from different semiconductors located in different planes in one device).
The listed disadvantages of optocouplers, as materials, technology, and circuitry are improved, are partially eliminated, but, nevertheless, for a long time they will be quite fundamental. However, their advantages are so high that they provide confident non-competitive optrons among other microelectronic devices.
Generalized block diagram
As a communication element, the optocoupler is characterized by a transmission coefficient K i , determined by the ratio of output and input signals, and a maximum information transfer rate F. Practically instead of F, the duration of the rise and fall of the transmitted pulses t nar (cn) or the cutoff frequency are measured. The capabilities of the optocoupler as an element of galvanic isolation are characterized by the maximum voltage and isolation resistance of U dec and R dec and through passage capacitance C dec .
In the structural diagram in Fig. 1 input device is used to optimize the operating mode of the emitter (for example, the displacement of the LED on the linear portion of the watt-ampere characteristic) and the conversion (amplification) of the external signal. The input unit must have a high conversion efficiency, high speed, a wide dynamic range of permissible input currents (for linear systems), a small value of the "threshold" input current, which ensures reliable transmission of information along the circuit.
Fig 1. Generalized block diagram of the optocoupler
The purpose of the optical medium is to transfer the energy of the optical signal from the emitter to the photodetector, as well as in many cases to ensure the mechanical integrity of the structure.
The fundamental possibility of controlling the optical properties of the medium, for example, by using electro-optical or magneto-optical effects, is reflected by the introduction of a control device into the circuit. In this case, we obtain an optocoupler with a controlled optical channel, functionally different from a “conventional” optocoupler: the output signal can be changed as input and control circuit.
In the photodetector, the information signal is “restored” from optical to electric; while striving to have high sensitivity and high speed.
Finally, the output device is designed to convert the signal of the photodetector into a standard form, convenient for influencing the cascades that follow the optocoupler. An almost mandatory function of the output device is signal amplification, since losses after double conversion are very significant. Often the gain function is also performed by the photodetector itself (for example, a phototransistor).
The general structural diagram of Fig. 1 is implemented in each specific device only part of the blocks. In accordance with this, there are three main groups of devices of optic technology; the previously named optocouplers (elementary optocouplers) using the blocks light emitter - optical medium - photodetector; optoelectronic (optronic) microcircuits (optocouplers with the addition of an output, and sometimes an input device); special types of optocouplers - devices that are functionally and structurally significantly different from elementary optocouplers and optoelectronic ICs.
A real optocoupler can be arranged and more complicated than the circuit in Fig. one; each of these blocks may include not one, but several identical or similar to each other elements connected electrically and optically, however this does not significantly change the fundamentals of the physics and electronics of the optocoupler.
As elements of galvanic isolation, optocouplers are used: for communication of equipment blocks, between which there is a significant potential difference; to protect the input circuits of measuring devices from interference and interference, etc.
Another important area of application of optocouplers is optical, non-contact control of high-current and high-voltage circuits. Launch of powerful thyristors, triacs, triacs, control of electromechanical relay devices.
A specific group of control optocouplers is composed of resistor optocouplers designed for low-current switching circuits in complex devices for visual display of information made on electroluminescent (powder) indicators, mnemonic diagrams, and screens.
The creation of "long" optocouplers (devices with an extended flexible fiber optic fiber) has opened up a whole new direction in the use of optocoupler products - communication over short distances.
Various optocouplers (diode, resistor, transistor) are also used in purely radio-technical modulation schemes, automatic gain control, etc. The influence of the optical channel is used here to bring the circuit to the optimal operating mode, for contactless tuning of the mode, etc.
The ability to change the properties of the optical channel under various external influences on it allows you to create a whole series of optocouplers: such are humidity and gas sensors, sensors for the presence of a particular liquid in the volume, sensors for the cleanliness of the surface treatment of an object, its speed, etc.
Quite specific is the use of optocouplers for energy purposes, i.e., the operation of a diode optocoupler in a photofan mode. In this mode, the photodiode generates electric power to the load and the optocoupler is to some extent similar to a low-power secondary power source, completely isolated from the primary circuit.
The creation of optocouplers with photoresistors, whose properties change under illumination according to a given complex law, allows you to simulate mathematical functions, is a step towards the creation of functional optoelectronics.
The versatility of optocouplers as elements of galvanic isolation and contactless control, the diversity and uniqueness of many other functions are the reason that the applications of these devices are computer technology, automation, communications and radio equipment, automated control systems, measuring equipment, control and regulation systems, medical electronics , devices for visual display of information.
Physical fundamentals of optronic technology
Elemental base and device of optocouplers
The elemental basis of optocouplers is photodetectors and emitters, as well as the optical medium between them. All these elements are subject to such general requirements as small dimensions and weight, high durability and reliability, resistance to mechanical and climatic influences, manufacturability, low cost. It is also desirable that the elements pass a fairly wide and lengthy industrial testing.
Functionally (as an element of the circuit) the optocoupler is characterized primarily by the type of photodetector used in it.
The successful use of a photodetector in an optocoupler is determined by the fulfillment of the following basic requirements: the efficiency of converting the energy of radiation quanta into the energy of mobile electric; the presence and effectiveness of internal built-in amplification; high speed; breadth of functionality.
Optocouplers use photodetectors of various structures that are sensitive in the visible and near infrared, since it is in this spectral range that there are intense radiation sources and photodetectors can be operated without cooling.
The most versatile are photodetectors with p - n junctions (diodes, transistors, etc.), in the vast majority of cases they are made on the basis of silicon and the region of their maximum spectral sensitivity is near l = 0.7 ... 0.9 μm .
Numerous requirements are also imposed on optocoupler emitters. The main ones: spectral matching with the selected photodetector; high efficiency of converting electric current energy into radiation energy; primary focus of radiation; high speed; simplicity and convenience of excitation and modulation of radiation.
Several types of emitters are suitable and available for use in optocouplers:
- Miniature incandescent bulbs .
- Neon bulbs that use the glow of an electric discharge of a neon-argon gas mixture.
These types of emitters are characterized by low light output, low resistance to mechanical stress, limited durability, large dimensions, and complete incompatibility with integrated technology. Nevertheless, in certain types of optocouplers they can find application.
- An electroluminescent powder cell uses finely crystalline grains of zinc sulfide (activated by copper, manganese, or other additives) suspended in a polymerized dielectric as a luminous body. When sufficiently high AC voltages are applied, the process of prebreakdown luminescence occurs.
- Thin-film electroluminescent cells . The glow here is due to the excitation of manganese atoms by "hot" electrons.
Both powder and film electroluminescent cells have low efficiency of converting electric energy into light, low durability (especially thin-film), and are difficult to control (for example, the optimal mode for powder phosphors is ~ 220 V at f = 400 ... 800 Hz). The main advantage of these emitters is structural and technological compatibility with photoresistors, the possibility of creating on this basis multifunctional, multi-element optocoupler structures.
The main most universal type of emitter used in optocouplers is a semiconductor injection light-emitting diode - LED. This is due to its following advantages: high value of the efficiency of conversion of electrical energy into optical; narrow emission spectrum (quasimonochromaticity); latitude of the spectral range covered by various LEDs; directivity of radiation; high speed; low values of supply voltages and currents; compatibility with transistors and integrated circuits; simplicity of modulation of radiation power by changing direct current; the ability to work, both in pulsed and continuous mode; linearity of the watt-ampere characteristic in a more or less wide range of input currents; high reliability and durability; small dimensions; technological compatibility with microelectronics products.
The general requirements for the optical immersion medium of the optocoupler are as follows: a high value of the refractive index n by it ; high value of resistivity r them ; high critical field strength E im cr , sufficient heat resistance Dq im slave ; good adhesion with crystals of silicon and gallium arsenide; elasticity (this is necessary, since it is not possible to ensure the matching of the elements of the optocoupler with the coefficients of thermal expansion); mechanical strength, since the immersion medium in the optocoupler performs not only light transmitting, but also structural functions; manufacturability (ease of use, reproducibility of properties, cheapness, etc.).
The main type of immersion medium used in optocouplers is polymer optical adhesives. Typically for them, n im = 1.4 ... 1.6, r im > 10 12 ... 10 14 Ohm cm, E im cr = 80 kV / mm, Dq im slave = - 60 ... 120 C. Glues have good adhesion to silicon and gallium arsenide, combine high mechanical strength and resistance to thermal cycling. Non-hardening petroleum jelly-like and rubber-like optical media are also used.
Physics of energy conversion in a diode optocoupler
Consideration of the processes of energy conversion in an optocoupler requires taking into account the quantum nature of light. It is known that electromagnetic radiation can be represented as a stream of particles - quanta (photons), energy. each of which is determined by the ratio:
E f = hn = hc / n l (2.1)
where h is the Planck constant;
c is the speed of light in vacuum;
n is the refractive index of the semiconductor;
n, l - oscillation frequency and wavelength of optical radiation.
If the density of the flux of quanta (i.e., the number of quanta flying through a unit area per unit time) is equal to N f , then the total specific radiation power will be:
P f = N f * E f (2.2)
and, as can be seen from (2.1), for a given N f it is the larger, the shorter the radiation wavelength. Since, in practice, P f (energy irradiation of the photodetector) is preset, it seems useful the following relation
N f = P f / E f = 5 * 10 15l P f (2.3)
where N f , cm -2 s -1 ; l , microns; P f , mW / cm.
Fig. 2. Energy diagram of a direct-gap semiconductor (using the GaAsP ternary compound as an example)
The mechanism of injection luminescence in an LED consists of three main processes: radiative (and non-radiative) recombination in semiconductors, injection of excess minority charge carriers into the base of the LED, and emission of radiation from the generation region.
The recombination of charge carriers in a semiconductor is determined, first of all, by its band diagram, the presence and nature of impurities and defects, and the degree of disturbance of the equilibrium state. The main materials of optocouplers (GaAs and ternary compounds based on it GaA1As and GaAsP) are direct-gap semiconductors, i.e. to those in which direct optical zone-zone transitions are allowed (Fig. 2). Each act of recombination of a charge carrier according to this scheme is accompanied by radiation of a quantum, the wavelength of which in accordance with the law of conservation of energy is determined by the ratio:
l out [μm] = 1.23 / E f [eB] (2.4)
It should be noted that there are competing nonradiative - recombination mechanisms. Among the most important of them are:
- Recombination in deep centers. An electron can transfer to the valence band not directly, but through certain recombination centers that form the allowed energy levels in the band gap (level E t in Figure 2).
- Auger recombination (or percussion). At very high concentrations of free charge carriers in a semiconductor, the probability of a collision of three bodies increases, the energy of a recombining electron-hole pair in this case is given to the third free carrier in the form of kinetic energy, which it gradually dissipates in collisions with a lattice.
Fig. 3. Electrical (a) and optical (b) LED models. A is the optically "transparent" part of the crystal; B is the active part of the crystal; C is the "opaque" part of the crystal; D - ohmic contacts; E is the space charge region
The relative role of various recombination mechanisms is described by introducing the concept of the internal quantum yield of radiation h int , which is determined by the ratio of the probability of radiative recombination to the total (radiative and nonradiative) probability of recombination (or, otherwise, by the ratio of the number of generated quanta to the number of minority carriers injected at the same time). The value of h int is the most important characteristic of the material used in the LED; obviously 0 h int one hundred%.
The creation of an excess concentration of free carriers in the active (emitting) region of the LED crystal is accomplished by injecting them with a pn junction shifted in the forward direction.
A “useful” component current supporting radiative recombination in the active region of the diode is the electron current I n (Fig. 3a) injected by the pn junction. The "useless" direct current components include:
- The hole component I p , due to the injection of holes into the n-region and reflecting the fact that there are no p - n junctions with one-sided injection, the fraction of this current is the smaller the stronger the doped n-region compared to the p-region.
- Recombination current (nonradiative) in the space charge region of the p - n junction of I rivers . In semiconductors with a large band gap at small forward biases, the fraction of this current can be noticeable.
- Tunnel current I tun due to the "leakage" of charge carriers through a potential barrier. The current is transferred by the main carriers and does not contribute to radiative recombination. The tunneling current is greater the narrower the p - n junction; it is noticeable with a strong degree of doping of the base region and with large forward biases.
- The surface leakage current I p , due to the difference in the properties of the surface of the semiconductor from the properties of the volume and the presence of certain shorting inclusions.
The efficiency of the p - n junction is characterized by an injection coefficient:
Obviously, the limits of a possible change in g are the same as for h int , i.e., 0 g one hundred%.
When radiation is removed from the generation region, the following types of energy losses occur (Fig. 3b):
- Losses on self-absorption (rays 1). If the wavelength of the generated quanta exactly corresponds to formula (2.4), then it coincides with the “red boundary” of absorption (see below), and such radiation is quickly absorbed in the bulk of the semiconductor (self-absorption). In fact, radiation in direct-gap semiconductors does not follow the ideal scheme given above. Therefore, the wavelength of the generated quanta is slightly longer than according to (2.4):
- Losses on total internal reflection (rays 2). It is known that when light beams fall on the interface between an optically dense medium (semiconductor) and an optically less dense one (air), for some of these rays the condition for total internal reflection is fulfilled, such rays reflected inside the crystal are ultimately lost due to self-absorption.
- Losses on back and end radiation (beam 3 and 4).
Quantitatively, the efficiency of the output of optical energy from a crystal is characterized by the output coefficient K opt determined by the ratio of the radiation power emerging in the desired direction to the radiation power generated inside the crystal. As for the coefficients h int and g , condition 0 is always satisfied To wholesale one hundred%.
g . An integral indicator of the emissivity of an LED is the magnitude of the external quantum yield h ext . From the foregoing it is clear that h ext = h intg K opt .
Let's move on to the receiving unit. The principle of operation of photodetectors used in optocouplers is based on the internal photoelectric effect, which consists in the separation of electrons from atoms inside the body under the influence of electromagnetic (optical) radiation.
Quantums of light, absorbed in a crystal, can cause the separation of electrons from atoms, both the semiconductor itself and the impurity. In accordance with this, one speaks of intrinsic (impurity) and impurity absorption (photoelectric effect). Since the concentration of impurity atoms is low, photoelectric effects based on intrinsic absorption are always more significant than those based on impurity. All photodetectors used in optocouplers "work" on an unparalleled photoelectric effect. In order for a quantum of light to cause separation of an electron from an atom, it is necessary to fulfill obvious energy relations:
E f1 = hn 1 E c - E v (2.6)
E f2 = hn 2 E c - E t (2.7)
Thus, the intrinsic photoelectric effect can take place only when a semiconductor is exposed to radiation with a wavelength less than a certain value of l gr :
l gr = hc / (E c - E v ) 1.23 / E g (2.8)
The second equality in (2.8) is valid if l g is expressed in micrometers, and the semiconductor band gap E g is expressed in electron volts. The value of l gr is called the long-wavelength or "red" border of the spectral sensitivity of the material.
The intensity of the photoelectric effect (in the spectral region where it can exist) depends on the quantum yield, which is determined by the ratio of the number of generated electron-hole pairs to the number of absorbed photons. An analysis of the experimental dependences on shows that in the spectral region interesting for optrons, b = 1.
The formation of free charge carriers under the action of irradiation manifests itself in the semiconductor in the form of two photoelectric effects: photoconductivity (increase in the conductivity of the sample upon exposure) and photovoltaic (the appearance of photo-emf at the p - n junction or another form of potential barrier in the semiconductor during illumination). Both effects are used in the practice of constructing photodetectors; for optocouplers, the use of the photo-emf effect is preferable and dominant.
The main parameters and characteristics of photodetectors (regardless of the physical nature and design of these devices) can be divided into several groups. Optical characteristics include the photosensitive surface area, material, dimensions and configuration of the optical window; maximum and minimum radiation power levels. To electro-optical - photosensitivity, the degree of uniformity of the distribution of sensitivity across the photodetector; sensitivity spectral density (dependence of the parameter characterizing sensitivity on wavelength); intrinsic noise of the photodetector and their dependence on the level of illumination and the range of operating frequencies; permissive time (speed); quality factor (a combined indicator that allows you to compare different photodetectors with each other); linearity indicator; dynamic range. As an element of the electric circuit, the photodetector is characterized, first of all, by the parameters of its equivalent circuit, the requirements for operating modes, the presence (or absence) of an integrated amplification mechanism, and the type and shape of the output signal. Other characteristics: operational, reliable, overall, technological - do not contain anything specifically “photo-receiving”.
Depending on the nature of the output signal (voltage, current), one speaks of the voltage or current photosensitivity of the receiver S, measured respectively in V / W or A / W. The linearity (or nonlinearity) of the photodetector is determined by the value of the exponent n in the equation linking the output signal with the input: U o (or I o ) ~ P f . At n 1 photodetector is linear; the range of values of P f (from P f max to P f min ) in which this is performed determines the dynamic range of linearity of the photodetector D , usually expressed in decibels: D = 10 log (P f max / P f min ).
The most important parameter of the photodetector, determining the threshold of its sensitivity, is the specific detection ability D, measured in W -1 m Hz 1/2 . For a known value of D, the sensitivity threshold (minimum detectable radiation power) is defined as
P f min = / D (2.9)
where A is the area of the photosensitive area; D f- range of working frequencies of the amplifier of photo signals. In other words, the parameter D plays the role of a photodetector quality factor.
Fig. 4. Measurement schemes and a family of current-voltage characteristics in the photodiode (a) and photoventile (b) diode operating modes
As applied to optocouplers, not all of the listed characteristics turn out to be equally important. As a rule, photodetectors in optocouplers operate at irradiations very far from the threshold, so the use of the parameters P f min and D is practically useless. Structurally, the photodetector in the optocoupler is usually "recessed" in immersion. the medium connecting it to the emitter, therefore, knowledge of the optical characteristics of the input window loses its meaning (as a rule, there is no such window specially). It is not very important to know the sensitivity distribution over the photosensitive area, since integral effects are of interest.
We consider the mechanism of operation of photodetectors based on the photovoltaic effect using planar-epitaxial photodiodes with a pn junction and a p-in structure, in which you can select an n + substrate, an n- or i-type base (weak conductivity n -type) and a thin p + -layer. When working in the photodiode mode (Fig. 4a), an externally applied voltage causes the moving holes and electrons to leave the p - n (p - i) junction; in this case, the picture of the field distribution in the crystal is sharply different for the two structures under consideration.
The light radiation, absorbed in the base region of the diode, generates electron-hole pairs that diffuse to the p - n junction, are separated by it and cause the appearance of an additional current in the external circuit. In p - i - n diodes, this separation occurs in the field of the i-o6 region and, instead of the diffusion process, charge carriers drift under the influence of an electric field. Each generated electron-hole pair passing through the p - n junction causes a charge equal to the charge of the electron to pass through the external circuit. The greater the irradiation of the diode, the greater the photocurrent. The photocurrent also flows when the diode is displaced in the forward direction (Fig. 4a), however, even at low voltages, it turns out to be much less than the direct current, therefore, its selection is difficult.
The working area of the current – voltage characteristics of the photodiode is the third quadrant in Fig. 4 a; Accordingly, the current sensitivity is the most important parameter.
The second equality in (2.10) was obtained under the assumption of a linear dependence I f = f (P f ), and the third under the condition of neglecting the dark current (I T << I F ), which is usually fulfilled for silicon photodiodes.
If you illuminate the photodiode without applying external bias to it, then the process of separation of the generated electrons and holes will proceed due to the action of its own built-in field of the p - n junction. In this case, the holes will flow into the p-region and partially compensate for the built-in field of the p - n junction. A certain new equilibrium (for a given value: P f ) state is created in which photo-emf U f appears on the external terminals of the diode. If you close the illuminated photodiode to some load, then it will give it a useful electric power P e .
The characteristic points of the volt-ampere characteristics of a diode operating in such a photo-fan mode are the open-circuit emf Uxx and the short-circuit current I kz (Fig. 4, b).
Schematically, the photodiode in the valve mode operates as a kind of secondary power source, therefore, its determining parameter is the efficiency of converting light energy into electrical energy:
Efficiency = P e / AP f = aU xx I short / A pf (2.11)
In the photofan mode, there is an important class of photovoltaic devices - solar panels.
Parameters and characteristics of optocouplers and optoelectronic integrated circuits
Classification of parameters of optronic products
When classifying optronic products, two points are taken into account: the type of photodetector and the design features of the device as a whole.
The choice of the first classification feature is due to the fact that almost all optocouplers have an LED at the input, and the functionality of the device is determined by the output characteristics of the photodetector.
As a second feature, a design is adopted that determines the specifics of the use of an optocoupler.
Fig. 5. To the determination of the pulse parameters of optocouplers
Using this mixed structural design principle of classification, it is logical to distinguish three main groups of products of optocoupler technology: optocouplers (elementary optocouplers), optoelectronic (optocoupler) integrated circuits and special types of optocouplers. Each of these groups includes a large number of types of devices.
The following abbreviations are used for the most common optocouplers: D - diode, T - transistor, R - resistor, Y - thyristor, T 2 - with a composite phototransistor, DT - diode-transistor, 2D (2T) - diode (transistor) differential.
The system of parameters of optocoupler products is based on the system of parameters of optocouplers, which is formed from four groups of parameters and modes.
The first group characterizes the input circuit of the optocoupler (input parameters), the second - its output circuit (output parameters), the third - combines the parameters characterizing the degree of exposure of the emitter to the photodetector and the related features of the signal passing through the optocoupler as a communication element (transmission characteristics), finally, the fourth group combines the parameters of galvanic isolation, the values of which show how close the optocoupler is to the ideal element of isolation. Of the four groups listed, the determining, specifically "optocoupler" parameters are the transfer characteristics and the parameters of the galvanic isolation.
The most important parameter of the diode and transistor optocouplers is the current transfer coefficient. The determination of the pulsed parameters of the optocouplers is clear from (Fig. 5). The reference levels when measuring the parameters t drug (cn) , t rear , and t on (off) are usually levels 0.1 and 0.9, the total time of the logical delay of the signal is determined by the level of 0.5 pulse amplitude.
The parameters of galvanic isolation. Optocouplers are: the maximum allowable peak voltage between input and output U raz p max ; maximum allowable voltage between input and output U ra max ; galvanic isolation resistance R raz ; passage capacity C rav ; maximum permissible rate of voltage change between input to output (dU un / dt) max . The most important parameter is U un n max . It is he who determines the electric strength of the optocoupler and its capabilities as an element of galvanic isolation.
The considered parameters of optocouplers are fully or with some modifications used to describe optoelectronic integrated circuits.
Fig. 6. Designation of optocouplers
Diode optocouplers (Fig. 6, a) to a large extent than any: other devices characterize the level of optronic technology. The value of K i can be judged on the achieved efficiency of energy conversion in the optocoupler; values of time parameters allow to determine the limiting speed of information distribution. The connection of various amplification elements to the diode optocoupler, which is very useful and convenient, cannot nevertheless give a gain either in energy or in limiting frequencies.
Transistor and Thyristor Optocouplers
Transistor optocouplers (Fig. 6, c) differ in their properties favorably with other types of optocouplers. This is primarily circuitry flexibility, manifested in the fact that the collector current can be controlled both via the LED circuit (optically) and the base circuit (electrically), as well as in that the output circuit can operate in both linear and key mode. The internal amplification mechanism provides large values of the current transfer coefficient K i , so that subsequent amplification stages are not always necessary. It is important that in this case the inertia of the optocoupler is not very large and for many cases is quite acceptable. The output currents of phototransistors are significantly higher than, for example, photodiodes, which makes them suitable for switching a wide range of electrical circuits. Finally, it should be noted that all this is achieved with the relative technological simplicity of transistor optocouplers.
Thyristor optocouplers (Fig. 6, b) are most promising for switching high-current high-voltage circuits: they are clearly preferable to T 2 optocouplers in combination of power switched in the load and speed. AOU103 type optocouplers are intended for use as contactless key elements in various electronic circuits: in control circuits, power amplifiers, pulse shapers, etc.
Resistor optocouplers (Fig. 6, d) fundamentally differ from all other types of optocouplers in physical and structural-technological features, as well as in the composition and values of the parameters.
The principle of the photoresistor is based on the effect of photoconductivity, i.e., changes in the semiconductor resistance under illumination.
Differential optocouplers for analog signal transmission
All of the above material relates to the transmission of digital information on a galvanically isolated circuit. In all cases, when it was a question of linearity, of analog signals, it was a question of the form of the output characteristic of an optocoupler. In all cases, the control over the emitter – photodetector channel was not described by a linear dependence. An important task is the transmission of analog information using an optocoupler, ie, ensuring the linearity of the input-output transfer characteristic . Only in the presence of such optocouplers does it become possible to directly disseminate analog information through galvanically isolated circuits without converting it to digital form (pulse train).
A comparison of the properties of different optocouplers in terms of parameters important from the point of view of analog signal transmission leads to the conclusion that if this problem can be solved, then only with the help of diode optocouplers with good frequency and noise characteristics. The complexity of the problem lies primarily in the narrow range of linearity of the transfer characteristic and the degree of this linearity in diode optocouplers.
It should be noted that in the creation of galvanically isolated devices suitable for transmitting analog signals, only the first steps have been taken, and further progress can be expected.
Optoelectronic circuits and other optocoupler devices
Optoelectronic circuits are one of the most widely used, developing, promising classes of optronic products. This is due to the complete electrical and structural compatibility of optoelectronic circuits with traditional circuits, as well as their wider functionality compared to elementary optocouplers. As among conventional microcircuits, the most widely used are switching optoelectronic microcircuits.
Special types of optocouplers are very different from traditional optocouplers and optoelectronic circuits. These include, first of all, optocouplers with an open optical channel. In the design of these devices, there is an air gap between the emitter and the photodetector, so that by placing certain mechanical obstacles in it, one can control the luminous flux and thereby the output signal of the optocoupler. Thus, optocouplers with an open optical channel act as optoelectronic sensors that record the presence (or absence) of objects, the state of their surface, the speed of movement or rotation, etc.
Scopes of optocouplers and optocoupler microcircuits
Promising areas of development and application of optronic technology have been largely determined. Оптроны и оптронные микросхемы эффективно применяются для передачи информации между устройствами, не имеющими замкнутых электрических связей. Традиционно сильными остаются позиции оптоэлектронных приборов в технике получения и отображения информации. Самостоятельное значение в этом направлении имеют оптронные датчики, предназначенные для контроля процессов и объектов, весьма различных по природе и назначении. Заметно прогрессирует функциональная оптронная микросхемотехника, ориентированная на выполнение разнообразных операций, связанных с преобразованием, накоплением и хранением информации. Эффективной и полезной оказывается замена громоздких, недолговечных и нетехнологичных (с позиций микроэлектроники) электромеханических изделий (трансформаторов, потенциометров, реле) оптоэлектронными приборами и устройствами. Достаточно специфическим, но во многих случаях оправданным и полезным является использование оптронных элементов в энергетических целях.
When transmitting information, optocouplers are used as communication elements, and, as a rule, do not carry an independent functional load. Their application allows for a very effective galvanic isolation of control devices and loads (Fig. 7) operating in various electrical conditions and modes. With the introduction of optocouplers, the noise immunity of communication channels increases sharply; “parasitic” interactions along the ground and power circuits are practically eliminated. Of interest is also the rational and reliable matching of digital integrated devices with a heterogeneous elemental base (TTL, ESL, I2L, CMOS, etc.).
Fig. 7. Scheme of inter-block galvanic isolation
The matching circuit of the element of transistor-transistor logic (TTL) with an integrated device on MIS transistors is built on a transistor optocoupler (Fig. 8). In a specific embodiment: E 1 = E 2 = 5 V, E 3 = 15 V, R 1 = 820 Ohms, R 2 = 24 kOhm - the optocoupler LED is excited by a current (5 mA), sufficient to saturate the transistor and confidently control the device on the MIS transistors.
Fig. 8. Scheme of coupling TTL and MIS elements on the optical channel
Optical communications are actively used in telephone devices and systems. Using optocouplers with technically simple means, it is possible to connect microelectronic devices intended for calling, displaying, monitoring and other purposes to telephone lines.
The introduction of optical links into electronic measuring equipment, in addition to the galvanic isolation of the object under study and the measuring device, which is useful in many respects, also makes it possible to drastically reduce the effect of noise acting on the ground and power circuits.
Of considerable interest are the capabilities and experience of using optoelectronic devices and devices in biomedical equipment. Optocouplers can reliably isolate the patient from the action of high voltages available, for example, in electrocardiographic devices.
Non-contact control of powerful, high-voltage circuits via optical channels is very convenient and safe in the difficult technical conditions that are characteristic of many devices and complexes of industrial electronics. In this region, the positions of thyristor optocouplers are strong (Fig. 9).
Fig. 9. AC load switching circuit
Receiving and displaying information
Optocouplers and optocoupler microcircuits occupy a strong position in non-contact remote technology for the rapid receipt and accurate display of information about the characteristics and properties of very different (in nature and purpose) processes and objects. Unique opportunities in this regard are optocouplers with open optical channels. Among them are optoelectronic choppers, which react to the intersection of the optical channel by opaque objects (Fig. 10), and reflective optrons, in which the action of light emitters on photodetectors is entirely associated with reflection of the emitted flux from external objects.
Fig. 10. Optoelectronic sensor
The range of applications of optocouplers with open optical channels is extensive and diverse. Already in the 60s, optocouplers of this type were effectively used to register objects and objects. With such registration, which is typical primarily for automatic monitoring and counting devices, as well as for detecting and indicating various kinds of defects and failures, it is important to clearly determine the location of the object or to reflect the fact of its existence. Optron registration functions are performed reliably and efficiently.
Electrical process control
The power of the radiation generated by the LED and the level of the photocurrent arising in linear circuits with photodetectors are directly proportional to the current of the electrical conductivity of the emitter. Thus, through optical (non-contact, remote) channels, it is possible to obtain quite definite information about the processes in electrical circuits galvanically connected to the emitter. Especially effective is the use of light emitters of optocouplers as sensors for electrical changes in high-current, high-voltage circuits. Clear information about such changes is important for the operational protection of energy sources and consumers from electrical overloads.
Fig. 11. Voltage stabilizer with a control optocoupler
Optocouplers successfully operate in high voltage voltage stabilizers, where they create optical channels of negative feedbacks. The stabilizer in question (Fig. 11) refers to a series-type device, the bipolar transistor being the regulating element, and the silicon zener diode acting as a source of the reference (reference) voltage. The comparing element is the LED.
If the output voltage in the circuit of Fig. 11 increases, then the conductivity current of the LED also increases. The optocoupler phototransistor acts on the transistor, suppressing possible instability of the output voltage.
Replacement of electromechanical products
In the range of technical solutions aimed at increasing the efficiency and quality of automation devices, radio engineering, telecommunications, industrial and consumer electronics, it is an expedient and useful measure to replace electromechanical products (transformers, relays, potentiometers, rheostats, push-button and keyboard switches) with more compact, durable ones, high-speed counterparts. The leading role in this direction is given to optoelectronic devices and devices. The fact is that the very important technical advantages of transformers and electromagnetic relays (galvanic isolation of control and load circuits, reliable operation in powerful, high-voltage, high-current systems) are also characteristic of optocouplers. At the same time, optoelectronic products significantly exceed electromagnetic analogues in terms of reliability, durability, transient and frequency characteristics. Compact and high-speed optoelectronic transformers, switches, relays are controlled confidently using integrated circuits of digital technology without special means of electrical matching.
An example of replacing a pulse transformer is shown in Fig. 12.
Fig. 12. Scheme of the optoelectronic transformer
In the energy mode, optocouplers are used as secondary sources of EMF and current. The efficiency of optronic energy converters is small. However, the possibility of introducing an additional voltage or current source into any circuit of the device without galvanic connection with the primary power supply gives the developer a new degree of freedom, which is especially useful in solving non-standard technical problems.