What are the functions of several resistors in the flyback power supply circuit?

kal9623287

What are the functions of several resistors in the flyback power supply circuit? [Copy link]

 

Teacher, please help me analyze this ACDC flyback power supply circuit. What are the functions of the three circled resistors? How should we choose the parameters of the three resistors?

乱世煮酒论天下

The first one looks like current limiting, and the second two are probably voltage divider sampling feedback.

Gen_X

The front one is called the starting resistor. At the moment of sending the point, the working state cannot be established, and this resistor is used to provide the "starting" energy.

qwqwqw2088

The function of the resistor R5 is mainly to control the opening and closing of the chip. It is generally around 4 MΩ. When the EN pin detects that the input current is less than 1uA, the chip disables the undervoltage protection function.

R4R6 belongs to the feedback circuit part

hjl2832

The role of R5 is purely to limit current and reduce voltage to ensure that the enable pin of the chip operates at a safe current and voltage. This can generally be found in the chip manual. The EN pin will have current requirements, so the resistance should be adjusted according to the actual current requirements and external voltage.

R4 and R6 are standard power supply reference feedback, which are used to stabilize the output voltage. This also needs to calculate the voltage divider value based on the internal reference voltage in the chip manual to determine the regulated output value of the output voltage (because PC817 is a linear optocoupler, different from the nonlinearity of ordinary optocouplers, its output and input are in a linear proportional relationship).

maychang

hjl2832 posted on 2024-6-3 08:48 The role of R5 is purely to limit current and reduce voltage to ensure that the enable pin of the chip works on the basis of safe current and voltage. This can generally be seen in the chip manual. The EN pin will...

(Because PC817 is a linear optocoupler, unlike the nonlinearity of ordinary optocouplers, its output and input are in a linear proportional relationship)

PC817 is not a linear optocoupler and its nonlinearity is very strong.

惠通科技

I saw it when I passed by and saw if I could learn some useful knowledge. It felt good and I learned something.

kal9623287

hjl2832 posted on 2024-6-3 08:48 The role of R5 is purely to limit current and reduce voltage to ensure that the enable pin of the chip works on the basis of safe current and voltage. This can generally be seen in the chip manual. The EN pin will...

Can R4 be removed?

hjl2832

maychang posted on 2024-6-3 08:55 (Because PC817 is a linear optocoupler, different from the nonlinearity of ordinary optocouplers, its output and input are in a linear proportional relationship) PC817 is not a linear...

In all power supply textbooks, PC817 is called linear optocoupler, including most electronic books, Introduction to Common Optocouplers and Common Models - Baidu Library (baidu.com) . Of course, if you insist on being entangled in true 100% linearity, there should be no such thing at present, as they all have some nonlinear characteristics.

maychang

hjl2832 posted on 2024-6-6 21:37 In all power supply textbooks, PC817 is called linear optocoupler, including most electronic books. Introduction to commonly used optocouplers and common models - Baidu...

I don't care what Baidu says. You should know that anyone can edit the entries on Baidu.

The original datasheet is more reliable.

Below is the original datasheet. Does it say "linear" in the datasheet?

PC817.pdf (489.21 KB, downloads: 0)

2024-6-6 21:51 上传

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maychang

hjl2832 posted on 2024-6-6 21:37 In all power supply textbooks, PC817 is called linear optocoupler, including most electronic books. Introduction to commonly used optocouplers and common models - Baidu...

[In all power supply textbooks, PC817 is called a linear optocoupler, including most electronics books]

Please give an example from a power supply textbook.

maychang

hjl2832 posted on 2024-6-6 21:37 In all power supply textbooks, PC817 is called linear optocoupler, including most electronic books. Introduction to commonly used optocouplers and common models - Baidu...

Let's take a look at this document, which is also an original datasheet. The first page clearly states High-Linerity Analog Optocouples.

HCNR200 201.pdf (244.37 KB, downloads: 0)

2024-6-6 22:07 上传

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