A flyback converter high voltage supply by 555

The circuit diagram of the flyback converter will hold no surprises (Fig.16). Literally the only difference with the boost converter is that the inductor is replaced by a transformer, and that the transistor has been replaced for a BUZ21. The BUZ21 has a much lower on resistance (Ron=0.085 ohm) as compared to the BUZ41A (Ron=1.5 ohm) but also a lower drain-source breakdown voltage (100V versus 500V)