Generally SMPS topology involves some fixed standard stages and criteria. Thgey may be listed in the following manner:
The first stage which is the input stage incorporates an obvious mains rectifiers stage, followed a few important protection components.
The above protection components may be in the form of an MOV , or an NTC or both of these for suppressing high voltage transients.
The next stage involves a mosfet based IC in conjunction with the primary of a small transformer for generating the required oscillations.
The IC is normally a state-of -the-art chip having many in built features and abilities.
Further on the secondary of the transformer is clamped with the mosfet IC through a optocoupler which takes the responsibility of controlling the output voltage to a predetermined fixed level.
However the proposed circuit of a cheapest SMPS circuit is rather free from all these complications and employs a very simple configuration.
The input does not involve any protection, which is rather replaced with the snubber network around the transistor. Moreover the rugged MJE13055 is assumably strong enough to take on most the situations.
The two winding on the primary side are so arranged that on switch ON the circuit immediately starts oscillating at around 100 kHz.
The secondary winding typically decides the output voltage and here no optos or zeners are introduced for the sake of simplicity.
Having said that, the circuit may be considered quite crude and therefore in some way might be vulnerable at some point of time in the long run.
Here's another identical simple 220V SMPS circuit design you would like to investigate: