How to build a variable input Voltage Power supply
This is about as simple a circuit that looks like it might work the way you want: -



About the only commonality between this and yours is the bridge rectifer if you used 1N4007 devices (rated at 1000 volts). Bare in mind that the circuit above has a lot of detail missing like the turns ratios of the high frequency flyback transformer (nothing at all like the transformer you have of course). There is also an opto coupler to sample the output side so that the TOPSwitch device can regulate the output voltage.

Here's one that uses a Viper 12 chip and this looks more reasonable in terms of all the components needed: -



You can get more details from this blog