A novice asks for advice, how can a switching power supply achieve a regulated output without using a 431?

太白金星

A novice asks for advice, how can a switching power supply achieve a regulated output without using a 431? [Copy link]

Please ask the experts: As shown in the figure below, the output is actually measured to be 30V. How to achieve this? Thank you!

maychang

There is a voltage regulator D12 on the right side of your picture, marked as 24V. This voltage regulator acts as a voltage reference. Of course, this method is quite inaccurate .

qwqwqw2088

U2 optocoupler and voltage regulator 24V work

okhxyyo

maychang published on 2020-5-26 11:22 There is a voltage regulator D12 on the right side of your picture, marked as 24V. This voltage regulator acts as a voltage reference. Of course, this method is quite inaccurate.

So how can the voltage be stabilized more stably?

maychang

okhxyyo posted on 2020-5-27 05:15 How can the voltage be stabilized more stably?

The diagram is not unusable. The working principle of this diagram is: the output voltage of the switching power supply (the upper end of R22 to the lower end of D12) increases for some reason --> the current flowing through U2A increases --> the current flowing through U2B increases --> the FB voltage decreases --> the duty cycle of the control chip output PWM decreases --> the output voltage of the switching power supply decreases.

maychang

okhxyyo posted on 2020-5-27 05:15 How can the voltage be stabilized more stably?

In the original poster's circuit, D12 has no amplification effect. If TL431 is used, because TL431 has a fairly stable voltage reference inside, and TL431 has an operational amplifier inside, the loop gain (from the upper end of R22 to the lower end of D12, through the optocoupler and the amplifier inside the control chip, and then the output voltage is controlled by the PWM duty cycle) is much larger than simply using the voltage regulator D12, that is, the negative feedback is very strong, and a small change in the output voltage can cause a large change in the PWM duty cycle, and the output voltage deviation value is relatively small, that is, the change in the output voltage due to load changes and other reasons is relatively small. The output voltage is more stable than simply using D12.

maychang

okhxyyo posted on 2020-5-27 05:15 How can the voltage be stabilized more stably?

The original poster only posted a small part of the circuit, and there may be errors in this small part.

The FB pin must be a voltage input pin. Even if it has an internal pull-up, the current is very small. The original U2B in the original image does not have a power supply or a pull-up resistor. The pull-up inside the chip is not enough to make the optocoupler work properly.

maychang

okhxyyo posted on 2020-5-27 05:15 How can the voltage be stabilized more stably?

Another reason why the original poster's circuit is inaccurate is that the temperature coefficient of the voltage regulator D12 is much larger than the temperature coefficient of the internal voltage reference of TL431. In this way, when the ambient temperature changes, the output voltage will also change accordingly.

okhxyyo

maychang posted on 2020-5-27 09:18 Another reason why the OP's circuit is inaccurate is that the temperature coefficient of the voltage regulator D12 is much larger than the temperature coefficient of the internal voltage reference of TL431. In this way, in the environment...

Great~~Thank you teacher for clearing up my doubts! I have learned a lot~

maychang

I didn't find the manual of the power control chip OB2283 in the original picture, and of course I couldn't find the characteristics and typical application circuit of the chip. I think it is a domestic chip modified from a foreign chip.

木犯001号

The feedback network composed of D12 and optocoupler has similar functions to 431. Assuming that the output voltage increases, the current of U2 increases, and the output current of the optocoupler increases according to the transmission ratio, the voltage of resistor R8 increases to realize feedback.