A Full-GaN Solution For High Power Density Chargers And Adapters

Due to continuous demand for high power density, USB-C fast chargers’ switching frequencies need to be increased to reduce the size of the transformers and the filter components. Emerging technologies based on Wide Band Gap (WBG) semiconductor materials enable new approaches to increase power density. At high switching frequencies, GaN HEMTs for synchronous rectifier (SR) switches have the advantages of lower charges and faster switching transitions. In addition, the size of a GaN HEMT is smaller than that of a Silicon (Si) MOSFET for the same voltage rating and on-resistance. Despite these clear advantages, state-of-the-art systems still use MOSFET SR. But what is the reason behind this? The relatively high third quadrant (body diode) mode voltage drop of GaN compared to MOSFETs introduces higher conduction losses during turn-on/-off delay times. These “dead-time” losses increase with higher switching frequencies. To minimize these losses, the controller circuit must be GaNoptimized to shorten the delay for turn-on and turn-off gate biasing. State-of-the-art SR controllers have delay times on the order of tens to hundreds of nanoseconds. This introduces significant losses in topologies like LLC, Active Clamp Flyback (ACF), and especially in the Quasi-Resonant (QR) Flyback. Even with a well-optimized controller, the delay times could be quite long, still in the order of tens of nanoseconds, due to PCB layout parasitics that affect the accuracy of drain-source voltage (VDS) sensing and reaction time of the SR controller. However, by addressing the issues during the design process, the full potential of GaN in SR applications can be realized. This paper presents the methodology of using GaN HEMTs for SR in USB-C PD charger and adapter designs. First, we provide an overview and comparison of the most common charger & adapter topologies used in the market. Then synchronous rectification characteristics are discussed, and design tips are provided regarding the usage of GaN HEMT as SR. Finally, we introduce Infineon’s 65 W Full Gan ACF converter Evaluation Board.

By Dr. Gökhan Sen, Senior Staff Engineer, Infineon Technologies

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