Hi People ,
This is a design I saw at here. Does this highlighted part a tiny step-up voltage converter after it has been stepped down by the transformer in order to prevent voltage drop when a heavy load is connected?

Thank you

 

that is part of conventional SMPS. it does not say if this is step up or step down. usually it is step down.
in the top left part of a transformer. which is followed by rectifier and filter.

rest of the highlighter circuit is simply part of feedback.

 

That looks like an mains-powered flyback converter. Inside your green area you have the rectifiers and filtering at the top.
Q1 and the zener next to it create a stable supply for the opto-isolator.
The resistor at the right is the top half of the feedback. There will be another resistor to ground which completes the feedback.
Just off picture to the bottom will probably be a TL431 which drives the opto-isolator.
The capacitors bottom right are frequency compensation.

 

It's a smpsu , looking at the circuit on the right it's a feedback regulator, i think you have cut off the Tl431 zener which puts on the optocoupler .