High Temperature DC-to-DC Converters for Downhole Applications

Space in downhole applications is limited in diameter to a few centimeters due to the outer tubing, whereas the length can be up to several meters. Generally, any equipment operated in downhole ambients has a large length to diameter ratio. Any tools operated downhole such as measurement and logging equipment, pumps, motors etc. are assembled within a cylindric housing. Cooling of such systems is challenging. In arti cial lift applications the pumped uid ow can be used whereas the uid temperature equals the reservoir temperature.

To ensure a reliable operation of such a DC/DC converter system in a high temperature environment the thermal management must be su cient to keep all device temperatures below a prede ned limit (e.g. maximum allowable junction temperature). Within this work the general cooling capability of downhole power electronic systems will be demonstrated. With the help of simulations and analytical calculations the performance of di erent cooling strategies especially for active devices will be analyzed. An important result of these calculations is the maximum allowable power loss of these devices to ensure a device temperature just below the maximum speci ed limit during operation of the system.

Based on the cooling capability and the maximum allowable power loss, the system can be designed electrically to meet these thermal requirements. The maximum power loss corresponds to a maximum transferable power of the DC/DC converter system. To meet these loss limits within the devices the silicon chip area has to be oversized in comparison to applications where a higher temperature di erence between junction and ambient is possible. An additional challenge is the operation of DC/DC converters with these large silicon chip areas especially at an operation at light loads.

The goal of this thesis is to analyze the behavior of a DC/DC converter over the complete power range and to present methods to meet these challenges. To validate the thermal analysis a measurement system has been developed which is able to measure the junction temperature of MOSFETs during operation based on electrical parameters. Besides the validation of simulations this method can be used to fully utilize semiconductor components up to their thermal limit. As soon as the thermal limit is reached, the transferred power can be reduced which generally corresponds to lower power losses and consequently limits the temperature rise within the semiconductor.