Abstarct: Implantable medical devices in the body need a power supply. These devices typically use small batteries implemented on them. The challenge of these devices is the limited energy of their batteries, which must be recharged or replaced every few years, and this requires surgery. The best way to power these devices is to use the body's natural energy sources. The human body is a good source of heat and the extraction of this energy can be the solution to this challenge. In this research, a power supply to power implantable devices has been designed and simulated with using thermal energy extraction circuit methods and a thermoelectric generator has been used to convert body heat energy into electrical energy. Depending on the temperature difference between the surface of the skin and the inside of the body, this generator can produce a voltage of about 20 to 200 mV (20 to 2000 µW). Because this voltage is too small to charge the batteries of implantable devices, a boost converter is designed to convert this voltage range to 1.8V. Optimal boost converter design has many challenges and the main reason for these challenges is converter losses. In recent research, MPPT and ZCS control methods have been commonly used to improve efficiency. In this research, by controlling the maximum inductor current with using the PFM method, the losses are reduced and a boost converter with high efficiency is presented. The proposed converter is designed and simulated in 180 nm technology and Cadence software. The minimum efficiency of this converter at 20 mV input voltage is 60.05% and its maximum efficiency is 92.52%. The achievements of this research include extracting and analyzing the equations of converter losses and optimal inductor current, providing a practical method for their implementation and designing a ZCS structure with low power consumption.