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Critical-mode-based Sof-switching Modulation for High-frequency, Three-phase, Bidirectional ac-dc Converters

Fig. 1. Prototype of 25-kW SiC-based three-phase ac-dc converter
For wide bangap (WBG) semiconductor devices based high-frequency power conversion systems, the high power density is achieved due to the size reduction of filters made up of passive components. Since WBG devices show negligible turn-off energy when compared with the turn-on energy, critical conduction mode (CRM) operation zero voltage switch (ZVS) soft switching is preferred in order to achieve high efficiency. However and according to the literature, in order to achieve independent CRM control and therefore ZVS soft switching in three-phase ac/dc extra components and connections are required. This requirement which adds to system complexity and leads to some limitations under high modulation index conditions, showing extremely wide switching frequency variation and the associated high switching related loss. Therefore, a novel CRM based soft-switching modulation is proposed for a bidirectional three-phase ac/dc converter. In this modulation, CRM operation with ZVS soft switching is still the foundation. With two total control freedoms among three phases, discontinuous pulse width modulation (DPWM) is adopted by clamping one phase to enable independent CRM control in the other two phases. Switching frequency synchronization implemented by discontinuous conduction mode (DCM) operation is applied to limit switching frequency variation. With the proposed modulation, the switching frequency range is 300 kHz - 500 kHz at full load. With the proposed modulation technique, at any line cycle operating point, the first phase is clamped and uncontrolled, the second phase operates at CRM, while the third phase operates at DCM. Under a high modulation index (%gt; 0.8), for each phase, ZVS turn-on of the control switch in each switching cycle is naturally achieved during CRM operation in the inverter mode. At the same time, the conclusion is totally opposite in the rectifier mode and therefore off-time extension is always required in the rectifier mode during CRM operation. The lower modulation index introduces the tendency of losing the capability of natural ZVS for the inverter mode operation during CRM, while introducing the tendency of gaining the capability of natural ZVS for the rectifier mode operation during CRM. For each phase during DCM operation, for both inverter and rectifier modes, valley switching (in most cases ZVS) turn-on is always achieved by applying a slight turn-on delay to the control switch. A 25 kW SiC based three-phase bidirecitonal ac/dc converter prototype is built, as shown in Fig. 1. This prototype is designed to operate at above 300 kHz switching frequency with the proposed modulation, achieving 127 W/in3 power density. Fig. 2 shows the efficiency of the converter prototype, and approximately 99 % peak efficiency is achieved for bidirectional operation even with above 300 kHz switching frequency.
Fig. 2. Tested efficiency of the converter