When Q1 conducts, current is sourced from VIN into ground through the lower half of the primary of the transformer. This creates a negative potential at the lower half of the primary, winding with respect to the primary center tap. In order to maintain the previously established current through Q2 , which has now been opened, the upper half of the primary turns positive with respect to the center tap of the primary. This voltage is now transferred to the secondary of the transformer according to the dot convention and the turns ratio of the transformer. CR1 is now forward biased and CR2 is reverse biased; this in turn causes a current to flow through the upper half of the secondary, passing through CR1 into C charging the capacitor and back into the secondary center tap.
