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A High-Efficiency High-Density Wide-Bandgap Device-Based Bidirectional On-Board Charger

Variable dc-link voltage structure
Fig. 1. Variable dc-link voltage structure without split capacitors.
  Due to concerns regarding increasing fuel costs and air pollution, plug-in electric vehicles (PEVs) are drawing more and more attention. PEVs have a rechargeable battery that can be restored to full charge by plugging in to an external electrical source. However, the commercialization of the PEV is impeded by the demands of a lightweight, compact, yet efficient on-board charger system. Since the state-of-the-art, level 2 on-board charger products are largely Si-based, they operate at less than 100-kHz switching frequency, resulting in a low power density at 3–12 W/in3, as well as an efficiency of not more than 92%–94%. Compared with Si devices, the absence of a reverse recovery charge in wide-bandgap (WBG) devices (GaN and SiC) enables bidirectional operation with a single converter. In addition, WBG devices have a much better figure of merit than Si devices. For a given ON-resistance and breakdown voltage, WBG devices require a much smaller die size, which can translate into a smaller gate charge and junction capacitance. Both of these characteristics are able to shorten the current and voltage transition interval, and thus reduce the switching loss. Therefore, by moving to WBG devices, the system switching frequency is increased to 100–300 kHz, and 95%–96% efficiency is achieved.
  For the ac-dc stage, a bridgeless totem-pole converter with CRM operation is adopted. Zero voltage switching (ZVS) is achieved for all fast switches, making it a promising candidate for high-frequency operation. Also, the symmetrical structure guarantees easy bidirectional operation. For the isolated dc-dc stage, the CLLC resonant converter featuring the same soft-switching technique as the LLC resonant converter is selected. It achieves ZVS for all switches under all load conditions. In addition, the symmetrical resonant tank makes it much more suitable for bidirectional operation. To optimize the converter over a wide output voltage range, a variable dc-link voltage structure is used, utilizing 1.2-kV SiC devices so that the second stage CLLC resonant converter is always working around its optimized point for all output voltages. As a result, very high efficiency is achieved during the whole charging cycle, from low battery voltage to high battery voltage. Also, a two-stage combined control strategy is proposed for both charging and discharging operations. The proposed control guarantees that the dc-dc stage switching frequency stays around resonant frequency and also eliminates the secondary order line-frequency ripple generated by the ac-dc stage. A 6.6 kW, 500 kHz (300 kHz for ac/dc) bidirectional on-board charger prototype with 37 W/in3 power density and over 96% efficiency is demonstrated to verify the proposed structure.
hardware prototype and efficiency
Fig. 2: The hardware prototype and tested efficiency.
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