With the great application potential of the Internet of Things (IoT), a reliable power supply solution for IoT plays an important role during the development of IoT and other advanced applications. Among all the options including the electricity grid, batteries, and energy harvesting systems, the energy harvesting systems are considered with priority for the following advantages such as longevity and maintenance-free, environmental sustainability, integration with other electronics, continuous Operation, etc. Among all the energy harvesting systems with different energy sources from the ambient environment, thermoelectric energy harvesting systems have attracted much attention from researchers due to the qualities of ubiquitous temperature difference and stable power supply. Another reason for choosing thermoelectric energy is that with a bipolar temperature difference across the material, an input voltage of either positive or negative polarity is generated to be harvested during the day and night. This is to say, with a bipolar input thermoelectric energy harvesting system, the IoT devices can be powered for a super long period of 90\% of the whole day. However, the system has to solve the problem of low input voltage from the thermoelectric generator(TEG).

To design a real thermoelectric energy harvesting system, there are several indispensable blocks including the start-up block, maximum power point tracking(MPPT), zero current switching(ZCS), and polarity detection. The start-up block is necessary to be implemented to provide a higher voltage available to turn on the transistors and help the system get rid of the cold state. The MPPT block is exploited to track the maximum power point where the equivalent resistance of the DC-DC converter is equal to the internal resistance of the TEG. The ZCS block is implemented is make sure after the discharging of the inductor inside the DC-DC converter, the power switch is precisely turned off to minimize the synchronization loss. The polarity detection block makes sure that the system always works in a proper state to harvest either positive or negative input voltage.

This article presents a 2-switch bipolar boost converter with 40 mV Photovoltaic(PV)-assisted startup voltage for a thermoelectric energy harvesting system. The power stage is configured as a bipolar boost converter for the bipolar input with the implementation of complementary transistors. And a buck-boost converter is configured for the input polarity transition. The proposed system was fabricated in a 180-nm BCD process. The measurement results show that the system can start up from the cold state with a TEG voltage as low as 40 mV. It boosts input voltages ranging from 40 mV to 0.3 V and from -40 mV to -0.3 V to a 1.2 V output voltage across the load. It achieves a peak end-to-end efficiency of 93.0\% with a 0.26V input voltage or 89.0\% with a -0.27V input voltage. Moreover, end-to-end efficiencies are higher than 80\% for input voltages from 120 mV to 0.3 V and from -120 mV to -0.3 V.