October / November 2021

14 POWER CONVERTERS https://toshiba.semicon-storage.com/ Issue 4 2021 Power Electronics Europe www.power-mag.com the two sides. The series inductor is not a requirement. In some cases the transformer can fulfill both roles, but this is typically at the expense of increase losses and a loss in efficiency. Both sides are controlled using complementary PWM control signals. Modification of the phase of the signal applied to the two sides defines the direction of energy transfer. The side connected to the high-voltage DC link is well suited to the capabilities of SiC MOSFETs. They support the high voltages being applied while supporting the high switching frequencies used. Thanks to the use of zero-voltage switching (ZVS), high-voltage Silicon MOSFETs are matched to the needs of the opposing side. This is the approach taken in a new bidirectional DC/DC power supply reference design (RD167). Supporting high-side voltages of 750 V DC, and outputting 380 V DC, the supply can deliver 5 kW at a power efficiency of 97 % in either direction (100 % step-up load) using a 50 kHz switching frequency. The design uses the 1200 V TW070J120B SiC MOSFET rather than IGBTs to take advantage of the low switching losses, and the low 70 m Ω RDS(ON). The gate threshold (Vth) lies between 4.2 V and 5.8 V, which contributes to the robustness of the design by making it less prone to gate voltage fluctuations and noise. On the low-voltage side, the design uses the 650 V TK49N65W5, a Silicon N- channel MOSFET, taking advantage of the performance improvements compared to IGBTs. Its high-speed parasitic diode, coupled with the DTMOS superjunction structure, contribute to the high efficiency thanks to the low switching losses and fast reverse recovery time (trr = 145 ns typical). With a low on-resistance of 0.051 Ω (typical), it can support drain DC currents (ID) of 49.2 A and drain pulse currents (IDP) of 192 A. To provide optimal gate control to both the SiC and Silicon MOSFET, the TLP5214A gate drive is used. Its 4 A sink and source capability provide adequate drive and discharge currents at elevated voltages and the high switching frequencies used. It also provides a safeguard to the design thanks to its over-current protection and under- voltage lock-out function. Conclusion Continuous innovation in the domain of power devices is supporting power converter engineers to attain ever higher efficiencies with their products. It also helps ensure that the move from fossil-fuel to electric energy makes optimal use of our available resources. Whether targeting higher powers, or looking to move to greater power densities, IGBTs are increasingly being exchanged for alternatives. At high voltages (> 1000 V) SiC MOSFETs provide lower losses and, thanks to their support for higher switching frequencies, enable more efficient power conversion. Around 650 V, superjunction Silicon MOSFETs, with low reverse recovery times, low on- resistance, and support for higher switching frequencies, are also displacing IGBTs. 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. Figure 3: The DAB bidirectional DC- DC converters based upon the TW070J120B SiC MOSFET and TK49N65W5 silicon superjunction MOSFET Figure 2: Role of bidirectional DC/DC converters in photovoltaic (left) and electric vehicle charging (right) applications