I've been trying to educate myself on general power supply operation and stumbled across several schematics for ATX power supplies. In general I understand how a SMPS operates, and an ATX PS is just one example. However, in reference to the following schematic, I am totally stumped on several fronts:
Why is the primary of T1 fed off the un-rectified input 'AC2'? Why should it not be connected to the + output from the bridge rectifier? From what I've seen on other SMPS, the primary side on the main switching transformer is fed by a DC source and switched on/off at several kHz usually by a low side power NFET.
I also don't understand the point of coupling the un-rectified AC line signal (through ZNR1) into the center point of the filter network formed by C1/C2/R2/R3
How can any current flow in T1's primary at all? Cap C9 blocks all DC on T1's primary.
I understand T2 is an isolation transformer; Q6 and Q5 run off the PWM from the controller, and by way of T2 provide current to turn driver transistors Q3 and Q4 on and off. What I don't understand is what's happening as a whole on T2's secondary: Is the pin #10 a center tap? An off-center tap? Why not just have one NPN turning the current through T1's primary on and off? Why is pin 5 connected to the lower half of T1's primary?
I understand I am lacking some basic knowledge here about how this works, so if you may be so kind please direct me to books or other tutorials on what is happening here. Thanks!