>> ... The method simply consists of PWMing an input (usually mains) 
>> 50/60 Hz AC waveform so that its amplitude is effectively reduced. 
>> The reduction is NOT achieved by converting to DC and then using 
>> high frequency Ac to reconstitute the waveform. There is no 
>> intentional flyback or energy storage at high frequency in the 
>> following transformer.

> Well, if we can do this then can we also discard the transformer, 
> pass the reduced voltage AC through a full-wave rectifier and get 
> controlled DC?  I guess the downside is that there is no isolation. 
> Is adding isolation with a 1:1 transformers, and are they  cheap?

Yes, you could definitely (try to) do this transformerless, but there 
are large advantages in doing it with a transformer when peak output 
voltage is much lower than mains voltage. If you want a 25 amp output 
at 2 volts then if you switch direct off mains you are trying to draw 
25 amp peaks for short periods off the mains. This is effectively 
supplied by the filter, but it's not nice. If you supply the 2.5V 
demand from say a 3 or 4v supply then the current drawn at peak load 
matches more what you'd see when the switch was run at 100% duty 
cycle. High power short term current peaks are much reduced.



        RM

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