I am trying to design a USB charger from scratch, and I have come across this schematic in the data sheet of a AC-DC converter that I would like to use (NCP1011):
Unfortunately the circuit does not have any values, but it looks like there is a transformer (with reversed polarity on the output) on the rectified DC output. Page 17 of the datasheet reveals that if you wanted a 12V output with a 120V rectified voltage, then you should use a transformer ratio of 1:0.1. So the transformer ratio is the same as if it were AC. Since I want 5V output, and I am using 240V (Australian mains power), the rectified voltage will be around 240Vdc, so I will use a transformer with a ratio of 1:0.02.
Also, why is the polarity reversed on the transformer? If the transformer is only converting the ripple, then why would you want that to be negative?
EDIT: Just realized a miscalculation. Australian mains will be closer to 339V (240 * sqrt(2)) after it is rectified, so my transformer would be something 1:0.014 (if such a thing exists)