Abstract
Soft switching for both primary and secondary side devices is available by using LLC converters. This resonant converter is an ideal candidate for today’s high frequency, high efficiency, and high power density applications like adapters, Uninterrupted Power Supplies (UPS), Solid State Transformers (SST), electric vehicle battery chargers, renewable energy systems, servers, and telecom systems. Using Gallium-Nitride (GaN)-based power switches in this converter merits more and more switching frequency, power density, and efficiency. Therefore, the present paper focused on GaN-based LLC resonant converters. The converter structure, operation regions, design steps, and drive system are described precisely. Then its losses are discussed, and the magnets and inductance characteristics are investigated. After that, various interleaved topologies, as a solution to improve power density and decrease current ripples, have been discussed. Also, some challenges and concerns related to GaN-based LLC converters have been reviewed. Commercially available power transistors based on various technologies, i.e., GaN HEMT, Silicon (Si) MOSFET, and Silicon Carbide (SiC) have been compared. Finally, the LLC resonant converter has been simulated by taking advantage of LTspice and GaN HEMT merits, as compared with Si MOSFETs.
Highlights
Resonant converters are soft switching topologies that can be employed either in Zero-Voltage Switching (ZVS) or Zero-Current Switching (ZCS) mode to reduce the turn-on or turn-off losses of power transistors [1]
Converter is operating at the step-up mode with a switching frequency lower than the resonant frequency; (2) conduction loss that happens in the on-state mode because of the on-state resistance of the transistor; (3) turn-off loss, especially when the LLC converter is working at the step-down mode with a switching frequency higher than the resonant frequency; and (4) gate drive loss, especially in high switching frequencies
Experimental tests have proved that the impact of parasitic capacitors and secondary side diodes is not limited to the ZVS condition and they can influence the waveforms of the following normal operation [74]
Summary
Resonant converters are soft switching topologies that can be employed either in Zero-Voltage Switching (ZVS) or Zero-Current Switching (ZCS) mode to reduce the turn-on or turn-off losses of power transistors [1]. They can operate at high switching frequencies, hundreds kHz to MHz range, without unacceptable switching losses [2]. Recent developments in LLC resonant converters with GaN transistors and novel magnetics present more and more switching frequency, power density, and efficiency [11,30,31,32]. There are some challenges GaN HEMTs face in LLC resonant converters, like digital control system design, PCB winding for MHz range switching frequency, flux cancellation techniques, EMI noise cancellation, and the current conductive limitation of a single GaN switch [11,30,34,35,36,37,38,39,40,41]
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