Technical characteristics and application of the h

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Technical characteristics and applications of optical couplers

1 Overview

opticalcoupler (abbreviated as OC) is also known as photoelectric isolator, or optocoupler for short. Optical couplers transmit electrical signals through light. It has a good isolation effect on input and output electrical signals, so it is widely used in various circuits. At present, it has become one of the optoelectronic devices with the most kinds and the most widely used. The optical coupler is generally composed of three parts: light emission, light reception and signal amplification. The input electrical signal drives the light-emitting diode (LED) to emit a certain wavelength of light, which is received by the photodetector to generate photocurrent, and then output after further amplification. This completes the electrical optical electrical conversion, thus playing the role of input, output and isolation. Because the input and output of the optocoupler are isolated from each other, and the electrical signal transmission has the characteristics of unidirectionality, it has good electrical insulation ability and anti-interference ability. Because the input end of the optocoupler is a low resistance element of current mode operation, it has a strong common mode suppression ability. Therefore, it can greatly improve the signal-to-noise ratio as a terminal isolation element in long-distance transmission of information. In computer digital communication and real-time control, the grounding Review Committee, as a signal isolation, believes that the standard preparation group has done a lot of work port devices, which can greatly increase the reliability of computer work

the main advantages of optical coupler are: signal unidirectional transmission, complete electrical isolation between input and output, no influence of output signal on input, strong anti-interference ability, stable operation, no contact, long service life and high transmission efficiency. Optocoupler is a new device developed in the 1970s. It has been widely used in electrical insulation, level conversion, interstage coupling, driving circuit, switching circuit, chopper, multivibrator, signal isolation, interstage isolation, pulse amplification circuit, digital instrument, long-distance signal transmission, pulse amplification, solid-state relay (SSR), instrument and meter, communication equipment and microcomputer interface. In the monolithic switching power supply, the optocoupler feedback circuit can be formed by using the linear optocoupler, and the duty cycle can be changed by adjusting the current at the control end to achieve the purpose of precise voltage stabilization

over the past decade, new optical couplers have been emerging to meet the requirements of various optical control. Its application scope has been extended to measuring instruments, precision instruments, industrial electronic instruments, computers and their external equipment, communicators, annunciators, road information systems that can significantly improve the air quality inside the trunk, electric machinery and other fields. This paper focuses on the working characteristics of the device, driving and output circuits and some practical application circuits

the linear optocoupler developed in recent years can transmit continuously changing analog voltage or analog current signals, which greatly expands its application field. The working principle and detection method of optocoupler are introduced below

2. Performance and type of optocoupler

the light-emitting device of the optocoupler used to transmit analog signals is a diode and the optical receiver is a photosensitive triode. When a current passes through the light-emitting diode, a light source is formed. The light source irradiates the surface of the photosensitive triode, causing the photosensitive triode to generate a collector current. The magnitude of the current is proportional to the intensity of the light, that is, the magnitude of the forward current flowing through the diode. Since the input and output ends of the optocoupler are transmitted through optical signals, the two parts are electrically completely isolated, and there is no feedback and interference of electrical signals, so the performance is stable and the anti-interference ability is strong. The coupling capacitance between the light-emitting tube and the photosensitive tube is small (about 2pF) and the withstand voltage is high (about 2.5KV), so the common mode rejection ratio is very high. The electrical isolation between input and output depends on the insulation resistance between the two power supplies. In addition, due to its small input resistance (about 10 Ω), the noise of high internal resistance source is equivalent to being short circuited. Therefore, the analog signal isolation circuit composed of optocouplers has excellent electrical performance

in fact, the optocoupler is a current transfer device controlled by photocurrent, and its output characteristics are similar to those of ordinary bipolar transistors. Therefore, it can be used as an ordinary amplifier to directly form an analog amplification circuit, and the input and output can be electrically isolated

however, this kind of amplification circuit has poor working stability and no practical value. There are two main reasons: first, the linear working range of the optocoupler is narrow and changes with temperature; The second is the common emitter current transmission coefficient of the optocoupler β And collector reverse saturation current Icbo (i.e. dark current) is obviously affected by temperature change. Therefore, in practical application, in addition to selecting optical couplers with wide linear range and high linearity to realize analog signal isolation, effective measures must be taken in the circuit to eliminate the influence of temperature change on the working state of the amplification circuit as far as possible

from the relationship between the transfer characteristics of the optical coupler and temperature, it can be seen that if the analog isolation circuit composed of the optical coupler is stable and practical, the influence of dark current (Icbo) should be eliminated as far as possible to improve the linearity, so that the static working point IFQ can be automatically adjusted with the change of temperature, so that the output signal can maintain symmetry, and the dynamic range of the input signal can be automatically changed with the change of temperature to offset β Value changes with temperature to ensure the stability of the working state of the circuit

2.1 types of optical couplers

optical couplers have dozens of sealed cultivation forms, such as tube type, dual in-line type and optical fiber type. There are dozens of types of optocouplers, mainly including general-purpose type (including no base lead and base lead), Darlington type, Schmidt type, high-speed type, optical integrated circuit, optical fiber, photosensitive thyristor type (including unidirectional thyristor and bidirectional thyristor), photosensitive field effect tube type. In addition, there are dual channel type (with two sets of tubes inside), high gain type, AC-DC input type, etc. Foreign manufacturers include the British isocom company and the Suzhou Semiconductor General Factory of domestic manufacturers

2.2 reduction of linear optocoupler human resource input product classification

typical products and main parameters of linear optocoupler are shown in Table 1, These optocouplers use photosensitive triode as the receiving tube

Table 1 main parameters of typical linear optocoupler

Product model ctr/%v (BR) Ce0/v manufacturer's packaging type

pc816a 80 ~ 160 70 sharpdzp-4 base is not led out

pc817a 80 ~ 160 35 sharp

sfh610a-2 63 ~ 125 70 simens

nec2501-h 80 ~ 160 40 nec

cny 63 ~ 125 70 Motorola dzp-4 base is not led out

cny 100 ~ 200 70 simens

sfh 63 ~ 125 70 Simens ens

sfh 100 ~ 20070 simens

cny75ga 63 ~ 12590 temicdzp-4 base is not led out

cny75gb 100 ~ 20090 te mic

MOC8101 50~80 30 Motoroln

MOC8102 73~117 30 Motoroln

3. The technical parameters of the optocoupler

the technical parameters of the optocoupler mainly include the forward voltage drop VF of the light-emitting diode, the forward current if, the current transmission ratio CTR, the insulation resistance between the input stage and the output stage, the Collector Emitter reverse breakdown voltage V (BR) CEO, and the collector emitter saturation voltage drop VCE (SAT)

in addition, parameters such as rise time, fall time, delay time and storage time need to be considered when transmitting digital signals

the most important parameter is the current amplification factor transfer ratio (CTR). It is usually expressed by DC current transmission ratio. When the output voltage remains constant, it is equal to the percentage of DC output current IC and DC input current if. When the current amplification factor HFE of the receiving tube is constant, it is equal to the ratio of the output current IC, which is usually expressed as a percentage. There is a formula:

ctr=ic/if × 100%

for the optocoupler with a photosensitive triode, the CTR range is mostly 20% - 30% (such as 4N35), while the PC817 range is 80% - 160%. Darling 6. The tensile strength, elongation at break, elastic modulus, stress, strain and other parameters of the material can be measured. The Dayton optocoupler (such as 4n30) can reach 100% - 500%. This indicates that to obtain the same output current, the latter only needs a smaller input current. Therefore, CTR parameters have some similarities with HFE of transistors. The ctr-if characteristic curve of ordinary optical coupler is nonlinear, and the nonlinear distortion is particularly serious when the if is small, so it is not suitable for transmitting analog signals. The ctr-if characteristic curve of linear optocoupler has good linearity, especially when transmitting small signals, its AC current transmission ratio( Δ CTR= Δ IC/Δ If) is very close to the value of DC current transmission ratio CTR. Therefore, it is suitable for transmitting analog voltage or current signals, and can make the relationship between output and input linear. This is its important characteristic. When designing the optocoupler feedback switching power supply, the model and parameters of the linear optocoupler must be correctly selected. The selection principles are as follows:

(1) the allowable range of the current transfer ratio (CTR) of the optocoupler is 50% - 200%. This is because when CTR is less than 50%, the LED in the optocoupler needs a large working current (if> 5.0mA) to normally control the duty cycle of the monolithic switching power supply IC, which will increase the power consumption of the optocoupler. If CTR > 200%, the single-chip switching power supply may be triggered by mistake in the starting circuit or when the load changes suddenly, affecting the normal output

(2) linear optocoupler is recommended, which is characterized by the linear adjustment of CTR value within a certain range

(3) 4N produced by British isocom company and American Motorola Company ×× Series (such as 4N25, 4n26, 4N35) optocouplers are widely used in China at present. In view of the switching characteristics of this kind of optocoupler, its linearity is poor, and it is suitable for transmitting digital signals (high and low levels), so it is not recommended to be used in switching power supply

4. One of the methods to distinguish general-purpose and Darlington optocouplers


in general-purpose optocouplers, the receiver is a silicon photoelectric semiconductor tube, so there is only a silicon PN junction between b-e. Darlington type is not. It is composed of a composite tube, and two silicon PN junctions are connected in series to form the emission junction of the composite tube. According to the above differences, it is easy to distinguish general-purpose and Darlington optocouplers. The specific method is to dial the multimeter to R × In gear 100, the black probe is connected to pole B and the red probe is connected to pole e. the forward voltage VBE of the emission junction is calculated by reading the voltage method. If vbe=0.55 ~ 0.7V, it is a Darlington optocoupler

example: use r of 500 multimeter × The VBE of 4N35 and 4n30 optocouplers were measured in 100 gears, and the measured data and conclusions are listed in Table 2

Table 2 test results

model RBE (Ω) n` (lattice) v be (V) calculation formula test conclusion

4n35 850 23 0.6 9V be=0.03n (V) general type

4n30 4.3K 40.5 1.2 15V be=0.03n` (V) Darlington type

4.2 method 2

the main difference between general type and Darlington type optoelectric coupling is the difference in the current amplification coefficient of the receiving tube

the HFE of the former is dozens to hundreds of times, and the latter can be thousands of times, with a difference of 1 ~ 2 orders of magnitude. Therefore, as long as the HFE value is accurately measured, it can be distinguished. Precautions during measurement:

(1) because the HFE value of Darlington optocoupler is very high, the number of deflection cells of the meter needle is very close. Accurately reading the lattice numbers of N1 and N2 is the key of this method, otherwise it will cause large errors. In addition, the ohm zero point should also be adjusted in advance

(2) if the transmitting tube in 4n30 is damaged, but no fault is found in the receiving tube, it can replace the super β Tube

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