Moonhyun Lee

Discontinuous conduction mode (DCM) and critical conduction mode (CRM) have held dominant positions in low-power boost power-factor-correction (PFC) control. Typically, two-level DCM and CRM suffer from high peak current and variable frequency at several-hundred kHz, respectively. The two factors can cause secondary effects such as high electromagnetic interference, current stresses, input filter design difficulty and switching losses. Only to take merits of DCM and CRM which are fixed frequency and lower peak current, this article utilizes three-level boost (TLB) and its three-level current-slope options. This paper proposes a hybrid-conduction-mode (HCM) control which adopts adaptive single-switch on-time to achieve fixed-frequency quadrangular CRM operation. At lower input voltage, low inductor current enters DCM so that TLB PFC experiences hybrid operation modes depending on varying ac input. Analyses and experiment results in this paper confirm the proposed HCM control can achieve huge reductions of switching losses and peak current stress, improving efficiency in wide-range input voltage and load at the cost of acceptable PF drop. Moreover, a simple voltage-balancing scheme is implemented to the proposed control and verified effective to resolve the inherent topology issue.

Discontinuous conduction mode (DCM) controlled boosting power-factor-correction (PFC) converters suffer from high differential-mode (DM) electromagnetic interference (EMI) and require large input filter due to high inductor current ripples at single switching frequency. To remedy the shortcomings, a downward spread-spectrum frequency modulation (SSFM) is proposed in this letter. The SSFM can be used by boosting PFC converters for reduced EMI and improved efficiency. On top of the frequency modulation, a novel adaptive three-level current (ATC) scheme is devised with three-level boost (TLB) PFC using multilevel capability to obtain further reductions of peak current and EMI amplitude. Analyses and experiments validate that the proposed SSFM and ATC scheme can bring improvements of EMI and efficiency at the cost of increased output-voltage ripple.

This article presents a single-sourced hybrid binary-cascaded multilevel inverter (BCMLI) with front-end high-frequency-link (HFL) converters. The purpose of HFL converters is to boost a dc-source voltage to three bus voltages in a binary ratio. In this article, one of the HFL converters is removed from the system and the voltage of the floating capacitor caused by the removal is controlled by a newly proposed floating-capacitor-voltage (FCV) control. Compared with the conventional BCMLI, the system size and cost can be significantly reduced while maintaining promising features, including 15-level output, high voltage gain, galvanic isolation, and reactive-power capability remain unchanged. The proposed FCV control utilizes one of the switching angles at the voltage-level transitions as the control variable in a single-loop design, which facilitates control simplicity in the proposed system. The hybrid BCMLI with the proposed FCV control shows relative advantages of voltage gain, isolation capability, reactive supply, and simple control system over the state-of-the-art hybrid inverter topologies with FCV control methods. The details of the hybrid BCMLI, modulation schemes, proposed FCV control design, and comparative assessment are included in this article. In order to validate the proposed system and FCV control, a set of experiments with a 1-kW BCMLI prototype has been conducted for wide-range load levels and power factors.

This brief presents a unified voltage balancing feedforward (UVBFF) for three-level boost (TLB) power-factorcorrection (PFC) circuit in discontinuous and critical conduction modes. In general, the topology in both modes puts emphasis on low-cost system. Thus, electrolytic capacitors with wide-tolerance are commonly used at outputs and voltage unbalance is inevitable even under symmetric operations of two boost cells. In this paper, three-level switching capability of TLB PFC is introduced and asymmetric on-time feedforward for main switches is proposed. The proposed UVBFF can be realized by a single equation and harmonized with conventional feedback voltage-loop so that easy implementation is the key merit. The effectiveness of UVBFF is proven by analyses and experimental results.

Conventional critical conduction mode (CRM) control for many boost-derived power-factor-correction (PFC) circuits forms triangular inductor current. For three-level boost (TLB) converter, the conventional method is unattractive because losses increase but waveform quality remains unchanged, compared to other topologies. In order to obtain higher efficiency and improved waveform qualities, this paper focuses on the distinctive structure of TLB and its inherent degree-of-freedom (DOF) in current-slope shaping. Based on the DOF, a new CRM control for TLB PFC is proposed and analyzed. Detailed design and digital implementation method are also provided. Different from conventional methods, the proposed CRM control divides each switching cycle into three parts including common on-time of two switches, additional on-time of one switch and common off-time. Accordingly, TLB inductor current is synthesized in quadrangular forms. The analyses and experimental results confirm that switching frequency and peak input current of TLB can be reduced by the proposed control. Due to the reductions, efficiency, total-harmonic-distortion (THD) and quality of input current are improved with practically unchanged power factor.

Pseudo-dc-link inverters are promising due to the single-stage configuration. The inverters can be isolated by high-frequency transformers, which also provide the freedom of step-up/down through winding turns ratio. Conventionally, a pseudo-dc-link inverter is based upon a pulsewidth modulation converter like flyback converter, but the accompanying switching losses can degrade efficiency and overall system performance. Alternatively, resonant converters (e.g., series resonant converter) seem to fit the need as they typically display soft-switching characteristics. However, the relevant research results are scarce because of two main reasons: high resonant current and limited soft-switching range. To overcome the difficulties, a hybrid modulation method is proposed in this article for a pseudo-dc-link inverter based on series resonant converter. In high-line region, the inverter operates with variable-frequency modulation, which leads to zero-voltage switching and relatively low-resonant current. In low-line region, the inverter operates with a short-pulse density modulation so that switching losses are greatly curtailed without excessive resonant current. The merits in both regions together contribute toward the high efficiency of the inverter. This article presents the operational principle of the proposed modulation as well as a comparison with other modulation methods. In addition, a 400 V-230 Vac, 2-kW prototype was built to verify the proposed modulation and the peak efficiency reaches 98.0%.

This paper presents modeling, control and system design of three-level boost (TLB) rectifier based medium-voltage solid-state transformer (SST) for dc fast charger application. In order to operate at medium-voltage with 1.2 kV SiC devices, the target system is configured by input-series output-parallel (ISOP) multi-module structure. Each module adopts TLB to front-end rectifier and stacked half-bridge LLCs to back-end isolated dc-dc converters. Operation principles of TLB rectifier and LLC converters are analyzed to derive equivalent model of entire system. Based on the detailed derivation, this paper aims to achieve three performance criteria including bus voltage regulation, input current regulation and voltage balancing. Basic feedback control-loops are designed to control TLB’s output voltage and input current in average manner, based on the derived system model. Then, ideal TLB duty-ratios in both continuous/discontinuous conduction mode are analyzed and duty-ratio feedforward control is proposed to improve low quality of input current due to inherent feedback control limitations. For voltage balancing, system design approaches are used in the SST prototype. With the proposed control and system design, a prototype of 4-module-stacked TLB SST was built and tested up to 3.8 kVrms and 16 kW conditions.

In photovoltaic applications, many previous researches have focused on pulse width modulation (PWM) resonant converters in order to achieve a high efficiency with a wide input voltage range. Conventional approaches utilized symmetric boosting modulation at the secondary side rectifier to obtain a symmetric operation, and they utilized two boosting modes in a switching period. Among various rectifier structures, the voltage doubler structure has a strong advantage due to a small number of components. However, it suffers from serious hard switching losses in the secondary side rectifier. In this paper, a new converter with a novel asymmetrical modulation is proposed and verified. The strong point of the proposed converter is that it eliminates hard switching turn on losses from the rectifier, while maintaining the minimized number of components. Although the proposed converter adopts an asymmetric modulation, the offset current on the transformer becomes zero inherently. Furthermore, a “forced half resonance” operation of the proposed converter keeps RMS current stresses at the same level as conventional converter although it has a higher peak current. Accordingly, the proposed converter achieves a superior efficiency with the minimum number of components at 35–25V input and 380V/300W output specification.

This paper presents a single‐phase differential‐type photovoltaic inverter named single inductor dual buck‐boost inverter (SIDBBI) based on improved half‐cycle PWM (HPWM). Conventional DBBI (CDBBI) is a good option for photovoltaic application due to step‐up/down ability for varying inputs. However, inherent high inductor current and corresponding increments of inductor size and device ratings have made it less attractive than other topologies. This paper proposes an HPWM scheme with active clamping switches to reduce inductor voltage swing, highly induced current and component stresses by eliminating output voltages offsets. Under the proposed modulation, component voltage stresses in CDBBI can be reduced by zero offset, and there arises a chance to integrate half‐utilized inductors, main switches and complementary diodes of two modules into one set, which is the proposed SIDBBI topology. 500 W prototypes of the conventional/proposed inverters have been built for verifications and comparison. Detailed analysis and experimental results under the proposed HPWM show that the size of SIDBBI can be reduced by 40.4% than CDBBI, maintaining comparable efficiency over 96% and wide input coverage by step‐up/down ability. Comparative results address that SIDBBI is competitive in terms of component counts, stresses and losses, compared with various kinds of buck‐boost inverter topologies.