Take the MIT Course on edX, on Circuits and Electronics. I think it comes in 3 or 4 parts now. That's the best place to start. It has a textbook, available online for free reading to those who sign up. Then, take the Power Electronics course on Coursera, by University of Colorado Boulder, that has a textbook on designing Power Electronics circuits, that's fairly easy reading, once you've completed the MIT course. The Power Electronics course has a recommended textbook that is not free, but it's worth the money. The professor asks you not to download any copies of this book that you might find around the internet, because they are pirated versions. So, do buy the book to support the prof and his team. Those two courses, and textbooks that go along with them are a good start.