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Critical-Conduction-Mode-Based Soft-Switching Modulation for Three-Phase PV Inverters with Reactive Power Transfer Capability

experimental waveform
Fig. 1. Typical experimental waveforms at different operating points.
  High-frequency power conversion systems based on wide bandgap (WBG) semiconductor devices can achieve high power density by reducing the size of passive-component filters. Since WBG devices show negligible turn-off energy compared with the turn-on energy, a critical conduction mode (CRM) operation, zero-voltage switch (ZVS) soft switching is preferred for achieving high efficiency. Both high density and high efficiency are desired in three-phase photovoltaic (PV) systems.
  In CRM operation, three-wire systems—different from the three-phase four-wire systems reported in the literature—cause extremely wide switching-frequency variations. Previously, a novel CRM-based, soft-switching modulation was proposed, where a discontinuous pulse width modulation (DPWM) was adopted for decoupled control in three-phase, three-wire systems and discontinuous conduction mode (DCM) operation was applied to limit switching-frequency variation. Using that modulation, switching frequency variation range is 300 kHz–500 kHz at full load.
  In this work, soft switching modulation is improved not only in the unity power factor condition, but also in non-unity power factor conditions. DPWM clamping is applied to the phase with the highest ac line-to-neutral voltage amplitude to avoid hard switching turn-on during DCM operation. Control switches in the other two phases are turned on in each switching cycle after the inductor current zero crossing has occurred in both phases, which determines the assignment of DCM and CRM operation modes in each phase, as well as the optimal DCM/CRM transition angle. A numerical model is proposed for predicting this optimal transition angle with sufficient accuracy. Based on the improvements, a generalized CRM-based, soft switching modulation is proposed for both unity and non-unity power factor operating conditions to minimize switching loss.
  The proposed soft switching modulation is digitally implemented with one low-cost microcontroller (MCU), and the benefits are experimentally verified on a 25 kW SiC (WBG)-based three-phase, bidirectional ac-dc converter prototype. This prototype is designed to operate above a 300 kHz switching frequency, achieving a 127 W/in3 power density. Fig. 1 shows some typical experimental waveforms to verify the zero-voltage, soft-switching turn-on at different operating points. Fig. 2 shows the tested efficiency before and after the improvements under different power factor conditions. Efficiency above 98% is achieved when the power factor varies between 0.8 (lagging) and 0.8 (leading), even with above-300 kHz high-frequency operation.
tested efficiency
Fig. 2. Tested efficiency of the prototype.
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