SMPS circuit to drop from 220V to 12V

[alec_t]

If your main purpose is just learning about SMPS you might find LTSpice (a free download) offers all you need. That program was developed with SMPS in mind and has examples which you can use as a basis for experimental simulations. Once you're happy with a simulated circuit then you can build a practical circuit for comparison.

 

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if you want to learn about smps then start with 12V to whatever-V you want, be it a buck, buck-boost, or boost, then its just a matter of getting a higher voltage in the input.
all the smps i saw always rectify the AC to DC, in my case the 22Vac to 311Vdc (220V is rms, it's 622Vp-p actualy)and then with a transistor oscilator and a mini tranformer as buck converter and isolator, to get it down to lets say 5Vdc.
ATX psus do the same but with some ICs instead of an oscilator

i started making one from a chinese atx psu parts, it was a buck-boost one, u need just an inductance, a mosfet, a 555 and a 393, and if you want to have a wide range of input voltage add a lm385 as voltage reference in the 393 and that's it, a smps
(the minimum imput is 6V as it's the minimum the 555 will work with)
do set the 555 to about 32kHz / 50%, if u set it to high from only one psu the peaks in the load (higher frecuency = higher current) will tilt the 555 making it do weird things, in my case it happenned above 75KHz.
U'll need an osciloscope for those frecuencies, but if you dont have one just an easy way to calibreate the 555 is to set a dummy load thatn will may the voltage drop some and a resistor before the inductance. the change the frecuency and the duty cyclewith some pots or 25 turns bourns till you get the higher posible voltage.