In Chaps. 8 and 9 , the optimal peak current and decoupled inductor current modulation strategy are presented respectively, and the gird side current power quality improvement, reactive power optimization, transformer turns-ratio optimization and conversion efficiency are studied. But the primary side switches cannot achieve ZVS on within some operating conditions. So in order to achieve full operating range ZVS on, in this chapter, an improved modulation and variable frequency control strategy for a single-stage high frequency isolated semi dual active bridge (semi-DAB) based AC-DC converter is proposed, which can achieve a full operating range (the entire ac line cycle and full load range) zero-voltage-switching (ZVS) and high efficiency. Compared with the basic variable frequency control, improved variable frequency strategy can reduce the envelope of the leakage inductor peak current and reduce the dependence and sensitivity to the load power. More accurate ZVS criteria in view of the magnetizing inductor Lm is proposed. Seven operating modes are adopted and the transition among them is seamless. Three control variables are used in every mode. Wide voltage conversion gain and high quality grid side AC current can be obtained without mandatory current tracking loops. In addition, detailed comparisons among different control strategies and different mode operating regions are analyzed. The design of the transformer and inductor is not affected at all despite the varying switching frequency. A 250 W experimental prototype was built to verify the effectiveness of the proposed improved variable frequency control strategy.