An off-line switch-mode power supply (SMPS) gets its average energy from the rectified DC rail. So it requires a rectifier and filter stage at the input. The main outlet charges the bulk capacitor only when its peak value passes above a DC level, called ripple voltage. As a result, the time needed to charge the bulk capacitor is quasi-instantaneous because of the short equivalent of the main outlet impedance. The input current is made of recurring short pulses of very high harmonic content. Ripple voltage calculation is very important to predict the low line operation range of the SMPS. In general, the designers are used to selecting the rectifier diode and capacitor based on previous experience. But, in real condition, the filter capacitor is of great importance. In addition to the control over input ripple, it influences the diode RMS current and power factor. This paper explains the detailed design of an input rectifier.