There is discontinuous input current or output current in most of the isolated DC-DC converter. If continuous current is required, generally we need to add additional bulky filter, which will reduce the system's power density. In this paper, the method of adding a built-in filter to the isolated converter to achieve a continuous input current was employed. The built-in filter is composed of transformer leakage inductor and cross-coupled capacitor. Two kinds of push-pull converters with built-in filter which are derived from the voltage-type push-pull converter are proposed in this paper. The working rules and the results of PSIM simulation are given. A prototype was built in our lab to verify the synthesizing theory. The experimental results have verified the theoretical analysis.