The design starts with selecting an appropriate bulk capacitor.
The primary-side bulk capacitor is selected based on the input power level and on the desired minimum bulk voltage level. The bulk capacitor value can be calculated by Equation 5.
Based on this equation, to achieve 75-V minimum bulk voltage, assuming 85% converter efficiency and 47-Hz minimum line frequency, the bulk capacitor must be larger than 127 µF. 180 µF was chosen in the design, considering the typical tolerance of bulk capacitors.