Hi,
After reviewing viper, power int., it seems getting special type of transformer is rather inconvenient. ST L4970A may be a good option to convert 230Vac (rectifying 28Vdc), 50hz to (0 to 25Vdc and 10A). And this is fed into pin 9, Vin. The question is what current input.
Referring to some design options, it appears the input should be
V0 times I0 div by Vin (V0 and I0 are output voltage and current) which is
25x10/40 =6.25, convert this to rms current 6.25/1.414 = 4.42 Amp ac.
Hence transformer VA is 28 x 4.42 about 124VA.
As this is the first time I am using this type of switching regulator, welcome any one who has this experience to review and comment.