Modulation and Control for High-Frequency GaN-Based Bidirectional AC--DC Converters

Abstract

Alternating-current--direct-current (AC--DC) converters serve as a critical interface between the electric grid and direct-current (DC) loads. With the increasing demand for data center power consumption, renewable energy systems, energy storage systems, electric vehicles (EVs), AC--DC converters are required to reach high efficiency, high power density, high reliability and also support both rectifier-mode and inverter-mode operations. Gallium nitride (GaN) high-electron-mobility transistor (HEMT) devices provide superior switching performance compared with conventional silicon (Si) metal--oxide--semiconductor field-effect transistors (MOSFETs), enabling high-frequency and high-power-density converter designs. Critical-conduction-mode (CRM) operation can further utilize the advantages of GaN devices by achieving zero-voltage switching (ZVS). However, as the switching frequency increases, new challenges arise in zero-current-detection (ZCD) sensing, digital modulation, closed-loop control and increased circulating current. Furthermore, the current distortion issue under non-unity power factor (PF) operation is another challenge for inverter mode operation. This dissertation investigates the sensing, modulation, and control of high-frequency GaN-based bidirectional totem-pole (TP) AC--DC converters. First, a reliable ZCD-sensing-based CRM modulation method is developed for GaN-based TP AC--DC converters. The noise propagation path in the ZCD sensing circuit is analyzed, and a noise-oriented circuit design method is proposed, including component selection guidelines and printed-circuit-board (PCB) layout rules. To avoid false triggering and clamp the maximum switching frequency, a window-based ZCD signal processing method is introduced. In addition, the closed-loop control challenge of high-frequency CRM TP-power factor correction (PFC) converters is addressed. Conventional constant-on-time (COT) control can suffer from input-current total harmonic distortion (THD) due to the resonance between the inductor and switch output capacitances, while variable-on-time (VOT) operation with voltage-mode control (VMC) is sensitive to parameter tolerances. Therefore, an average-current-mode control (ACMC) method with adaptive compensator gain is proposed to directly regulate the AC current and improve the dynamic response near the AC voltage zero-crossing region. The proposed ZCD sensing circuit and closed-loop control methods are verified on a 1~kW GaN-based TP-PFC prototype. Experimental results show noise-free ZCD operation with negligible sensing delay, and the measured input-current THD is 1.97% at full load and remains below 5% over the tested operating range. Second, a sensor-less CRM modulation method is proposed to eliminate the dedicated ZCD sensing circuit and reduce the impact of system response delay. Although improved ZCD circuits can achieve reliable operation, practical converter systems may still suffer from severe electromagnetic-interference (EMI) noise, increased bill-of-materials (BOM) cost, and delay-induced performance degradation, especially when the switching frequency is pushed even higher. Quantitative analysis shows that even a response delay at the 50~ns level can significantly increase the inductor-current ripple under ultra-high-frequency operation. To address this issue, a model-based CRM modulation method is developed, where the synchronous-rectifier (SR) conduction time is calculated based on the sensed average inductor current. The proposed model avoids the complex timing calculation of the second-order resonance between the inductor and switch output capacitances while maintaining good accuracy. The tolerance analysis shows that the model has low sensitivity to the output-capacitance tolerance. With printed-circuit-board (PCB) winding inductors, the proposed method has strong potential for mass-production applications. Experimental comparison with ZCD-based modulation shows reduced inductor current and smaller circulating current. The model-based method is further applied to a 2.2~kW two-channel interleaved TP-PFC prototype with PCB inductors, achieving a measured peak efficiency of 99.02%. Finally, high-frequency CRM operation is extended to inverter-mode applications. Under non-unity-PF operation, the zero-crossing instants of the AC current and AC voltage no longer coincide, which causes an extremely high switching frequency near the AC current zero crossing. To solve this issue, discontinuous-conduction-mode (DCM) modulation with stepwise resonant-dead-time adjustment is proposed to clamp the switching frequency. A ZCD window is also applied to avoid undesired triggering caused by DCM current ringing. In addition, the AC-voltage zero-crossing blanking time used in conventional unipolar modulation can cause current distortion in inverter-mode operation. Although bipolar modulation can avoid this issue, it increases the switching frequency and reduces the benefit of CRM operation. Therefore, a unipolar--bipolar hybrid modulation method is proposed. In this method, blanking time is applied around the AC current zero crossing instead of the AC voltage zero crossing, and bipolar modulation is only used near the voltage zero-crossing region. A compare-value adjustment method is further proposed to achieve seamless transitions between modulation modes without using conventional action-qualifier software forcing. Hardware experiments verify the effectiveness of the proposed DCM frequency-clamping method and hybrid modulation strategy. Overall, this dissertation provides a systematic study of sensing, modulation, and control techniques for high-frequency GaN-based bidirectional TP converters. The proposed methods improve the reliability of CRM operation, reduce current distortion, mitigate delay and tolerance issues, and extend CRM operation from rectifier-mode PFC applications to bidirectional applications.