Flyback Converter Using a D-Mode GaN HEMT Synchronous Rectifier

Abstract

The flyback converter with its active cell balancing topology for charging lithium-based batteries in Electrical Vehicles (EV) have been adopted recently into the industry. Electrical Vehicle battery charging requires high current operation in continuous current mode and hence, the power loss on the Schottky diode rectifier on the secondary side determines the power conversion efficiency. The depletion mode (D-mode) GaN HEMT synchronous rectifier proposed in this paper has been used to replace the Schottky diode on the secondary side of the flyback converter in order to improve the power conversion efficiency. This synchronous rectifier regulates the forward voltage drop of an external switch to about 100 mV per ampere of current flow with no concern to threshold voltage. The first challenge of converting the D-mode GaN HEMT as a synchronous rectifier is that the normally-on device must be off when the primary side inductor of the flyback converter is initially charging the magnetic energy. That is, the rectifier must behave as the normally-off device during its initialization stage. The second challenge is that the D-mode GaN HEMT must switch off as soon as the secondary current becomes zero. The third challenge is posing a fast recovery feature to reduce the drain-source voltage rise on the primary side switch, which suffices to be the main reason as to why the D-mode GaN HEMT is used instead of MOS devices. The proposed depletion mode GaN HEMT synchronous rectifier is verified to be able to overcome all challenges and in result becomes a candidate for the synchronous rectifier.