switch mode power supply How does this circuit regulate voltage

Posted on Feb 6, 2014

An old Nokia phone charger with 5. 6 V output, so I tried to adjust it to 12 V. By changing the Zener diode though I manged to do that, but I can`t understand its circuit (I replaced the 9v1 with 16v Zener). I would really appreciate any help to understand it. I already understand most of the circuit, but the feedback and switching action isn`t clear. i think Q1 with the upper winding and R5 manage the switching

switch mode power supply How does this circuit regulate voltage
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action of the half rectified ac voltage (by D1 and C1). Q2 with the lower winding sense the voltage as when the D1 break conducting current to Q2 base which in turn will open conducting current away from Q1 base so as to turning it off. but what i miss what the remaining components do(C3, R7, C2, R4, C4, D2) Mostfa Mahmod Mar 25 `13 at 7:56 I`m sorry but i cant figure why it isn`t isolated from line voltage i think it is isolated by the means of T1. and thank you for your time. Mostfa Mahmod Mar 25 `13 at 8:11 According to the schematic, the secondary winding of T1, diode D3 and capacitor C5 are isolated. The rest of the circuit isn`t. Kaz Mar 25 `13 at 8:18 i think because it is a cheap small power supply so it was more cost efficient to isolate the output circuit only not the switching circuit. however isolation isn`t my concern now as i seek understanding the switching action Mostfa Mahmod Mar 25 `13 at 8:36

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