A power conversion circuit is provided. The power conversion circuit includes a square wave generator, a resonant tank, a transformer and a controller. The square wave generator is for generating a square wave power. The resonant tank is for receiving the square wave power and outputting a first resonant power. The resonant tank includes a capacitor, an inductor and a controllable capacitive device. The transformer is coupled with the controllable capacitive device for receiving the first resonant power and outputting a second resonant power. A rectified second resonant power is provided to a load. The controller is for generating a switching signal to the controllable capacitive device based on a comparison result between a threshold signal and a signal calculated based on at least one detected signal of the load. A method for controlling a power conversion circuit and a resonant tank are also provided.