A while back I made a supply much like this.
I rectified and filtered the power line to get 140VDC. The built a small isolated 0 to 30V DC supply that sat on top of the 140V. In this case we never needed to go below 140 and never above 170. So the switching supply was very simple and small. (low voltage parts)
ronsimpson , Would you show how this was done, please? Didn't want to hijack the thread but do want to know how this is done, for an application of mine. Everyone says it won't work, that the highest voltage source will cancel the lowest. I need to have a continuous 100VDC with a periodic 170VDC spike. Any help in doing this will be much appreciated.