The above placed circuit by gevv is simple but it is good for only testing purpose. Simplicity is not always good in smps. The circuit is uncontrolled and risky as flyback circuit requires lot of transient suppression because in this circuit, peak current is 5.5 times than the nominal current and since no suppressor is used in this circuit, one should be very careful in testing this circuit.
tAhm1D I agree with your observation. The above circuit and the associated diagram is very good for someone who is learning about SMPS and how it operates. In the real world you'll need more than just a 555 IC.
@ gevv, how fast is the output switching? Remember the faster the output is switching the more the energy that is dissipated in the MOSFET driver. Additionally, one also needs to keep in mind the saturation of the core, which results in decreased efficiency of the SMPS as drive frequency increases.