Good explanations, Russell. It is very difficult to directly
derive anything above 48V to something around 1V. The efficiency 
suffered a lot here due to low duty ratio.

This is the same in the VRM world. 5V (1st generation Pentium I/II)
VRMs are easier, 12V VRMs are much more difficult (Pentium III/IV)
but can still be achieved without a transformer. In 48V VRMs (
server/workstation/dual core??), you need a transformer here if
not resorting back to (48V -> 12V --> 1V). Efficiency is the 
key here.

Sorry about the VRM stuff if you think they are not relevant here
but I think they are really related.

Xiaofan

 
>Date: Tue, 01 Feb 2005 16:32:27 +1300
>From: Russell McMahon <apptech@paradise.net.nz>
>
>>> ... 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.
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