Application of the hottest optocoupler in digital

The application of optocoupler in digital switching power supply

with the rapid development of switching power supply technology and green power supply, APFC technology has become the focus of current research. Electronic switching power supply technology has been mature, and there are quite a few control methods. At present, people are carrying out the research and development of digital switching power supply, and digital switching power supply products with power factor correction have been listed. For digital switching power supply, isolation technology and anti-interference technology are very important. With the rapid development of electronic components, the linearity of optocoupler is getting higher and higher. Optocoupler is the most used isolation anti-interference device in MCU and switching power supply at present. Optical coupler (abbreviated as OC) is also called photoelectric isolator or photoelectric coupler, which is called optocoupler for short. It is a device that uses light as the medium to transmit electrical signals. Usually, the light emitter (infrared light-emitting diode LED) and the light receiver (photosensitive semiconductor tube) are packaged in the same shell. When an electric signal is added to the input end, the light emitter emits light, and the light receiver receives the light and generates photocurrent, which flows out of the output end, thus realizing the "electric optical electric" conversion. Optocoupler, which uses light as the medium to couple the input signal to the output, is widely used in digital circuits because of its small size, long service life, contactless, strong anti-interference ability, insulation between output and input, unidirectional signal transmission and other advantages. Because of its nonlinearity, the application of optocouplers in analog circuits is limited to the isolated transmission of small signals with higher frequencies. Ordinary optocouplers can only transmit digital (SWITCHING) signals and are not suitable for transmitting analog signals. In recent years, linear optocouplers can transmit continuously changing analog voltage or analog current signals, which greatly expands their application fields

performance characteristics of optical coupler

the main advantages of optical coupler are unidirectional signal transmission, complete electrical isolation between input and output, strong anti-interference ability, long service life and high transmission efficiency. It is widely used in level conversion, signal isolation, interstage isolation, switch circuit, long-distance signal transmission, pulse amplification, solid-state relay (SSR), instrumentation, communication equipment and microcomputer interface. Because the input impedance of the optocoupler is relatively small compared with that of the general interference source, the interference voltage of the partial voltage at the input end of the optocoupler is small, and the current it can provide is not large, so it is not easy to make the semiconductor diode emit light except its own tube; Since the shell of the optocoupler is sealed, it is not affected by external light; The optocoupler has a large isolation resistance (about 1012 Ω) and a small isolation capacitance (about a few PF), so it can prevent electromagnetic interference caused by circuit coupling. The optocoupler working in linear mode applies a control voltage to the input end of the optocoupler, and a voltage for further controlling the circuit of the next stage will be generated in proportion at the output end. The linear optocoupler is composed of light-emitting diodes and photosensitive triodes. When the light-emitting diodes are turned on, the photosensitive triodes are turned on. The optocoupler is current driven and needs a large enough current to turn on the light-emitting diodes. If the input signal is too small, the light-emitting diodes will not turn on, and its output signal will be distorted. In switching power supply, especially in digital switching power supply, linear optocoupler can be used to form optocoupler feedback circuit, and the duty cycle can be changed by adjusting the current at the control end to achieve the purpose of precise voltage stabilization

the technical parameters of the optocoupler mainly include the forward voltage drop VF of the LED, 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

current transfer ratio is an important parameter of optocoupler, which is usually expressed by DC current transfer ratio. When the output voltage remains constant, it is equal to the percentage of DC output current IC and DC input current if. The formula is:

for the optocoupler with a photosensitive triode, the CTR range is mostly 20% - 300% (such as 4N35), while the PC817 is 80% - 160%, and the Darlington optocoupler (such as 4n30) can reach 100% - 5000%. 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 typical ctr-if characteristic curves of linear optocoupler and ordinary optocoupler are shown as the dotted line and solid line in Figure 1 respectively

Figure 1 ctr-if characteristic curve of the two optocouplers

it can be seen from Figure 1 that the ctr-if characteristic curve of the ordinary optocoupler 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

the optocoupler is mainly used to provide isolation between input circuit and output circuit. When designing the circuit, the following principles must be followed: the selected optocoupler must comply with domestic and international standards on isolation breakdown voltage; 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, which is suitable for transmitting digital signals (high and low levels), and can be used for the output isolation of single chip microcomputer; The selected optocoupler must have a high coupling coefficient

products and selection principles of linear optocouplers

typical products and main parameters of linear optocouplers are shown in Table 1. These optocouplers use photosensitive triodes as receiving tubes

in the isolation of switching power supply and the design of optocoupler feedback switching power supply, the model and parameters of linear optocoupler must be correctly selected. In addition to the selection principle of ordinary optocoupler, the following principles must be followed:

1. The allowable range of current transfer ratio (CTR) of 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. If the optocoupler is driven by the amplifier circuit, it must be carefully designed to ensure that it can compensate for the temperature instability and drift of the coupler

3. Linear optocoupler is recommended, which is characterized by linear adjustment of CTR value within a certain range

under the working mode of the optocoupler used above, add a control voltage to the input end of the optocoupler, and a voltage for further controlling the next level circuit will be generated in proportion at the output end. It is the closed-loop regulation and control of the single chip microcomputer, which plays a role in stabilizing the power output

design of optocoupler circuit in switching power supply

in the switching power supply in Figure 2, the voltage looping we use will also be the line closed-loop regulation that our manufacturer does not want to see to achieve the stable output of output voltage. Ps2501 optocoupler of NEC company is selected as the isolation device between input sampling, feedback signal, output drive and single chip microcomputer. On the one hand, optocoupler can isolate the ground wires of two systems, Make the power supplies of the two systems independent of each other, and eliminate the influence of different ground potentials. On the other hand, the LED of the optocoupler is a current driver, which can form the transmission form of the current loop. The current loop is a low impedance circuit with low sensitivity to noise, which improves the anti-interference ability of the system, plays the role of electromagnetic compatibility and isolation anti-interference, and will not interfere with the single chip microcomputer due to the electromagnetic interference of the high-frequency current in the circuit, At the same time, the damage caused by the wrong triggering of the switch tube caused by electromagnetic interference is eliminated, and the linearity is also better. Amway invested to build a new production base in Vietnam. In Figure 2, the single chip microcomputer and analog circuit use independent power supplies respectively. COM1 and com2 are two different power supplies to eliminate the interference of analog circuit to the single chip microcomputer

Figure 2 Application of optocoupler in digital switching power supply

tl431 is a reference voltage regulator circuit, which can be used as a programmable reference amplifier with low temperature coefficient, allowing a large injection current of 100mA. There is a 2.5V reference voltage inside the TL431, so its reference input voltage can be provided by the voltage division of the DC output voltage, which can make it present a good working state. It has very low output noise and a temperature coefficient of only 50ppm/C. It is ideal to be used as a reference power supply. In Figure 2, the resistance capacitance complex composed of resistance R11 and capacitance C6 is essential in practical application circuits, which is mainly used for product rate compensation

in order to completely block the interference signal from entering the system, not only the signal path should be isolated, but also the input or output circuits should be isolated from the power supply of the system, that is, these circuits use mutually independent isolated power supplies. For common mode interference, isolation technology is adopted, that is, using transformer or linear optocoupler to disconnect the input ground from the output ground, so that the interference has no loop and is suppressed. In the switching power supply, the optocoupler is a non important peripheral device. The designer can make full use of its input and output isolation to carry out anti-interference design for the single chip microcomputer, and carry out closed-loop voltage regulation to reduce the dead weight of the converter. (end)