Why is SMPS' bridge diode rectifier before the transformer which won't/shouldn't work having a DC input ?
Can anyone crystal clearly but succinctly explain how or which is the processes of DC step down ?

 

Show us the circuit you are talking about.

 

There are at least three different power conversion schemes I can think of that you could be talking about. Which one do you have in mind?

 

Likely the bridge rectifies the AC input voltage to provide DC for the switching circuit that drives the transformer with a square-wave.
The output of the transformer then also has one or more diodes to generate the output DC voltage.

 

220 VAC is first converted into 220 VDC. The DC is chopped to make high frequency. The high frequency chopped ac lead reduction in transformer sizing. The output of the transformer is rectified to get desired DC voltage. The ferrite core transformer is used for this application.

Read More : Ferrite core transformer and applications

 

The DC is chopped to make high frequency. The high frequency chopped ac lead reduction in transformer sizing.

Would you please elaborate it to an extend which'd be an informative modern electronic technology knowledge

 

hi abdulbadii.
Look at this link. it shows the method.
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https://www.smps.us/power-supply.html