Bi-directional operation is becoming increasingly important as consideration is given to using energy stored in vehicle batteries to power the home at a time when electricity consumption is expensive. If some diodes are replaced by switches, all topologies shown will support this mode of operation.
Silicon carbide (SiC) MOSFETs with low internal resistance RDSon (in the <40 mΩ range) are the preferred solution, especially for the higher power range, as they are more efficient compared to silicon MOSFETs. They are ideal for use in PIMs (Power Integrated Modules) as these integrated solutions offer higher performance, simplified design, reduced system size and increased reliability. switches such as IGBTs.
The DC/DC converter stage uses two main topologies: a resonant full bridge LLC and a full bridge with ZVS (Zero Voltage Switching). the LLC enables ZVS on the primary side and usually ZCS (Zero Current Switching) on the secondary side, resulting in very high efficiency when operating near the resonant frequency. Where operating frequencies are limited, LLC converters can be an efficient solution – although parallel operation is challenging due to the difficulties associated with current sharing and synchronisation.
Phase-shifted full-bridge topologies are also common and benefit from simpler control than the LLC approach. With ZVS, efficiency is high over a wide range of output voltages, making it ideal for supporting 400 and 800V battery voltages.
Despite the significant differences, the LLC and phase-shifted full bridge have similar component counts and achieve similar power densities. Both methods are suitable for synchronous secondary rectification and therefore also for bi-directional operation.
onsemi offers a range of building blocks for high-efficiency EV charging systems, including SiC diodes and MOSFETs, SiC power modules and associated gate drivers.
SiC Schottky diodes offer better switching performance and higher reliability than silicon, partly due to their excellent thermal performance. onsemi SiC diodes are available in 650, 1200 and 1700 V versions, with no reverse current and temperature-independent switching characteristics.
SiC MOSFETs are fast and rugged, with 10 times higher dielectric breakdown field strength, 2 times higher electron saturation rate, 3 times higher energy band gap and 3 times higher thermal conductivity. They are available in 650, 900 and 1200 V versions.
The system advantages of SiC diodes and MOSFETs include the highest possible efficiency through lower power consumption, higher power density, higher operating frequency, operation at high temperatures and reduced EMI.
SiC-based boost/boost converters contain SiC diodes and SiC MOSFETs for the DC/DC converter stage. The fully integrated approach reduces parasitic effects and improves thermal performance, simplifying design and reducing board space.
In addition to these solutions, onsemi also offers isolated gate drivers for WBG (wide band gap) switching to improve system efficiency and reliability.
An extensive network of DC fast charging stations is seen as a decisive factor in the growth of electric vehicle sales in the coming years. There are multiple standards from different industry bodies and although there are significant differences, there is one common denominator: the need for higher performance and efficiency.
Silicon carbide devices are the key to fast charging, which will enable the success and popularity of electric vehicles. with a comprehensive range of SiC diodes, MOSFETs, power modules and associated gate drivers, Onsemi is a leader in this field.