Can you help with some circuit?

[salova322]

Hello, can you please explain the purpose of some parts of the circuit? Why are they needed and are they needed at all?

This circuit was generated on a Texas Instruments site. 12V 8A, uc3844a. Q1 is a 9 V stabilizer, but why they included it in the circuit, I saw 12 V circuits in which it does not. The circuit with a zener diode of 0.55 volts. I do not understand why it is here.

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https://imgur.com/a/IWWg8

 

[betwixt]

Its difficult to see on the diagram but I think the 'wings' on the diode symbol are indicating it's a Schottky diode not a Zener. The voltage would then make more sense.

Brian.

 

[salova322]

I do not know what happened to the topic.

Hello, can you please explain the purpose of some parts of the circuit? Why are they needed and are they needed at all?

This circuit was generated on a Texas Instruments site. 12V 8A, uc3844a. Q1 is a 9 V stabilizer, but why they included it in the circuit, I saw 12 V circuits in which it does not. The circuit with a zener diode of 0.55 volts. I do not understand why it is here

https://imgur.com/a/IWWg8

It's Shottky, ok.

 

[d123]

Hi,

I'm guessing so no doubt wrong about both interpretations. Is the Schottky diode intended to protect the opto LED? Is Q1 performing some kind of (additional) voltage regulation by its turning on more when voltage rises and so sinking more and vice versa? They're my best guesses, unless Q1 is for current regulation, but as I said, I think its for voltage regulation.

 

[Pudu]

The 8.5V regulator around Q1 provides the feedback circuit with a regulated and relatively clean supply voltage. Strictly it shouldn't be necessary, but maybe without it there was too much switching frequency ripple making the optocoupler transmit a pulsing signal that caused instability in the regulation loop...

The circuit around D21 turns on the optocoupler when the output voltage is rising, just after switch-on. So this provides a soft-start function. Once the power supply is up and running, it has no further effect.

And the circuit around Qsc provides slope compensation, required for loop stability in this kind of circuit, whenever the duty cycle can exceed 50%. Google for slope compensation to understand this matter. It's interesting.

Manfred