Bi-Directional Charging for SiC MOSFETS Circuits Use in Electric Vehicles


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The success of electric vehicles (EVs) and more generally on electric mobility depends largely on the time required to charge the batteries. Long considered one of the weaknesses of electric vehicles, charging time has been gradually reduced, with advanced solutions such as fast charging that only takes a few minutes. Onboard charging systems (OBCs), which are connected directly to AC mains, require at least four hours per charge. Conversely, fast charging systems operating in direct current can reduce the charging time to less than 30 minutes. In charging systems, electrical MOSFETs based on silicon carbide (SiC) play an essential role. SiC is a wide bandgap semiconductor, which, compared to silicon, offers advantages such as high efficiency, power density, high reliability and durability, reducing solution cost and size. As shown in Figure 1, although with different power requirements and technical specifications, both charging systems can benefit from the use of SiC MOSFETs, which can manage the wide voltage range (typically between 200V and 800V) of batteries installed in electric vehicles, Reduce energy loss by up to 40%, increase energy density by 50%, halve the number of active ingredients, and reduce the total cost of the solution. Wolfspeed’s 1.2-kV SiC MOSFET series not only meets these requirements, but operates a bi-directional charge/discharge process, replacing the IGBT transistors used in current charging circuit topologies. Figure 1: Comparison of the fast charging system and the fast charging system. The DC converter can be adapted to both OBC charging systems and DC fast chargers. The proposed solution, based on 1,200-V SiC MOSFETs with RDS (operating) = 32 mM (Fig. 2), provides very high energy density (4.6 kW/L) and efficiency (>98.5%) at a lower cost. Figure 2: Simple two-level SiC AFE designed by Wolfspeed In contrast to other standard architectures, such as the design based on six-switch IGBT (a simple but less efficient and energy-intensive solution) and a T-type switch (the most complex and expensive solution), SiC AFE provides a simple control and interface Driver-driven operation, which supports bi-directional operation with fewer parts. C3M0032120K, 1.2 kV 32 mV SiC MOSFET with Kelvin-source package, helps reduce switching loss and interference while allowing easy driving voltages from -3- to 15-V Vgs. The AFE design is optimized to use magnetic elements, achieving a high switching frequency (45 kHz) with less power loss in both the core and coil. AFE also uses a digital control circuit capable of supporting three-phase and single-phase PWM schemes, balancing switching losses and improving thermal performance, efficiency and reliability. Furthermore, DC junction variable voltage control enables high system efficiency by varying the DC bus output voltage based on the sensor battery voltage and ensuring that the CLLC operates close to its resonant frequency. Figure 3 (top) shows single-phase waveforms when charging (operating the totem pole) and discharging (interleaving process). The waveforms in Figure 3 (bottom), which have a total harmonic distortion of less than 5%, instead indicate a three-phase AFE formation. Figure 3: (Top) Test result for single-phase AC AC/DC waveforms and test results for three-phase AC AC/DC waveforms (bottom) Compared with conventional IGBT-based solution (with maximum efficiency of 96%), SiC MOSFET efficiency reaches to 98.5%, which reduces power loss by up to 38%. In addition, SiC allows for lower operating temperatures and therefore better thermal management. Under maximum power conditions (22 kW), 89.4 °C was measured at the case, 112.4 °C (calculated) at the junction, and 65 °C for the base plate. Figure 4 shows the efficiency curves related to the results obtained from the tests. Figure 4: Single-phase AFE charge/discharge efficiency diagrams and three-phase charge modes Full-bridge CLLC DC/DC converter with 1.2 kV SiC MOSFETs Another interesting application scheme is the full-bridge CLLC DC/DC converter, where 1.2-capacity SiC MOSFET circuits can be used. kV in a single-level high-efficiency transformer scheme (Fig. 5), which reduces the number of parts and the cost of the system. Operating currents on the DC jumper side (900V) reach 22.6 ARMS, while on the battery side (800V) they reach 28.5 ARMS. Figure 5: SiC-based Single Bi-level Transformer Coupled with a SiC AFE block, a full-bridge DC/DC design takes advantage of the variable DC bus voltage supplied by the AFE based on the voltage of the sensor battery to be charged. This allows the CLLC to operate close to the resonant frequency, achieving high system efficiency. When the battery voltage drops, the control will switch to phase change mode, reducing the circuit gain without passive operation outside the resonant frequency range. At a lower output voltage (slightly above 400 V), the primary CLLC operates as a half-bridge, reducing system gain and keeping the resonant transformer in an effective operating area. Half-bridge mode has some limitations in the overall power range but provides a robust peak efficiency of 98%, even for low-voltage batteries. Please visit the eBook for the full article.


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