I haven't built a ballast like you are talking about, have only had experience with old magnetic ballast HID ballasts, and they were Halide type bulbs. HPS is of course different in starting parameters. Lots of companies building electronic ballast, but not many schematics on these newer microcontroller based designs out there on the net in my looking around. The only place I can think you might find usefull info to check out is international Rectifier for ideas. Here is an article from their site
http://www.irf.com/pressroom/articles/683EPD1005.pdfWhole section of their site devoted to this area here to look through
http://www.irf.com/product-info/lighting/Few other link from IRF site
http://www.irf.com/technical-info/appnotes/an-995a.pdfhttp://www.irf.com/pressroom/articles/667ede1006.pdfMicrochip has a reference design, but it's for an automotive HID bulb setup, not HPS and not dimmable. In case you might be interested in having a look
http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1406&dDocName=en553545