We designed an LCC series-parallel resonant converter for the energy storage capacitor of the Pulse Forming Unit（PFU）. In order to solve the problem of low current output and amplitude damping under traditional constant-current control scheme

to meet the power supply requirements of charging efficiency and pulse to pulse repeatability

we studied the characteristics of LCC resonant converter operating in discontinuous-current-mode. Moreover

the critical switching cycle curve was fitted approximately by an approximate fitting method of piecewise linear function

and the switching cycle was corrected with the constantly changing output voltage. The results show that the critical switching cycle curve can be tracked online through digital-signal-processor

the duty ratio of the output current under critical frequency is increased

and the capability of current output is improved greatly. In order to obtain a constant current output

a charging current fed digital PI control scheme was designed. The designed power supply achieves zero current soft-switching in the whole operating process

to ensure the charging efficiency and charging voltage linearity. The validity of the theoretical analysis and the feasibility of the control scheme are verified by experimental data.