A novel high efficiency transformer-based flyback dc-dc converter with maximum power point tracking (MPPT) and zero current switching (ZCS) control is proposed in this paper. By adding a new primary side, the transformer works in flyback mode and the ordinary n-well process is used rather than twin-well. The startup process is divided into two modes: oscillation mode and constant duty-cycle flyback mode, greatly reduce startup time. The MPPT and ZCS control block work periodically, maximizing the energy delivered to the output capacitors. And the control methods are based on successive approximation digital logic. This converter is implemented in UMC 0.18μm CMOS process. Simulation results show that the current consumption of controller is only 0.6μA. Compared to previous work, the efficiency increase from 61% to 91% at V_IN=300mV, and from 35% to 75% at V_IN=20mV.