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Basic SMPS power supply design

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samEEEf

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I want to design a basic power supply using flyback or bridge converter topology. It seems okay from my understanding point of view if the input voltage is low (say 12 or 24). But what usually do in the practical design when the input voltage is the line voltage (230v ac)? As we will have to make the power supply from the line supply, how the gate control circuit will be powered up?
 

treez

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Typically it powers up slowly from a bleed resistor into a vcc capacitor, then the chip starts switching, and just before the vcc cap discharges too low, the bias winding of the flyback powers up and comes to the rescue and starts powering the controller from there onwards.........

or here is a external start up chip for a offline flyback..LR745

https://ww1.microchip.com/downloads/en/DeviceDoc/20005394A.pdf
 

treez

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if you use from power integrations then they have the PI expert software which will design it all for you and give the schem so you can see it all...you download the free software and it do it all for you.
 

samEEEf

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PI expert uses their own chip. Actually I wanted to learn to design it myself using some power semiconductors (mosfet, igbt ) generating gate signal from microcontroller.
 

treez

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ok but then you have to do the error amplifier and the pwm comparator...microchip actually have a chip though that has them and all you do is shovel in the oscillator signal with a micro
 

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