ADVANCED BASE DRIVERS FOR SILICON CARBIDE BJTs
POZO ARRIBAS, ALEJANDRO
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This thesis focuses on the optimization of base drivers for SiC BJTs and presents a novel driver topology that targets minimum power consumption. SiC BJTs have been studied for over a decade, during which time, they have been proven to have superior performance than Si IGBTs and even other normally-off SiC devices such as MOSFETs. Despite this, SiC BJTs are the least popular among the family of SiC power switches. As current controlled devices, BJTs require a continuous sup- ply of current through the base during the on-time. And, even though current gains over 100 have been reported, the base current required translates into a considerable amount power consumed by its driver, compared to its competitors. This power can affect the overall efficiency of a converter if the driver circuit is not designed properly. Since, the driver represents a key system for the success of SiC BJTs as power semiconductor devices, this thesis conducts a comprehensive evaluation of previous solutions and an analysis of the driver power losses to identify the optimal driver configuration. As a conclusion of this study, a novel topology is proposed, designed and built for its latter validation through experimental tests. The proposed solution allows the replacement of a SiC MOSFET or Si IGBT and driver with a SiC BJT and driver without the need of a current sensor or a dedicated DSP/FPGA. The driver power consumption is minimized with a proportional base current design based on a MHz synchronous buck converter operating as a Class D amplifier. This switched mode power amplifier uses a reference signal to provide a voltage that causes a base current proportional to the instantaneous collector current. The reference signal is generated with a high bandwidth sensor that measures the instantaneous voltage drop across the BJT (vCE) during the on-time. Hence, current sensors are avoided. Different alternatives for a voltage sensor are discussed and analyzed through simulations and experimental results. Moreover, the use of vCE to estimate the instantaneous collector current makes the proposed driver a temperature-sensitive design. For the first time, a proportional base current driver generates a base current proportional to the instantaneous collector current taking into account the effect of temperature on the DC current gain. Moreover, all this is achieved with solely analog electronics in a standalone solution. A 1.5kW Boost converter was built to validate the proposed driver under different collector currents and operating temperatures. In order to show the performance improvement offered by the proposed solution, the same Boost converter was operated with a commercial base current driver. This exercise showed a reduction of the driver power consumption by up to a factor of 4 without affecting the efficiency of the Boost converter. The switching behavior of a SiC BJT operated with the proposed driver and some of its limitations are discussed. These have, in fact, motivated additional research to develop efficient, isolated MHz regulators for faster operating frequencies of the SiC BJT. In addition, a new over-current protection integrated into the proposed driver is suggested and tested with interrupt times of less than 500ns for a collector current of 50A.