Important parameters of optocoupler and common design considerations

Important parameters of optocoupler and common design considerations

Optocoupler (signal transmission)

(2021.3.29) Reasons: Optocouplers are used when making fingerprint access control, and one is used for electromagnetic interference generated by the isolation relay switch (can be used, pay attention to the voltage of pin 1 )
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One place is used as a switch (I have encountered a problem, let us analyze the problem of this circuit through the understanding of the optocoupler system)

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data:

1. The advantages of optocouplers (source Baidu):

The signal is transmitted in one direction, and the input terminal and the output terminal are completely electrically isolated. The output signal has no effect on the input terminal. It has strong anti-interference ability, stable operation, no contact, long service life, and high transmission efficiency. Optocouplers are a new type of device developed in the 1970s, and are now widely used in electrical insulation, level conversion, inter-stage coupling, drive circuits , switching circuits , choppers, multivibrators, signal isolation, inter-stage isolation, Pulse amplifier circuit, digital instrument, long-distance signal transmission, pulse amplifier, solid state relay (SSR), instrumentation, communication equipment and microcomputer interface.

2. Type:

  • Linearity: It is often used in switching power supplies. In a single-chip switching power supply , a linear optocoupler can be used to form an optocoupler feedback circuit . The duty cycle can be changed by adjusting the control terminal current to achieve the purpose of precision voltage stabilization and signal reception and transfer .
  • Non-linearity (switching)
  • There are also a bunch of various balabalas about the transmission signal speed, signal type, power, and type of photodiode, which will not be introduced.

3. Technical parameters (see Baidu Library for specific parameters)

  • The technical parameters of optocouplers mainly include light-emitting diode forward voltage drop VF, forward current IF, current transmission ratio CTR, insulation resistance between the input stage and output stage, collector-emitter reverse breakdown voltage V(BR )CEO, 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.

  • Focus : CTR (Current Transfer Ratio) Ic/If

    eg. CTR of TLP181 optocoupler

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    It should be noted that: the front CTR refers to the amplification effect, the Saturate CTR is the CTR that we want in the saturation state, which is used as the switch.

    The allowable range of the current transfer ratio (CTR) of the optocoupler is 50% to 200%. This is because when CTR<50%, the LED in the optocoupler needs a larger operating current (IF>5.0mA) to normally control the duty cycle of the monolithic switching power supply IC, which will increase the power of the optocoupler. Consumption. If CTR>200%, the single-chip switching power supply may be triggered by mistake when the circuit is started or when the load changes suddenly, which will affect the normal output. 2. If an amplifier circuit is used to drive the photocoupler, it must be carefully designed to ensure that it can compensate for the temperature instability and drift of the coupler.

    The linear optocoupler is characterized by the linear adjustment of the CTR value within a certain range.

    The photocoupler used above works in linear mode. A control voltage is applied to the input end of the photocoupler, and a voltage used to further control the next level circuit is proportionally generated at the output end. The single-chip microcomputer performs closed-loop adjustment control. , To stabilize the output of the power supply.

    In order to completely block interference signals from entering the system, not only the signal path must be isolated, but also the input or output circuit and the power supply of the system must be isolated, that is, these circuits use independent isolated power supplies . For common mode interference, isolation technology is used, that is, a transformer or a linear photocoupler is used to disconnect the input ground from the output ground, so that the interference is suppressed without a loop. In the switching power supply, the photocoupler is a very important peripheral device. The designer can make full use of its input and output isolation to design the anti-interference of the single-chip microcomputer, and perform closed-loop voltage regulation and regulation on the converter.


Personal experience and summary

Two commonly used isolation

  1. The single-chip microcomputer controls the isolation of the steering gear (we often see such usage, but we all know that the signal end PWM of the small steering gear does not require a large current. Before this, my board used the IO port to output PWM directly to the signal end. Facts have proved that the interference effect is not very large, at least not obvious on stm32/stm8. What really needs us to pay attention to is that it will generate a relatively large reverse current at the moment of stalling or rapid rotation, but we When using only one steering gear, there may be no need to waste optocouplers, one zener tube may be enough )

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  2. Single-chip microcomputer optocoupler isolation relay drive (the principle is still relatively simple and will not be explained)

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  3. Switching power supply (to be learned)

The pits stepped on, the understanding of pull-up resistors and CTR

Reference: CSDN boss

​ 1. Let's go back to my circuit problem at the beginning. As shown in the figure, header 6 is a fingerprint module. The working principle of the module will not be mentioned for the time being. You only need to know that the working voltage of pin 4 is 3.3V and the working current is relatively large. (It is estimated that the rated working current is 10mA), we need to control its on and off.

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​ When we follow the circuit design of the schematic diagram. We will encounter a situation where the voltage cannot reach 3.3V. This is because we made a huge mistake. We want to use an optocoupler to transmit the power signal, and it cannot pass a large current, unless you want to add a design to follow. The amplifier amplifies the current, it is unnecessary to use the LDO enable terminal control.

  1. Here I will talk about my understanding of pull-up resistors. If this pin 4 is not a power port, but SPI communication and signal transmission, then pull-ups can be used completely, so we often hear that the pull-up resistors are all in digital circuits.
  2. Modify the circuit to use the method of pulling down to a low level and the optocoupler conducts power supply? After some changes, I tried this idea. The fact is that it doesn’t work. When I calculated the resistance of R7 and the pull-down resistor, and watched the datasheet repeatedly to understand the CTR, the voltage of its power supply port successfully reached 3.12V, but the current was far Not up to my request.

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Origin blog.csdn.net/weixin_48723396/article/details/115307353