Need help with Buck+boost or SEPIC circuit

[AlxDroidDev]

Hello again.

I am trying to build a small power supply that will output 12V and 1A, with the input from a suitable wall wart. I already have that working with a LM7812, but that limits me to wallwarts that output at least 15V.

The I thought: why not have a circuit that will accept any wall wart from 6V to 20V (not precisely), and will output 12V/1A, as long as the wall wart has the proper current rating?

Then I though of 2 alternatives:
1) I have some MC34063 (and MC33063) ICs on my bin, but its datasheet only has circuits either for a buck or boost regulation, not for both. Is there any known schematic for this IC that will have a variable input and a fixed output? It will have to work as both buck and boost configuration.

2) Use a MAX668 in SEPIC configuration. Using Maxim's EE-Sim online tool, it proposed the circuit in the image below. Do you guys think this circuit will do what I want? Another thing: how the hell do I get a suitable resistor for R4 = 45.3mR ?

Thanks in advance.

[chris_leyson]

You have got several choices, buck boost (flyback), boost buck (Cuk) or Sepic which is variation of the Cuk converter, they will step up or down. The Cuk converter option would have to be transformer isolated as it is an inverting topology so not much good Sepic is non inverting. Another option might be a transformer isolated buck or forward converter, you could 1:2 step up in the transformer which would help at low voltages but at higher input voltages the duty cycle is quite low. Wurths' WEFLEX six winding transformers are useful for step up or down and also flyback. An EFD15 OR ER14 core will handle 12W easy.

[AlxDroidDev]

Thank you, Chris, and sorry for being so late for a feedback.

In the last few days I did some search on the options you mentioned, and all of them are a lot more complex than my limited knowledge allows me do to design on my own.