October / November 2021

https://toshiba.semicon-storage.com/ POWER CONVERTERS 13 www.power-mag.com Issue 4 2021 Power Electronics Europe Obtaining Highest Efficiency in DC/DC Converter Applications While improvements in Silicon power devices have been incremental, the introduction of wide-bandgap devices, such as Silicon Carbide (SiC), allow a jump in performance to be attained. Thanks to reference designs, such as Toshiba’s bidirectional DC/DC power supply, design engineers can significantly speed-up evaluation of suitable design approaches and topologies and get up to speed on the intricacies of using SiC MOSFETs. Dr. Matthias Ortmann, Chief Engineer, Application Support,Toshiba Electronics Europe As the world weans itself from its dependency on fossil fuels, there has been a focus on innovative electronic systems to deliver clean, efficient electrical power. Government initiatives to reduce vehicle emissions have seen the automotive industry move to electric drivetrains. This requires a charging infrastructure that is efficient and robust to keep this method of mobility on the move. Electricity generation has also moved to renewable sources of energy, such as solar and wind. Unlike fossil fuel and nuclear alternatives, such power generation is dependent on the weather and time of day. Since these do not always match with grid demand, storage of energy such sources in batteries helps to improve its effectiveness in the energy supply mix. Over the years, Silicon devices have advanced enormously, demonstrating continuous improvement in their capabilities. Microcontrollers offer clever pulse-width modulated (PWM) timers coupled with synchronous analogue to provide engineers with highly customizable platforms that can be programmed to meet exacting power conversion needs. Simultaneously, Silicon power devices have been optimized in terms of on-resistance and their parasitics to minimize their losses. A jump in performance SiC MOSFETs offer significant improvements in switching losses when compared to silicon IGBTs. Thanks to the high drain- source voltage supported, they are increasingly displacing IGBTs in power factor correction and other high-voltage power conversion stages. The SiC-based integrated diode incorporated into these devices is also support high surge currents, making them a robust component in the design. Perhaps the most desirable characteristic is their high switching speed compared to IGBTs. Not only does this significantly reduce turn-on and turn-off losses, it allows higher switching frequencies to be used. In turn, this leads to a reduction in size of inductors, resulting in compacter designs when targeting the same output power compared to IGBT- based converters. Under the same conditions, the Toshiba TW070J120B SiC MOSFET has a turn-on loss of just 0.6 mJ compared to a similarly specified IGBT, which required 2.5 mJ (Figure 1). Bidirectional DC/DC converters Bidirectional DC/DC converters enable power stored in batteries to be used for other purposes once charging is complete. For EVs, there is interest in providing Vehicle-to-Grid (V2G) capability, allowing vehicles to provide power during outages or even to stabilize the grid locally when required. Renewable energy plants also make use of this capability, storing energy generated during optimal weather conditions and delivering back to the grid when it is required. As a result, fossil-fuel power sources, such as diesel generators, are required less often or not at all. Efficiency is essential in such designs. One approach to construct two separate converters, each dedicated to the needs of the application, but this results in a bulky solution with a high component count. To achieve higher power density, designers turn to the Dual Active Bridge (DAB) topology (Figure 2). This allows the use of soft-switching, a lower device count, attain high efficiencies, while also providing galvanic isolation – often a critical design requirement – at a more attractive total system cost. The DAB topology consists of two full- bridges connected by an inductor and a high-frequency transformer (Figure 3). The transformer’s primary and secondary windings set the conversion ratio between Figure 1: Compared to latest-generation IGBTs, the TW070J120B SiC MOSFET shows considerably faster switching speeds that deliver higher efficiencies in power converters