Optocouplers and their application
The idea of creating and using optocouplers dates back to 1955, when the Loebner EE Optoelectronic devices network proposed a whole series of devices with optical and electrical connections between elements, which allowed amplification and spectral conversion of light signals, creating devices with two stable states - bistable optocouplers, optoelectronic devices for the accumulation and storage of information logic circuits, shift registers. The term "optocoupler" was also proposed there, which was 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 realization, 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 also imperfect: low-temperature and temporal stability of parameters, insufficient resistance to mechanical stress. Therefore. At first, the optocoupler remained only an interesting scientific achievement not finding application in engineering.
Only in the mid-60s of the development of semiconductor light-emitting diodes and technologically advanced high-performance high-speed silicon photodetectors with pn-junctions (photodiodes and phototransistors) the elementary base of modern optronic technology began to be created. By the beginning of the 1970s, the production of optocouplers in the leading countries of the world had become an important and rapidly developing branch of electronic technology, successfully complementing traditional microelectronics.
Optocouplers call such optoelectronic devices in which there are a source and receiver of radiation (a light emitter and a photodetector) with one or another type of optical and electrical connection between them, structurally related to each other.
The principle of operation of any kind of optocouplers is based on the following. In the emitter, the energy of the electrical signal is converted into light, in the photodetector, on the contrary, the light signal causes an electrical response.
Practically, only optocouplers, which have a direct optical connection from the emitter to the photodetector, have received widespread use and, as a rule, all types of electrical connection between these elements are excluded.
According to the degree of complexity of the structural scheme, there are two groups of devices among the products of optocouplers. An optocoupler (also referred to as an “elementary optocoupler”) is an optoelectronic semiconductor device consisting of radiating and photo-receiving elements, between which there is an optical link that provides electrical insulation between the input and the output. An optoelectronic integrated circuit is a chip 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 electrical (galvanic) isolation of the input and output is performed.
Distinctive features of optocouplers
The advantages of these devices are based on the general optoelectronic principle of using electrically neutral photons for information transfer. The main ones are as follows:
- the possibility of providing an ideal electrical (galvanic) isolation between the input and output; for optocouplers, there are no fundamental physical or structural constraints on achieving arbitrarily high voltages and decoupling resistances and arbitrarily small pass-through capacitances;
- the possibility of implementing contactless 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 a reverse response of the receiver to the emitter;
- wide frequency bandwidth of the optocoupler, the absence of restrictions on the part of low frequencies (which is typical of pulse transformers); the possibility of transmitting on the optocoupler circuit, as a pulse signal, and the constant component;
- the ability to control the output signal of the optocoupler by influencing (including non-electric) on the material of the optical channel and the resulting possibility of creating a variety of sensors, as well as a variety of instruments for transmitting information;
- the possibility of creating functional microelectronic devices with photodetectors, whose characteristics when illuminated change according to a complex, given law;
- immunity of optical communication channels to the effects of electromagnetic fields, which in the case of "long" optocouplers (with an extended optical fiber between the emitter and receiver) determines their protection from interference and information leakage, and also eliminates mutual pickups;
- physical and structural-technological compatibility with other semiconductor and microelectronic devices.
The 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 increased temperature and penetrating nuclear radiation;
- more or less noticeable temporary degradation (deterioration) of parameters;
- relatively high level of intrinsic noise, due, like the two previous drawbacks, to the peculiarities of LED physics;
- the complexity of the implementation of feedbacks caused by electrical disconnection of the input and output circuits;
- constructive-technological imperfections associated with the use of hybrid non-planar technology (with the need to combine in one device several - separate crystals from different semiconductors located in different planes).
The mentioned disadvantages of optocouplers are partially eliminated as materials, technology, and circuitry are improved, but, nevertheless, for a long time they will be of a rather fundamental nature. However, their advantages are so high that they ensure confident non-competitive optocouples among other microelectronic devices.
Generalized structural diagram
As an element of communication, the optocoupler is characterized by the transmission coefficient K i , determined by the ratio of the output and input signals, and the maximum information transfer rate F. Practically, instead of F, the duration of the rise and fall of the transmitted pulses t drug (cn) or the cutoff frequency is measured. The capabilities of an optocoupler as an element of galvanic isolation are characterized by maximum voltage and resistance of the isolation U development and R development and passing capacity C development .
In the block diagram in fig. 1 input device serves to optimize the operating mode of the emitter (for example, the displacement of the LED to the linear portion of the watt-ampere characteristic) and convert (amplify) 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 an optocoupler
The purpose of the optical medium is to transfer the energy of the optical signal from the emitter to the photodetector, and in many cases to ensure the mechanical integrity of the structure.
The principal possibility of controlling the optical properties of the medium, for example, by using electro-optical or magneto-optical effects, is reflected by introducing a control device into the circuit. In this case, we obtain an optocoupler with a controlled optical channel that is functionally different from the "normal" optocoupler: the output signal can be changed as input and control circuit.
In the photodetector, the information signal is “restored” from the optical to the electrical one; while striving to have high sensitivity and high speed.
Finally, the output device is designed to convert the photodetector signal into a standard form, convenient for influencing the subsequent stages of the optocoupler. A practically required function of the output device is signal amplification, since the losses after double conversion are very significant. Often, the photodetector itself (for example, a phototransistor) also performs the amplification function.
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 for optocoupler technology; The previously named optocouplers (elementary optocouplers) using blocks light emitter - optical medium - photodetector; optoelectronic (optocoupler) chips (optocouplers with the addition of the output, and sometimes the 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 diagram in fig. one; Each of the indicated blocks may include not one, but several elements, identical or similar to each other, connected electrically and optically, but this does not substantially change the fundamentals of physics and electronics of the optocoupler.
As elements of galvanic isolation, optocouplers are used: for the communication of equipment units, between which there is a significant potential difference; to protect the input circuits of measuring devices from interference and interference, etc.
Another major area of application for 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 consists of resistor optocouplers designed for low-current switching circuits in complex information display devices made on electroluminescent (powder) indicators, mnemonic diagrams, and screens.
The creation of "long" optocouplers (devices with an extended flexible fiber optic light guide) has opened up a completely new direction in the use of products of optocoupler technology - communication over short distances.
Various optocouplers (diode, resistor, transistor) are used in purely radio modulation schemes, automatic gain control, etc. The impact on the optical channel is used here to bring the circuit to the optimum operating mode, for contactless mode tuning, 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: these are the humidity and gas sensors, the sensor for the presence in the volume of a liquid, sensors for the surface finish of the object, the speed of its movement, etc.
The use of optocouplers for energy purposes is quite specific, that is, the operation of a diode optocoupler in the photo-ventilating mode. In this mode, the photodiode generates electrical power to the load and the optocoupler is, to a certain extent, similar to a low-power secondary power source, completely isolated from the primary circuit.
The creation of optocouplers with photoresistors, the properties of which, when illuminated, vary according to a given complex law, makes it possible to model 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 computers, automation, communication and radio equipment, automated control systems, measuring equipment, control and regulation systems, medical electronics , visual information display devices.
Physical basics of optoelectronic technology
Element base and device optocouplers
The elemental basis of the optocouplers are photodetectors and emitters, as well as the optical medium between them. General requirements such as small dimensions and weight, high durability and reliability, resistance to mechanical and climatic influences, manufacturability, low cost are imposed on all these elements. It is also desirable that the elements undergo a sufficiently wide and long industrial approbation.
Functionally (as an element of the circuit), an optocoupler is characterized primarily by what type of photodetector it uses.
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 electrical; the presence and effectiveness of an internal gain embedded; high speed; breadth of functionality.
In the optocouplers used photodetectors of various structures that are sensitive in the visible and near-infrared region, since it is in this range of the spectrum there are intense sources of radiation and the operation of photodetectors without cooling is possible.
The most versatile are photodetectors with p - n-junctions (diodes, transistors, and t, p.), In most cases they are made on the basis of silicon and their region of maximum spectral sensitivity is near l = 0.7 ... 0.9 μm .
Numerous requirements are imposed on the emitters of optocouplers. The main ones are: spectral matching with the selected photodetector; high efficiency of energy conversion of electric current into radiation energy; primary radiation directivity; high speed; simplicity and convenience of excitation and modulation of radiation.
Several types of emitters are suitable 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 radiators are characterized by low light output, low resistance to mechanical stress, limited durability, large dimensions, complete incompatibility with integrated technology. However, they may find application in certain types of optocouplers.
- The powder electroluminescent cell uses fine-crystalline zinc sulfide grains (activated by copper, manganese or other additives) suspended in a polymerizing dielectric as a luminous body. When a sufficiently high ac voltage is applied, a pre-breakdown luminescence process is in progress.
- Thin-film electroluminescent cells . The glow here is associated with the excitation of manganese atoms by “hot” electrons.
Both powder and film electroluminescent cells have low efficiency of conversion of electrical energy into light, low durability (especially thin-film), are difficult to manage (for example, the optimum mode for powder phosphors ~ 220 V at f = 400 ... 800 Hz). The main advantage of these emitters is the constructive-technological compatibility with photoresistors, the possibility of creating on this basis multi-functional, 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 the following advantages: high value of the efficiency of conversion of electric energy into optical energy; narrow emission spectrum (quasi-monochromatic); the latitude of the spectral range overlapped by various LEDs; radiation directivity; high speed; small values of supply voltages and currents; compatibility with transistors and integrated circuits; ease of modulation of the radiation power by changing the direct current; the ability to work, both in pulsed and in 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 an optocoupler are the following: a high value of the refractive index n for them ; high value of specific resistance r im ; high critical field strength E im kr , sufficient heat resistance Dq im slave ; good adhesion with crystals of silicon and gallium arsenide; elasticity (this is necessary because it is not possible to ensure the coordination of the elements of the optocoupler according to the coefficients of thermal expansion); mechanical strength, since the immersion medium in the optocoupler performs not only light transmitting, but also structural functions; manufacturability (usability, reproducibility of properties, low cost, etc.).
The main type of immersion medium used in optocouplers are polymeric optical adhesives. For them, typically 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. Adhesives have good adhesion to silicon and gallium arsenide, combine high mechanical strength and resistance to thermal cycling. Non-hardening vaseline-like and rubber-like optical media are also used.
Physics of energy conversion in a diode optocoupler
Consideration of energy conversion processes in the 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 Planck's constant;
c is the speed of light in vacuum;
n is the refractive index of a semiconductor;
n, l is the oscillation frequency and the wavelength of optical radiation.
If the flux density of quanta (i.e., the number of quanta flying through a unit area per unit time) is 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 greater, the shorter the radiation wavelength. Since in practice a given is P f (energy irradiance of a photodetector), the following relation seems to be useful
N f = P f / E f = 5 * 10 15l P f (2.3)
where N f , cm -2 s -1 ; l , micron; P f mW / cm
Fig. 2. Energy diagram of a direct-gap semiconductor (by the example of the ternary compound GaAsP)
The mechanism of injection luminescence in a LED consists of three main processes: radiative (and nonradiative) recombination in semiconductors, injection of excess minority charge carriers into the base of the LED, and radiation output from the generation region.
The recombination of charge carriers in a semiconductor is determined primarily by its band diagram, the presence and nature of impurities and defects, the degree of disturbance of the equilibrium state. The main materials of optocoupler emitters (GaAs and ternary compounds based on it GaA1As and GaAsP) belong to direct-gap semiconductors, i.e. to those in which direct zone-to-zone optical transitions are allowed (Fig. 2). Each act of carrier recombination according to this scheme is accompanied by the emission of a quantum, the wavelength of which, in accordance with the law of conservation of energy, is determined by the relation:
l rad [μm] = 1.23 / E f [eB] (2.4)
It should be noted that there are competing nonradiative - recombination mechanisms. The most important of these include:
- Recombination at deep centers. An electron can pass into the valence band not directly, but through those or other recombination centers, which form allowed energy levels in the forbidden band (the level E t in Figure 2).
- Auger recombination (or shock). At very high concentrations of free charge carriers in a semiconductor, the probability of three-body collisions increases, the energy of the recombining electron-hole pair is given to the third free carrier in the form of kinetic energy, which it gradually spends on collisions with the lattice.
Fig. 3. Electric (a) and optical (b) LED models. A - 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 - space charge region
The relative role of various recombination mechanisms is described by introducing the concept of internal quantum yield h int defined by the ratio of the probability of radiative recombination to the total (radiative and nonradiative) probability of recombination (or, otherwise, the ratio of the number of generated quanta to the number of injected non-core charge carriers). The value of h int is the most important characteristic of the material used in the LED; obviously 0 h int 100%.
The creation of an excess concentration of free carriers in the active (emitting) region of the LED crystal is carried out by injecting them with a pn junction shifted in the forward direction.
The “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" components of direct current 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 d-region is doped compared to the p-region.
- The recombination current (nonradiative) in the space charge region of the p – n junction of rivers I. In semiconductors with a large band gap with small forward biases, the fraction of this current can be noticeable.
- Tunneling current I tun , due to the "leakage" of charge carriers through the potential barrier. The current is carried by the main carriers and does not contribute to radiative recombination. The tunneling current is greater than the p-n junction, it is noticeable with a high 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 semiconductor surface from the properties of the volume and the presence of certain short-circuiting inclusions.
The efficiency of the p-n-junction is characterized by the injection rate:
Obviously, the limits of the possible change of g are the same as that of h int , i.e. 0 g 100%.
When outputting radiation from the generation region, the following types of energy loss occur (Fig. 3, b):
- 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 border” of absorption (see below), and such radiation is quickly absorbed in the thickness of the semiconductor (self-absorption). In fact, radiation in direct-gap semiconductors does not follow the above ideal scheme. Therefore, the wavelength of the generated quanta is somewhat larger than that given by (2.4):
- The loss of total internal reflection (rays 2). It is known that when rays of light fall on the interface of an optically dense medium (semiconductor) with optically less dense (air) for a part of these rays, the condition of total internal reflection is fulfilled. Such rays reflected inside the crystal are ultimately lost due to self-absorption.
- Back and end radiation losses (beam 3 and 4).
Quantitatively, the efficiency of optical energy output from a crystal is characterized by the output factor K opt determined by the ratio of the radiation power going out in the right direction to the radiation power generated inside the crystal. As for the coefficients h int and g , the condition 0 is always satisfied. To wholesale 100%.
g . The integral indicator of the emissivity of the LED is the external quantum yield h ext . From what has been said it is clear that h ext = h intg K opt .
Let's go to the receiving unit. The principle of operation of photo-detectors used in optocouplers based on the internal photoelectric effect, which consists in the separation of electrons from atoms inside the body under the action of electromagnetic (optical) radiation.
Quanta of light, being absorbed in a crystal, can cause the detachment of electrons from atoms, both of the semiconductor itself and of the impurity. In accordance with this, one speaks of one’s own (pure) and impurity absorption (photo effect). Since the concentration of impurity atoms is low, photoelectric effects based on self-absorption are always more significant than those based on impurity. All photodetectors used in optocouplers “work” on a pure photoelectric effect. In order for a quantum of light to cause a detachment of an electron from an atom, it is necessary to fulfill the obvious energy relations:
E F1 = hn 1 E c - E v (2.6)
E ф2 = hn 2 E c - E t (2.7)
Thus, the intrinsic photoelectric effect can occur only when a semiconductor is exposed to radiation with a wavelength less than a certain value l g :
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 band gap of a semiconductor is E g in electron volts. The value of l gr is called the long-wave or "red" boundary 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, 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 b = 1 in the spectral region of interest to optocouplers.
The formation of free charge carriers under the action of irradiation manifests itself in a semiconductor in the form of two photoelectric effects: photoconductivity (increasing the conductivity of the sample under illumination) and photovoltaic (occurrence of photo-emf at the pn junction or another form of potential barrier in the semiconductor when illuminated). Both effects are used in the practice of designing photodetectors; for optocouplers, the use of the photo emf effect is preferred 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 area of the photosensitive surface, the material, dimensions and configuration of the optical window; maximum and minimum levels of radiation power. Electro-optical - photosensitivity, the degree of uniformity of the distribution of sensitivity in the photodetector area; sensitivity spectral density (dependence of the parameter characterizing sensitivity on the wavelength); photodetector own noise and their dependence on the level of illumination and operating frequency range; allow time (speed); quality factor (a combined indicator that allows you to compare different photodetectors with each other); linearity index; dynamic range. As an element of an electrical circuit, a photodetector is characterized, first of all, by the parameters of its equivalent circuit, the requirements for operating modes, the presence (or absence) of the built-in amplification mechanism, the type and form of the output signal. Other characteristics: performance, reliability, dimensional, technological - do not contain anything specifically "photodetector".
Depending on the nature of the output signal (voltage, current), they indicate the voltage or current photosensitivity of the receiver S, measured in V / W or A / W, respectively. The linearity (or nonlinearity) of the photodetector is determined by the value of the exponent n in the equation relating the output signal to the input: U out (or I out ) ~ P f . With n 1 photodetector is linear; the range of P f values (from P f max to P f min ) in which it is performed determines the dynamic range of the linear sensor D , usually expressed in decibels: D = 10 lg (P f max / P f min ).
The most important parameter of the photodetector, which determines the threshold of its sensitivity, is the specific detecting ability D, measured in W -1 m Hz 1/2 . With a known value of D, the threshold of sensitivity (the minimum recorded radiation power) is defined as
P f min = / D (2.9)
where A is the area of the photosensitive site; D f is the operating frequency range of the photo signal amplifier. In other words, the parameter D plays the role of the quality factor of the photodetector.
Fig. 4. Measurement circuits and families of current-voltage characteristics in photodiode (a) and photo-ventilator (b) diode operation modes
When applied to optocouplers, not all of the listed characteristics are equally important. As a rule, photodetectors in optocouplers work at irradiations very far from the threshold ones, therefore the use of the parameters P f min and D turns out to be practically useless. Structurally, the photodetector in the optocoupler is usually "drowned" in the immersion. the medium connecting it with the radiator, therefore, knowledge of the optical characteristics of the input window loses its meaning (as a rule, there is no such window specifically). It is not very important to know the sensitivity distribution over the photosensitive site, since integral effects are of interest.
The mechanism of photodetectors based on the photovoltaic effect, we consider the example of planar-epitaxial photodiodes with pn-junction and p-in-structure, in which you can select the n + substrate, n-base or i-base (weak conductivity n -type) and thin p + -layer. When operating in the photodiode mode (Fig. 4, a), the voltage externally applied causes movable holes and electrons to move away from the p – n (p – i) junction; in this case, the pattern 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 additional current in the external circuit. In p - i - n-diodes, this separation occurs in the i-o6 field and, instead of a diffusion process, charge carriers drift under the influence of an electric field. Each generated electron-hole pair that has passed through the p – n junction causes a charge equal to the electron charge to pass through the external circuit. The greater the diode irradiance, the greater the photocurrent. The photocurrent also flows when the diode is biased in the forward direction (Fig. 4, a), but already at low voltages it turns out to be much less than the direct current, therefore its selection turns out to be difficult.
The working area of the current-voltage characteristics of the photodiode is III quadrant in Fig. 4, a; accordingly, current sensitivity is the most important parameter
The second equality in (2.10) was obtained assuming a linear relationship I f = f (P f ), and the third - under the condition of neglecting the dark current (I T << I F ), which is usually true for silicon photodiodes.
If the photodiode is illuminated without an external bias applied to it, then the process of separation of the generated electrons and holes will proceed due to the action of its own built-in p-n junction field. In this case, the holes will flow into the p-region and partially compensate for the built-in p-n-junction field. A new equilibrium state is created (for a given value: P f ) a state in which photo-emf U f arises at the external terminals of the diode. If you close the lighted photodiode for some load, it will give it the useful electrical power P e .
The characteristic points of the current-voltage characteristics of the diode operating in this - photo-ventilating mode are the open-circuit voltage EMF Uxx and the short-circuit current I CC (Fig. 4, b).
Schematically, the photodiode in the valve mode works as a kind of secondary power source, therefore its decisive parameter is the efficiency of conversion of light energy into electrical energy:
Efficiency = P e / AP f = aU xx I CC / A pf (2.11)
In the photo-ventilating mode, an important class of photovoltaic devices - solar panels.
Parameters and characteristics of optocouplers and optoelectronic integrated circuits
Classification of parameters of optocoupler products
When classifying the products of optocoupler technology takes into account two points: the type of photodetector device and the design features of the device as a whole.
The choice of the first classification sign is due to the fact that almost all the optocouplers at the input have a LED, and the functionality of the device is determined by the output characteristics of the photoreceiver.
As a second feature, a design was adopted that determines the specifics of using an optocoupler.
Fig. 5. To the determination of the pulse parameters of optocouplers
Using this mixed design and schematic classification principle, it is logical to distinguish three main groups of products of optocoupler technology: optocouplers (elementary optocouplers), optoelectronic (optocouplers) integrated circuits and special types of optocouplers. Each of these groups includes a large number of types of devices.
For the most common optocouplers, the following abbreviations are used: D - diode, T - transistor, R - resistor, U - thyristor, T 2 - with composite phototransistor, DT - diode-transistor, 2D (2Т) - 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 optocoupler input circuit (input parameters), the second group describes its output circuit (output parameters), the third group combines the parameters characterizing the degree of the emitter effect on the photodetector and the associated signal passing characteristics through the optocoupler as a coupling element Finally, the fourth group combines galvanic isolation parameters, the values of which show how close the optocoupler is to the ideal decoupling element. Of the four listed groups, the parameters of the transfer characteristic and the parameters of electrical isolation are defining, specifically "optocouplers".
The most important parameter of the diode and transistor optocouplers is the current transfer ratio. The determination of the pulse parameters of optocouplers is clear from (Fig. 5). The reference levels when measuring the parameters t drug (cn) , t cd , and t on (off) are usually levels 0.1 and 0.9, the total logical time 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 the input and the output of U development n max ; the maximum allowable voltage between the input and the output U ra max ; galvanic resistance R raz ; passing capacity C development ; maximum permissible rate of change of voltage between the input to the output (dU razd / dt) max . The most important is the parameter U ra p max . It is he who determines the electrical strength of the optocoupler and its capabilities as an element of galvanic isolation.
The considered parameters of optocouplers are used in full or with some changes to describe optoelectronic integrated circuits.
Fig. 6. Symbols of optocouplers
Diode optocouplers (Fig. 6, a) to a large extent than any other: other devices, characterize the level of optoelectronic 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 maximum speed of information distribution. The connection to the diode optocoupler of various amplifying elements, 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), in a number of their properties compare favorably with other types of optocouplers. This is primarily circuit flexibility, which is manifested in the fact that the collector current can be controlled both by the LED circuit (optically) and by the base circuit (electrically), and also that the output circuit can operate in linear and in key mode. The internal amplification mechanism provides large values of the current transfer coefficient K i , so that subsequent amplifying stages are not always necessary. It is important that in this case the inertia of the optocouplers is not very large and, in many cases, is quite acceptable. The output currents of phototransistors are much 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 the most promising for switching high-current high-voltage circuits: by the combination of power switched in the load, and speed, they are clearly preferable to T 2 -optopar. Optocouplers of type AOU103 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) are fundamentally different from all other types of optocouplers by physical and structural-technological features, as well as the composition and values of the parameters.
The principle of the photoresistor is based on the effect of photoconductivity, i.e. changes in the resistance of a semiconductor under illumination.
Differential Optocouplers for Analog Signal Transmission
All the above material relates to the transfer of digital information on a galvanically isolated circuit. In all cases, when it was spoken about linearity, about analog signals, it was a question of the type of output characteristic of an optocoupler. In all cases, control over the channel emitter-photodetector was not described by a linear relationship. An important task is the transmission of analog information using an optocoupler, that is, ensuring the linearity of the input-output transfer characteristic . Only in the presence of such optocouplers is it possible to directly distribute analog information over galvanically isolated circuits without converting it to digital form (a sequence of pulses).
Comparison of the properties of various optocouplers by parameters important from the point of view of transmitting analog signals leads to the conclusion that if this problem can be solved, it is 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 linearity of the diode optocouplers.
It should be noted that in the creation of electrically isolated devices suitable for the transmission of analog signals, only the first steps have been made, and further progress can be expected.
Optoelectronic microcircuits and other devices of optocoupler type
Optoelectronic microcircuits are one of the most widely used, developing, promising classes of products of optocoupler technology. This is due to the full electrical and structural compatibility of optoelectronic microcircuits with traditional microcircuits, as well as their broader functionality as compared to elementary optocouplers. As well as among ordinary microcircuits, switching optoelectronic microcircuits are most widely used.
Special types of optocouplers differ sharply from traditional optocouplers and optoelectronic microcircuits. These include, above all, optocouplers with an open optical channel. In the design of these devices between the emitter and photodetector there is an air gap, so that by placing certain mechanical obstacles in it, it is possible to control the luminous flux and thereby the output signal of the optocoupler. Thus, optocouplers with an open optical channel act as optoelectronic sensors that detect the presence (or absence) of objects, the state of their surface, the speed of movement or rotation, etc.
Applications of optocouplers and optocoupler chips
The promising directions for the development and application of optocoupler technology are largely determined. Optocouplers and optocoupler chips are effectively used to transfer information between devices that do not have closed electrical connections. The positions of optoelectronic devices in the technique of obtaining and displaying information remain traditionally strong. Optocoupler sensors designed to monitor processes and objects that are quite different in nature and purpose have an independent significance in this direction. A noticeable progress is made in functional optocoupler microcircuitry, focused on performing various operations related to the transformation, accumulation and storage of information. Efficient and useful is the replacement of bulky, short-lived and low-tech (from the standpoint of microelectronics) electromechanical products (transformers, potentiometers, relays) with optoelectronic devices and devices. The use of optocoupler elements for energy purposes is quite specific, but in many cases justified and useful.
When transmitting information, optocouplers are used as communication elements, and, as a rule, do not carry independent functional load. Their use allows for highly efficient galvanic isolation of control and load devices (Fig. 7) operating in various electrical conditions and modes. With the introduction of optocouplers, the noise immunity of communication channels increases dramatically; "parasitic" interactions along the circuits of the "earth" and food are practically eliminated. Of interest is also the rational and reliable coordination of digital integrated devices with a heterogeneous element base (TTL, ECL, I2L, CMOS, etc.).
Fig. 7. Scheme of inter-unit galvanic isolation
The matching circuit of an 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 kΩ - the LED of the optocoupler is energized by a current (5 mA) sufficient to saturate the transistor and confidently control the device on the MIS transistors.
Fig. 8. Diagram of coupling TTL and MDP elements via optical channel
Optical communications are actively used in telephone devices and systems. Using optocouplers using technically simple means, it is possible to connect microelectronic devices for calling, indicating, controlling, and other purposes to telephone lines.
The introduction of optical links into the electronic measuring equipment, in addition to galvanic isolation of the object and the measuring device, which is useful in many respects, also makes it possible to drastically reduce the influence 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, such as those found in electrocardiographic devices.
Contactless control of high-power, high-voltage circuits via optical channels is very convenient and safe in complex technical conditions characteristic of many devices and industrial electronics complexes. The positions of the thyristor optocouplers are strong in this area (Fig. 9).
Fig. 9. AC load switching circuit
Receiving and displaying information
Optocouplers and optocoupler chips occupy strong positions in contactless remote technology to quickly obtain and accurately display information about the characteristics and properties of very different (in nature and purpose) processes and objects. In this regard, optocouplers with open optical channels have unique capabilities. Among them are optoelectronic interrupters reacting to the intersection of the optical channel with opaque objects (Fig. 10), and reflective optocouplers, in which the action of light emitters on photodetectors is entirely due to the reflection of the radiated 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 characteristic primarily of devices for automatic control and counting of objects, as well as for detecting and indicating various types of defects and failures, it is important to clearly determine the location of the object or reflect the fact of its existence. The registration functions of the optocouplers perform reliably and efficiently.
Control of electrical processes
The power of the radiation generated by the LED, and the level of the photocurrent arising in linear circuits with photodetectors, is directly proportional to the current of the electrical conductivity of the emitter. Thus, using optical (contactless, remote) channels, you can get quite definite information about the processes in electrical circuits galvanically connected to the radiator. Especially effective is the use of light emitters of optocouplers as sensors for electrical changes in high-current, high-voltage circuits. Clear information on such changes is important for the operational protection of energy sources and consumers from electrical overloads.
Fig. 11. Voltage stabilizer with controlling optocoupler
Optocouplers successfully operate in high-voltage voltage regulators, where they create optical channels of negative feedback. The stabilizer under consideration (Fig. 11) refers to a device of the series type, the bipolar transistor being the regulating element, and the silicon Zener diode acts as a source of the reference (reference) voltage. The comparison element is the LED.
If the output voltage in the circuit Fig. 11 increases, then the conduction current of the LED increases. The phototransistor of the optocoupler acts on the transistor, suppressing possible instability of the output voltage.
Electromechanical product replacement
In the complex of technical solutions focused on improving the efficiency and quality of automation devices, radio engineering, telecommunications, industrial and consumer electronics, it is reasonable and useful measure to replace electromechanical products (transformers, relays, potentiometers, rheostats, push-button and keyboard switches) with more compact, durable, high-speed counterparts. The leading role in this direction is assigned to optoelectronic devices and devices. The fact is that very important technical advantages of transformers and electromagnetic relays (galvanic isolation of control circuits and loads, confident operation in high-power, high-voltage, high-current systems) are also characteristic of optocouplers. At the same time, optoelectronic products significantly exceed electromagnetic analogues in reliability, durability, transitional and frequency characteristics. Control of compact and high-speed optoelectronic transformers, switches, relays confidently carried out 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 optoelectronic transformer
In the energy mode, optocouplers are used as secondary sources of emf and current. The efficiency of optocoupler energy converters is small. However, the possibility of introducing an additional voltage source or current into any circuit of the device without galvanic coupling to the primary power source provides the developer with a new degree of freedom, especially useful in solving non-standard technical problems.