The RC across the transformer is not good, it will cause high losses in the transistor (should be a diode off the collector and then an RC in parallel to the top of the transformer - but not 1.1uF this is far too high), driving a transistor direct from the 555 is no good as there is no limiting resistor for the base current, the transistor is not very efficient in this circuit, better to use a mosfet (logic level to accept the 5V drive), What is the value of Vcc? how much power do you want to get on the output side? What is the primary inductance of the transformer? What peak current has the primary been designed for? (peak flux density?) If the transformer is saturating due to the low frequency then you will get additional losses, also the diode in parallel with the transistor serves no purpose. Regards, Orson Cart.