A Buck-Boost transformerless DC-DC converter based on IGBT modules for fast charge of electric vehicles

A transformer-less Buck-Boost DC-DC converter in usage for the fast-charge of electric vehicles, based on powerful high-voltage IGBT (Isolated Gate Bipolar Transistor) modules is analyzed, designed and experimentally verified. The main advantages of this topology are: simple structure on the converter’s power stage; a wide range of the output voltage, capable to support nowadays vehicles on-board battery packs; efficiency and power density accepted to be high enough for such class of hard-switched converters. A precise estimation of the loss, dissipated in the converter’s basic modes of operation – Buck, Boost, and Buck-Boost is presented. The analysis shows an approach of loss minimization, based on switching frequency reduction during the Buck-Boost operation mode. Such a technique guarantees stable thermal characteristics during the entire operation, i.e. battery charge cycle. As the Buck-Boost mode takes place when Buck and Boost modes cannot support the output voltage, operating as a combination of them, it can be considered as critically dependent on the characteristics of the semiconductors. With this, the necessary duty cycle and voltage range, determined with respect to the input-output voltages and power losses, require additional study to be conducted. Additionally, the tolerance of the applied switching frequencies for the most versatile silicon-based powerful IGBT modules is analyzed and experimentally verified. Finally, several important characteristics, such as transients during switch-on and switch-off, IGBTs voltage tails, critical duty cycles, etc., are depicted experimentally with oscillograms, obtained by an experimental model.