SiC 기반 LLC 공진형 컨버터의 PFM-Burst 전류제어에 관한 연구 = A Study on a PFM-Burst Current Control Method for an SiC-Based LLC Resonant Converter

다국어 초록 (Multilingual Abstract)

This dissertation proposes a hybrid PFM–Burst Current control strategy for an SiC-based full-bridge LLC Resonant Converter to improve light-load operating characteristics and reduce unnecessary switching activity over a wide load range. Conventional LLC Resonant Converters exhibit high efficiency near the Resonant operating point under rated-load conditions; however, under light-load and no-load conditions, the switching frequency may excessively increase to regulate the output, resulting in increased switching loss, circulating Current, and standby power consumption. To address these limitations, this study combines variable-frequency PFM control with Burst Mode operation.
The proposed Converter adopts a full-bridge LLC Resonant topology with a 750[V] DC input. The Resonant tank consists of a Resonant Inductor （）, a Resonant capacitor （）, and a magnetizing inductance （）, and the operating frequency range was set from 38[kHz] to 150[kHz]. Under normal and medium-load conditions, the Converter operates in continuous PFM Mode, whereas under light-load conditions it automatically transitions to Burst Mode when the switching frequency reaches the maximum operating limit. The Burst Mode controller intermittently disables PWM switching based on the control error, thereby reducing average switching activity while maintaining output stability.
PSIM simulations and hardware experiments were conducted to verify the proposed control method. The simulation results confirmed that output power can be regulated by adjusting the Burst Duty and that the Closed-loop Burst Mode controller can regulate output Current under different reference Current and load conditions. In the low-Current reference range from 1[A] to 10[A], intermittent Burst operation was observed, and the average output Current followed the reference value. At the 1[A] condition, the average output Current was approximately 1.01[A]. As the load increased, the Burst-On interval and energy transfer duration increased, indicating a gradual transition from Burst Mode to continuous PFM operation.
Additional load variation experiments from 50[A] to 260[A] were performed to evaluate Current regulation under medium- and high-load conditions. The Current regulation error remained within 2.82[%] over the tested load range. At the maximum load condition of 260[A], the measured output Current was 257.35[A], corresponding to an error of approximately 1.02[%]. Efficiency measurements under a 750[V] input condition showed an efficiency range of approximately 94.18[%] to 94.78[%], with an average efficiency of about 94.49[%].
Overall, the proposed PFM–Burst Current control method reduces average switching activity in the light-load region while maintaining Current regulation capability over a wide load range. The proposed method is applicable to high-Current DC power conversion systems requiring precise Current control, such as water electrolysis systems, water treatment power supplies, industrial rectifiers, energy storage systems, and electric vehicle charging systems.

목차 (Table of Contents)

• Ⅰ. 서 론 1
• A. 논문의 배경 및 필요성 1
• B. 선행연구 및 기존 기술의 한계점 4
• C. 논문의 구성 7
• Ⅱ. 이론적 배경 8
• A. LLC 공진형 컨버터 개요 8
• 1. LLC 공진형 컨버터의 구조 8
• 2. LLC 공진형 컨버터의 동작 원리 13
• 3. 소프트 스위칭(Soft Switching) 특성 15
• B. SiC 소자 특성 18
• 1. 스위칭 특성 및 손실 분석 18
• 2. 고속 스위칭 장점 및 LLC 컨버터 적용성 21
• C. 저전류 및 경부하 영역 제어 문제 23
• 1. 경부하 효율 저하 원인 23
• 2. 저전류 영역에서의 연속 스위칭 문제 24
• D. 주파수 변조(PFM) 제어 28
• 1. PFM 제어의 기본 원리 28
• 2. 경부하 영역에서의 PFM 제어 한계 29
• E. Burst Mode 제어의 원리 31
• 1. Burst Mode 기본 동작 31
• 2. Burst Mode 적용 시 장점 및 고려사항 34
• Ⅲ. 제안하는 저전류 정밀제어 방법 34
• A. 제안된 LLC 공진형 컨버터의 구조 34
• B. 제안된 LLC 공진형 컨버터의 동작 원리 37
• C. PFM Mode 제어 설계 44
• D. Burst Mode 제어 설계 46
• E. PFM-Burst Mode 제어 설계 48
• Ⅳ. 시뮬레이션 51
• A. 시뮬레이션 회로 구성 및 실험 조건 51
• B. 시뮬레이션 결과 54
• 1. Open-Loop Mode Control 54
• 2. Closed-Loop Mode Control 61
• Ⅴ. 실험 및 결과 65
• A. LLC 공진형 컨버터 설계 및 제작 65
• 1. 공진 커패시터 설계 65
• 2. 공진 인덕터 설계 66
• 3. 고주파 변압기 설계 67
• 4. 제작 하드웨어 구성 69
• B. 실험 결과 71
• 1. 실험 구성 71
• 2. SiC 기반 LLC 컨버터의 스위칭 주파수 특성 73
• 3. 저전류 영역에서의 Burst Mode 동작 특성 75
• 4. 넓은 부하 범위에서의 전류제어 성능 81
• 5. LLC 컨버터 효율분석 86
• VI. 결 론 92
• 참고문헌 94
• 부록 100