In this paper, swop fly-back converter, a new type of fly-back converter, was proposed. It is controlled by constant-on time, a strategy of pulse frequency modulation with boundary conduction mode. Due to the structure of swop fly-back converter and the boundary conduction mode control strategy, the energy delivery to battery is proportional to the voltage of coupled capacitor of swop fly-back converter which changed the position of the metal-oxide-semiconductor field-effect transistor and the capacitor instead of measuring input voltage and input current. Thus, the energy was estimated by measuring the voltage of coupled capacitor. The previous test shows that the voltage of coupled capacitor sensing which is measured by 10-bit ADC is good. The peak voltage is 4.036 V, the ADC value is 1009 digi, and it induced average power 1.25W for striding and 2W for jogging.