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Flyback converter question

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hithesh123

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I am new to SMPS, I couldn't find any good articles that explain the feedback part of the flyback converter.
From what I understand, the feedback controls the pulse width of the PWM driving the switching Mosfet. If the load needs more current, the pulse width of the PWM just increases. Correct?
I was wondering if there is any way of controlling the output voltage of the converter(Other than changing the transformer).
 

The output circuit has a filter capacitor storing energy,when output voltage drops,pulse width of the PWM driving the switching Mosfet increase,consequenly, the energy charged in the flyback transformer increase,when the Mosfet is turned off,more energy dispatched to output capacitor and thus the output voltage increase.
 

Great. So output voltage can be controlled by the duty cycle of the PWM.
How does the feed back circuit work. Say the input voltage is 220V, the pulse width will be 30%. Now if the input changes to 110V, the pulse width be 60%?
How will the feedback circuit know the difference?

In my design, the primary to secondary transformer ratio is 50:10. For the feedback ckt, the ratio is 50:6(12V output). It uses a TL431 shunt regulator for feedback.
Wouldn't the TL431 reference voltage change when the input changes from 220v to 110v?
How does the TL431 work, is it like a zener diode.
 

I use ic pwm, but I get the output signal which does not square, what causes it?
please help
 

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