Please tell me the circuit composed of TL431 directly and optocoupler inside the power supply

灞波儿奔

Please tell me the circuit composed of TL431 directly and optocoupler inside the power supply [Copy link]

In this circuit, the secondary circuit voltage regulator TL431 is used as the reference and feedback error amplifier, sampling the output, generating an error voltage, which is converted into an error current through an optocoupler and coupled to the primary. The main control chip generates a duty cycle through this input to control the power switch. This design uses the feedback amplifier inside the 431, and there is also an op amp in the UC3842.

Why not use the op amp inside the main controller, and directly connect the error to the output of 3842? Why is this done?

I would like to ask the power master, this design is to shorten the feedback signal time? What is the significance, or I understand it wrong, can you please briefly analyze it?

Isn't TL431 a power supply reference? How should we analyze the circuit composed of TL431 directly and optocoupler in this circuit?

The internal circuit diagram of TL431 is attached below

Below is a detailed diagram of the internal structure of UC3842

maychang

"Why not use the op amp inside the main control, and directly connect the error to the output of 3842? Why is that?"

To get the "error", we need to compare the output voltage with the reference voltage. The TL431 in the first picture is a voltage reference plus a comparison amplifier circuit. After getting the "error", the error signal is sent to the output terminal (comp terminal) of the internal op amp of UC3842 through an optical coupler.

maychang

Using the internal op amp and internal voltage reference of UC3842, the "error" signal cannot be sent to UC3942, but the output voltage (after voltage division and through optocoupler) is sent to UC3842. However, the output voltage is very uncertain after the optocoupler, the dispersion of optocoupler parameters is quite large, and the transmission coefficient of optocoupler varies greatly with temperature and decays with time.

maychang

If you want to use the internal op amp and voltage reference of UC3842 and omit TL431 to reduce cost, you cannot use optocoupler isolation.

qwqwqw2088

Why is the error directly connected to the output of 3842?

Direct connection, eliminating feedback signal transmission time and increasing response speed

Gen_X

The significance of this circuit is isolation.

As for the significance of amplification, you are right about everything you say, but none of it has anything to do with isolation.

shipeng

Your questions remind me of the time when I first started to learn electronics. As a person who has experienced it, I think your current state is exactly the same as mine. Here I would like to give you a suggestion that you should first deeply understand the principle of negative feedback. Looking at this answer from Baidu may give you a sense of sudden enlightenment https://zhidao.baidu.com/questio ... ?entry=qb_uhome_tag