Study on high power quality digital power factor correction control methods for single phase continuous conduction mode boost power factor correction converter = 단상 연속 도통 모드 부스트 역률 보상 회로의 높은 전력 품질을 위한 디지털 제어기법 연구

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Due to dramatic improvement of the information technology and explosive use of mobile devices, the amount of the processing data has been greatly increased. To handle this numerous data, a lot of data center has been built more and more, and the number of server computers that are installed in data center to process the data directly is continuously growing up. In accordance with this tendency, the power consumption of the server power supply which provide electric power to the sever computers has been also increased considerably. Thus, the high power quality, such as high PF (power factor) and low THD (total harmonic distortion), is necessary for the server power supply to minimize power loss resulting from the circulating current and prevent damage of line voltage and current. In addition, the server power supply has to be easy to maintain and have to meet the stringent requirement of reliability to prevent loss of data. As a result, the server power supply adopts the redundant structure for a high reliability, and it normally operates under 50% load conditions. Moreover, the data traffic of the data center during nighttime is significantly lower than the maximum data traffic at daytime, the sever power supply operates under 10% load conditions over 8 hours per day. For this reason, the 80 PLUS incentive program and Climate Saver Computing Initiative gradually reinforce requirements about efficiency and PF from full load conditions to 10% load conditions, and these requirements tend to be extended to even 5% load conditions. To satisfy these stringent requirements, the continuous conduction mode (CCM) boost power factor correction (PFC) converter is commonly used in sever power supply, because of low conduction loss and small EMI filter size resulting from small current ripple. However, the power quality of CCM boot PFC converter is severely decreases at high line and light load conditions. Therefore, it is hard to meet the reinforced requirement with CCM boost PFC converter that uses conventional average current control (ACC) method. This is caused by two reasons. The first reason is the phase leading circulating current caused by the EMI and input capacitor in CCM boost PFC converter. Since the EMI and input capacitor is parallel connected with CCM boost converter, this phase leading circulating current is increased larger and larger as line voltage is increased. As a result, in high line and light load conditions, the phase leading circulating current charges large proportion of the line current, and it degrade displacement PF. The second one is operation in discontinuous conduction mode (DCM) and mixed conduction mode (MCM), which is CCM-DCM transition mode. In DCM mode, the unpredictable current oscillation caused by parasitic components of switch and diode leads to inductor and line current distortion, and in MCM mode, dynamics change between CCM and DCM results in current distortion. Therefore, a considerable phase difference between voltage and current, and current distortion degrade power quality especially in high line and light load conditions. Nowadays, due to the reduced cost and design flexibility, the digital control for power converters has gained remarkable attention, and a lot of control algorithms are developed and implement to control CCM boost PFC converter. However, aforementioned problems have not been deal with till now. Thus, in order to overcome drawbacks of the conventional control methods, several digital control methods that compensate the phase leading circulating filter current and the minimize current distortion by predictive peak current control are proposed and verified in this dissertation. Therefore, the power quality of the proposed control methods greatly improved compared with other conventional control methods. Part 1. A Digital Phase Leading Filter Current Compensation Technique for CCM Boost PFC Converter to Improve PF in High Line Voltage and Light Load Conditions In order to relieve the switching noise of the power supply and prevent disrupt nearby equipment, the EMI filter should be employed in front of PFC converters. Since, the EMI filter is parallel connected with the CCM boost PFC converter, the capacitor in EMI filter causes the phase leading circulating current, and it is not only independent of the load conditions, but also larger and larger as line voltage increases. In addition, conventional control methods are not take into account the phase leading circulating current, and it make the inductor current follow to the line voltage. As a result, the displacement PF is considerably decreases in high line and light load conditions, because the phase leading circulating current is added to the line current, and it is hard to meet requirement in high line and light load conditions with conventional control methods, To cope with this drawback, a digital phase leading filter current compensation method for CCM boost PFC converter to improve power quality is proposed in this dissertation. The proposed control method analyzes and derives the magnitude and phase of the phase leading current, and it compensates this phase leading current from the current reference correction and enhanced duty-ratio feed-forward. Because of these reasons, the proposed control method modulates the line current in phase with the line voltage can be achieved, and the switching dead-zone near the zero crossing of line voltage can be implemented naturally. Therefore, a high PF and low THD as well as high efficiency in high line and light load conditions can be achieved by the proposed control method. To verify the effectiveness of the proposed control method, the prototype of universal input ($85-265V_{AC}$) and 750W output CCM boost PFC converter for server power supply is experimented. As a result, the PF is improved greatly, and over 0.95PF which fulfills the reinforced requirement is achieved in high line and light load conditions. Furthermore, it slightly improves the efficiency under 10% load conditions. Part 2. A Digital Predictive Peak Current Control for Power Factor Correction with Low-Input Current Distortion Analysis In case of CCM boost PFC converter, it operates in DCM in high line and light load conditions due to the low inductance for a high power density, although it operates in CCM in low line or heavy load conditions. To modulate the sinusoidal inductor current, average current control (ACC) methods are commonly used for CCM boost PFC converter. However, with conventional ACC methods, the operation in DCM of CCM boost PFC converter suffers from the unpredictable and high frequency current oscillation caused by the parasitic component, and it results in the current distortion in DCM. Moreover, while the CCM boost PFC converter operate in MCM, it induces a large current distortion due to abrupt dynamics change. These are because that the limited current control bandwidth and gain induced by S&H and time delay of the digital controller insufficient to reject the disturbance and properly compensate the dynamics change. As a result, the current distortion in DCM and MCM is unavoidable with conventional digital average current control methods. To solve the aforementioned problems, a digital predictive peak current control (PCC) method for CCM boost PFC converter to eliminate current distortion is proposed in this dissertation. The proposed control method estimates the duty, operational region, and peak inductor current corresponding to the input and output conditions by using DC-conversion ratio in CCM and DCM. Moreover, the slope ramp and current reference for PCC is predicted to cancel the improper sub-harmonic oscillation of inductor current and guarantee the stability of the proposed control method. Thus, since the proposed control method constrain the peak inductor current, it dramatically reduces the current oscillation in DCM, and it also removes the current distortion in MCM. Furthermore, the current compensator which requires a high control bandwidth and gain is not needed in the proposed control method, and the burden of DSP is reduced due to the predictive peak current control. To verify the validity of the proposed control method, the prototype of universal input and 750W output for server power supply is implemented. Therefore, THD is significantly improved in DCM and MCM, and almost 7% THD is achieved at high line 10% load conditions. In this dissertation, the high PF and low THD digital PFC control methods for single phase continuous conduction mode (CCM) boost PFC converter are presented, and a high power quality is achieved by proposed control methods. Therefore, proposed control methods are very promising and can be widely adopted for server power supply as well as power supplies which use AC line input.
Advisors
Moon, Gun-Wooresearcher문건우researcher
Description
한국과학기술원 :전기및전자공학부,
Publisher
한국과학기술원
Issue Date
2017
Identifier
325007
Language
eng
Description

학위논문(박사) - 한국과학기술원 : 전기및전자공학부, 2017.2,[vi, 95 p. :]

Keywords

Average current control; Digital control; duty ratio feed-forward-control; power factor correction; predictive control; 평균 전류 제어; 디지털 제어; 시비율 전향 보상 제어; 역률 보상; 예측제어

URI
http://hdl.handle.net/10203/242035
Link
http://library.kaist.ac.kr/search/detail/view.do?bibCtrlNo=675833&flag=dissertation
Appears in Collection
EE-Theses_Ph.D.(박사논문)
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