The main purpose of bulk capacitors on the input of a design is to control input spikes when there is an output current transient. It is important to keep in mind that when there is an output load transient this energy must come from the input of the regulator. Bulk capacitors can control the voltage ripple at the input when the converter is responding to a load transient. The size of the capacitor is determined by the size of the potential output current transient and the allowable input voltage ripple of the design specification. The necessary capacitance needed can often be quite large and therefore Electrolytic Capacitors are often used for bulk capacitance.