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High-Frequency, High-Efficiency GaN Based Interleaved CRM Bi-directional Buck/Boost Converter with Coupled Inductor

Fig. 1. Buck/boost converter with coupled inductor.
The bidirectional buck/boost converter is widely used in power electronics systems due to its simplicity and high efficiency. Some applications include on-board charger/dischargers for plug-in hybrid electric vehicles, and interfaced converters for energy storage in a dc-based nano-grid. Conventional silicon-based bidirectional buck/boost converters are usually intended to be operated in discontinuous current mode (DCM) in order to alleviate reverse-recovery issues and to be able to use a small inductor. However, the DCM operation greatly increases the turn-off loss because the main switch is turned off at least twice during the load current. As a result, the switching frequency can barely be pushed to the hundreds of kilohertz due to power loss.

In recent years, the GaN HEMT has emerged as a promising device for high-frequency, high-efficiency, and high-density power conversion due to its better figure of merit than comparable Si and SiC transistors. The switching frequency has been continuously pushed up to several MHz to both reduce passive component size and increase power density. Previous research concludes that the turn-on switching loss is dominant due to the reverse-recovery charge or the junction capacitor charge of the free-wheeling device under hard-switching conditions. On the other hand, the turn-off loss is negligible because of the intrinsic current-source driving mechanism in the cascode structure. These important switching characteristics indicate that zero-voltage-switching (ZVS) is still desired for GaN devices in high-frequency applications, while the turn-off current is no longer a big concern for cascode GaN devices.

The critical current mode (CRM) operation is the most simple and effective way to achieve ZVS and is widely used in medium-low power applications. Conventional CRM operation has some limitations in high frequencies and requires further improvement. The concept of a coupled inductor has been applied successfully in the interleaved voltage regulator modules to improve efficiency and transient response. A two-phase buck/boost converter with a coupled inductor is shown in Fig. 1, and the converter's key waveforms are shown in Fig. 2. The resonant period in CRM operation is less with the coupled inductor than with the non-coupled condition, which is beneficial for high-frequency operation. The soft-switching range and the circulating energy are both improved with the coupled inductor in CRM. The coupled inductor prototype efficiency is 98.5% at 1MHz, which is 0.5% higher than the efficiency of the non-coupled converter.

Fig. 2. Key waveforms.
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