Seems they first full rectify mains. Stepping voltage way up to next bringing down to 5V ? Is there a reason why a half way rectifier is not used instead ?

 

Is there a reason why a half way rectifier is not used instead ?

Because the input capacitor must be much much larger with half wave rectifier.

 

Switch mode power supplies. Much more efficient than linear supplies and cheaper/smaller because of the relatively tiny transformer needed for high frequencies.

 

Assuming a large enough smoothing capacitor, the voltage would be the same.

 

Because the input capacitor must be much much larger with half wave rectifier.

Isn't the size also dependent on voltage, not only capacity ?

 

Assuming a large enough smoothing capacitor

Those tiny common 5V supplies

Tiny and Large are opposites.

Isn't the size also dependent on voltage, not only capacity

Yes, Full rectify and half wave gives you about the same voltage.

 

Likely a full-wave bridge is smaller (and possibly cheaper) than the larger capacitor needed for a half-wave rectifier at the same current level.

 

Isn't the size also dependent on voltage, not only capacity ?

 

Thosetiny 5 volt supplies that work adequately are proof that the FW bridge and a tiny switcher work well. REALITY WINS!!!

 

Seems they first full rectify mains. Stepping voltage way up to next bringing down to 5V ? Is there a reason why a half way rectifier is not used instead ?

View attachment 363969

Probably because it is illegal. it is in most countries under EMC regulations.
And regardless of whether if it full- or half-wave rectified, the peak rectified voltage will be the same, just with more ripple.

 

The mains voltage IS NOT "stepped up", but it rises towards the waveform peak value. Then it feeds a switching regulator that chops it into narrow pulses that are filtered to produce the required output voltage. Of course the design of the switching regulator circuit that does al of that requires a whole lot of careful design effort. Not many projects are more challenging than designing a good switcher supply. EVERY PART MATTERS!!

 

EVERY PART MATTERS!!

And the layout is critical.

 

5 V cube charger schematic diagram:
https://danyk.cz/reverz28_en.html

 

https://www.righto.com/2012/05/apple-iphone-charger-teardown-quality.html

 

Here's an interesting filter circuit, just right for an SMPS converter, as it can operate over a wide range of supply voltages.
If the mains voltage is 230VAC, the voltage at the filter output varies from 320 to 160 volts, and within these limits, the converter can operate normally.
At the same time, the voltage across electrolytic capacitors changes little if their capacitance is sufficiently large, and this is a good conditions for electrolytic capacitors, which are sensitive to large ripples.
Power factor is high.

Valley-fill circuit - Wikipedia

 

The optocoupler on post #13 'applies the brakes' to 13001, to regulate output voltage to desired setting per the zener or if unloaded, I think. What is the 1nF/1KV capacitor above it for? Doesn't it go against isolation by exposing mains to touch ?

 

What is the 1nF/1KV capacitor above it for?

It shorts out switching noise. Which is pretty bad in this switcher. (EMI filter)