Voltage and current stress reduction in single-stage power factorcorrection AC/DC converters with bulk capacitor voltage feedback

Single-stage power factor correction (PFC) AC/DC converters integrate a boost-derived input current shaper (ICS) with a flyback or forward DC/DC converter in one single stage. The ICS can be operated in either discontinuous current mode (DCM) or continuous current mode (CCM), while the flyback or forward DC/DC converter is operated in CCM. Almost all single-stage PFC AC/DC converters suffer from high bulk capacitor voltage stress and extra switch current stress. The bulk capacitor voltage feedback with a coupled winding structure is widely used to reduce both the voltage and current stresses in practical single-stage PFC AC/DC converters. This paper presents a detailed analysis of the bulk capacitor voltage feedback, including the relationship between bulk capacitor voltage, input current harmonics, voltage feedback ratio, and load condition. The maximum bulk capacitor voltage appears when the DC/DC converter operates at the boundary between CCM and DCM. This paper also reveals that only the voltage feedback ratio determines the input current harmonics under DCM ICS and CCM DC/DC operation. The theoretical prediction of the bulk capacitor voltage as well as the predicted input harmonic contents is verified experimentally on a 60 W AC/DC converter with universal-line input     

基于开环的临界续断模式交错并联Boost PFC

文中主要研究的对象是开环控制的交错并联BOOST PFC,且工作于临界续断模式,它的从变换器与主变换器在开通时同步,且主从变换器都工作在电流模式。文章指出只有这种主从方式能提供一个稳定的开环工作点。仿真实验设计了一台输入功率为400W,宽范围输入电压,400V输出电压的实验样机,实验结果验证了理论分析的正确性。     

Single-stage three-phase buck-boost type AC-DC converter with highpower factor

A new control process for single-stage three-phase buck-boost type AC-DC power converters with high power factor, sinusoidal input currents and adjustable output voltage is proposed. This converter allows variable power factor operation, but this work focus on achieving unity power factor. The proposed control method includes a fast and robust input current controller based on a vectorial sliding mode approach. The active nonlinear control strategy applied to this power converter, allows high quality input currents. Given the comparatively slow dynamics of the DC output voltage, a proportional integral (PI) controller is adopted to regulate the converter output voltage. The voltage controller modulates the amplitudes of the current references, which are sinusoidal and synchronous with the input source voltages. Experimental results from a laboratory prototype show the high power factor and the low harmonic distortion characteristics of the circuit     

Analysis and design of a single-stage single-switch bi-flyback ac/dc converter

An analysis and design of single-stage, single-switch bi-flyback ac/dc converter is presented. The main flyback stage controls the output power from the link capacitor voltage with Discontinuous Conduction Mode (DCM) or Continuous Conduction Mode (CCM) operation, while an auxiliary flyback stage supplies the power to the output directly from ac line input with DCM operation.

This scheme can effectively reduce the voltage stress on the link capacitor and can achieve the power factor correction (PFC) without a dead band at line zero-crossings, which reduces the harmonic distortion in ac line current. Theoretical analysis of the converter is presented and design guidelines to select circuit components are given. The experimental results on a 60?W (15?V, 4?A), 100?kHz ac/dc converter show that maximum link voltage and maximum efficiency are around 415?V and 82%, respectively. The power factor is above 0.96 under universal line input and load conditions.     

Analysis and Design of a Novel Three-Phase AC–DC Buck-Boost Converter

This paper proposes a novel three-phase ac-dc buck-boost converter. The proposed converter uses four active switches, which are driven by only one control signal. This converter is operated in discontinuous conduction mode (DCM) by using the pulsewidth modulation (PWM) technique, and the control scheme very easily and simply achieves purely sinusoidal input current, high power factor, low total harmonic distortion of the input current and step-up/down output voltage. Also, the proposed converter provides a constant average current to the output capacitor and load in each switching period. Thus, the ripple component of sixth times line frequency will not appear in the output voltage. Therefore, a smaller output capacitor can be used in the proposed converter. Moreover, the steady-state analysis of voltage gain and boundary operating condition are presented. Also, the selections of inductor, output capacitor and input filter are depicted. Finally, a prototype circuit with simple control logic is implemented to illustrate the theoretical analysis.     

Soft switching resonant converter with duty-cycle control in DC micro-grid system

Resonant converter has been widely used for the benefits of low switching losses and high circuit efficiency. However, the wide frequency variation is the main drawback of resonant converter. This paper studies a new modular resonant converter with duty-cycle control to overcome this problem and realise the advantages of low switching losses, no reverse recovery current loss, balance input split voltages and constant frequency operation for medium voltage direct currentgrid or system network. Series full-bridge (FB) converters are used in the studied circuit in order to reduce the voltage stresses and power rating on power semiconductors. Flying capacitor is used between two FB converters to balance input split voltages. Two circuit modules are paralleled on the secondary side to lessen the current rating of rectifier diodes and the size of magnetic components. The resonant tank is operated at inductive load circuit to help power switches to be turned on at zero voltage with wide load range. The pulse-width modulation scheme is used to regulate output voltage. Experimental verifications are provided to show the performance of the proposed circuit.     

Implementation of single-phase boost power-factor-correctioncircuits in three-phase applications

A three-phase rectifier employing three single-phase boost power-factor-correction circuits is analyzed. Each converter operates in the continuous conduction mode (CCM), which allows a high power factor and a small EMI filter. Current sharing is ensured by a common voltage loop driving the individual current loops of the three converters. A suitable circuit arrangement is devised to limit phase interaction. The zero-voltage-transition technique (ZVT) is successfully applied to each converter, in order to obtain zero turn on losses and soft turnoff of the freewheeling diodes. Results of a 1800-W 100-kHz experimental prototype are reported, which confirm the theoretical forecasts     

Analysis and Design for a Novel Single-Stage High Power Factor Correction Diagonal Half-Bridge Forward AC–DC Converter

By means of components placement, the buck-boost and diagonal half-bridge forward converters are combined to create a novel single-stage high power factor correction (HPFC) diagonal half-bridge forward converter. When both the PFC cell and dc–dc cell operate in DCM, the proposed converter can achieve HPFC and lower voltage stress of the bulk capacitor. The circuit analysis of the proposed converter operating in$ DCM+ DCM$mode is presented. In order to design controllers for the output voltage regulation, the ac small-signal model of the proposed converter is derived by the averaging method. Based on the derived model, the proportional integral (PI) controller and minor-loop controller are then designed. The simulation and experimental results show that the proposed converter with the minor-loop controller has faster output voltage regulation than that with the PI controller despite the variations of line voltage and load. Finally, a 100-W prototype of the proposed ac–dc converter is implemented and the theoretical result is experimentally verified.     

Inductorless DC-to-DC converter with high power density

A new type of switching-mode power supply containing no inductors or transformers is proposed. The controlled transfer of energy from a unregulated DC source to a regulated output voltage is realized through a switched-capacitor (SC) circuit. A duty-cycle control is used; the driving signals of the transistors in the SC circuit are determined by the feedback circuit. The absence of magnetic devices makes possible the realization of power converters of small size, low weight and high power density, able to be manufactured in IC technology. High efficiency, small output voltage ripple and good regulation for large changes in the input voltage and/or load values are other positive features of the new type of DC-to-DC power converter. The input-to-output voltage conversion ratio is flexible; the same converter structure can provide a large range of constant desired values of the output voltage for a given input voltage, by predetermining the steady-state conversion ratio. The frequency response shows good stability of the designed converter. The experimental results obtained by using a prototype of a step-down SC-based DC-to-DC converter confirmed the theoretical expectations and the computer simulation results.<>     

Novel Current-Mode AC/AC Converters With High-Frequency AC Link

A novel circuit-topology family of the current-mode AC/AC converter with high-frequency AC link, based on a Flyback converter, is proposed. These circuit topologies, which can transfer one unregulated sinusoidal voltage with high total harmonic distortion (THD) into another regulated constant-frequency sinusoidal voltage with low THD, are composed of input cycloconverter, high-frequency storage transformer, and output cycloconverter. The circuit-topology family includes single four-quadrant power switch mode, push-pull mode, half-bridge mode, and full-bridge mode circuits. The single four-quadrant power switch mode and push-pull mode converters are suitable for low input voltage fields, but the half-bridge mode and full-bridge mode converters are suitable for high input voltage fields. The operational mode, steady principle, and transient voltage feedback control strategy of the kind of converter are investigated. The output characteristic curve, its relation to internal resistance, and the design criteria for the key circuit parameters are given. The theoretical analysis and the test result of the 500 VA 220 V 15% 50 HzAC/220 V 50 HzAC prototype have shown that the converters have advantages such as high-frequency galvanic isolation, simple topology, two-stage power conversion [low frequency alternating current (LFAC)/high frequency alternating current (HFAC)/LFAC], bidirectional power flow, high efficiency, high power density, low THD of the output voltage, strong adaptability to various loads, higher line power factor, low audio noise, etc.     

Operating a three-phase diode rectifier with a low-input currentdistortion using a series-connected dual boost converter

This paper describes a technique for shaping the input current to a three-phase diode rectifier using a two-switch series-connected dual boost converter and a three-phase bidirectional switch circuit. Circuits are described for generating a single voltage DC output, “single DC-rail”, or a dual output DC voltage using center-tapped capacitors, “split DC-rail”. Both rectifier types can be operated with the boost inductors located either on the DC or the AC side of the rectifier. The resultant rectifier circuit configurations have an excellent immunity to the “shoot-through” fault condition and use active switching elements with low per-unit current ratings and low switching losses. These features increase the reliability factor and lower the cost penalty associated with unity fundamental power factor three-phase rectifiers. Test results are presented for the rectifiers using simulation and experimental results     

Input-Current Distortion of CCM Boost PFC Converters Operated in DCM

When power-factor correction (PFC) converters designed for operation in continuous-conduction mode (CCM) at full power are operated at reduced load, operation in discontinuous-conduction mode (DCM) occurs in a zone that is close to the crossover of the line voltage. This zone will gradually expand with decreasing load to finally encompass the entire line cycle. Whereas, in CCM, the parasitic capacitances of the switches only cause switching losses, in DCM, they are a source of converter instability, resulting in significant input-current distortion. In this paper, this source of input-current distortion is analyzed, and a solution is proposed. Experimental results are obtained using a digitally controlled boost PFC converter, which is designed to operate in CCM for 1 kW     

A Single-Phase Boost Rectifier System for Wide Range of Load Variations

Converters operated in discontinuous-conduction-mode (DCM) and in continuous-conduction-mode (CCM) are suitable for lighter and higher loads, respectively. A new, constant switching frequency based single-phase rectifier system is proposed, which operates in DCM and in CCM for outputs less than and greater than 50% rated load, respectively, covering a wide range of load variation. The power circuit and the control circuit of the proposed rectifier are easily configurable for DCM and CCM operations. The measured load current is used to select the desired operating mode. The peak device current under DCM is limited to rated device current under CCM without using a device of higher current rating. The input current shaping under CCM and DCM are based on the comparison of measured input current with linear and nonlinear carriers, respectively. A load current feedforward scheme is presented to improve the system dynamic performance and also to ensure a smooth transition between the two operating modes. All the necessary control operations are performed without using multiplication, division and square-root operation. The proposed rectifier shows improved input current characteristics over the existing CCM converters for the above load range. This is validated on a 600-W rectifier prototype. Simulation and experimental results are presented     

A Comparative Study of Boundary Control With First- and Second-Order Switching Surfaces for Buck Converters Operating in DCM

A performance comparison of boundary control with the first-(sigma¹) and second-order(sigma²) switching surfaces for buck converters operating in discontinuous conduction mode (DCM) is presented in this paper. Performance attributes under investigation include the average output voltage, output ripple voltage, switching frequency, parametric sensitivities to the component values, and large signal characteristics. Due to the presence of the output hysteresis band, an additional switching boundary formed by the zero-inductor-current trajectory is created. This phenomenon causes a shift of the operating point in converters with sigma¹. Conversely, the operating point remains unchanged in converters with sigma². As well as in continuous conduction mode (CCM), sigma² can make the converter revert to the steady-state in two switching actions in DCM and gives better static and dynamic responses than in both CCM and DCM. Most importantly, its control law and settings are applicable for both modes. Experimental results of a prototype are found to be in good agreement with theoretical predictions.     

Performance Comparison of Single-Stage Three-Level Resonant AC/DC Converter Topologies

In this paper, the performance of different three-level resonant converters is studied for single-stage power factor correction operation. These converters are suitable for power ranges higher than that in the currently available single-stage converters, due to their high efficiency and reduced component stresses. All the converters presented here are characterized by their ability to regulate the output voltage as well as the dc bus voltage. This leads to lower voltage stresses, wider input voltage range, higher output power applications, and improved efficiencies compared to existing single-stage topologies. Due to the availability of more degrees of freedom in the presented converters, two types of control strategies can be used for this purpose: variable frequency asymmetrical pulsewidth modulation control and variable frequency phase-shift modulation control. Three resonant converters will be studied in this paper and their performances as well as the applicability of the aforementioned control methods for each converter are compared. A 2.3-kW, 48-V converter with input voltage range of 90-265 V_rms is used to study the performance of each case.     

New zero voltage switching DC converter with flying capacitors

A new soft switching converter is presented for medium power applications. Two full-bridge converters are connected in series at high voltage side in order to limit the voltage stress of power switches at V_in/2. Therefore, power metal–oxide–semiconductor field-effect transistors (MOSFETs) with 600 V voltage rating can be adopted for 1200 V input voltage applications. In order to balance two input split capacitor voltages in every switching cycle, two flying capacitors are connected on the AC side of two full-bridge converters. Phase-shift pulse-width modulation (PS-PWM) is adopted to regulate the output voltage. Based on the resonant behaviour by the output capacitance of MOSFETs and the resonant inductance, active MOSFETs can be turned on under zero voltage switching (ZVS) during the transition interval. Thus, the switching losses of power MOSFETs are reduced. Two full-bridge converters are used in the proposed circuit to share load current and reduce the current stress of passive and active components. The circuit analysis and design example of the prototype circuit are provided in detail and the performance of the proposed converter is verified by the experiments.     

反激式开关电源的能量分析

结合能量和电路分析方法,详细探讨了反激式开关电源在DCM和CCM两种工作模式下的能量状态与电路状态以及影响输出功率的因素。研究表明,工作于CCM模式下的开关电源,其占空比并不是严格不变,在主线路电压大范围变化时,系统会自动调整占空比以保证功率的恒定输出。在输出负载变化时,系统占空比保持基本恒定,但通过自动调整变压器初级的初始电流使输出功率满足要求,以保证输出电压的稳定。DCM模式下通过减小变压器的初级电感量或降低振荡频率,可以获得较大的电感储能使输出功率增加。     

The discontinuous conduction mode Sepic and Cuk power factorpreregulators: analysis and design

Sepic and Cuk power converters working as power factor preregulators (PFP) in the discontinuous conduction mode (DCM) present the following desirable characteristics for a PFP: (1) the power converter works as a voltage follower (no current loop is needed); (2) the theoretical power factor is unity; and (3) the input current ripple is defined at the design stage. Besides, input-output galvanic isolation is easily obtained. This paper analyzes the operation of both power converters as DCM-PFP. Design equations are derived, as well as a small-signal model to aid the control loop design. Both simulation and experimental results are presented that are in agreement with the theoretical analysis and complement the work     

Uniform Voltage Distribution Control for Series Connected DC–DC Converters

This paper investigates applications of current-mode, shared-bus commercial-off-the-shelf (COTS) dc-dc converters to power system architectures configured as parallel-input, series-output (PISO) and series-input, parallel-output (SIPO). By employing a PISO (or SIPO) architecture, current-mode COTS converters can transform their system input voltage to higher (or lower) system output voltage, provide ease and flexibility of power expansion, and preserve system efficiencies equal to those of standalone converters. Nonuniform output (or input) voltages still exist within a PISO (or SIPO) power system using identical converters when the system lacks proper distribution control of the series connected output (or input) voltages-and thus, system reliability suffers from thermal overstress to the converters that contribute a greater portion of the output power. Through unified approaches of voltage distribution control for the PISO and SIPO architectures, a series-connected converter power system attains robust stability and reliability. Two effective approaches to uniform voltage distribution control-the central-limit and maximum-limit voltage distribution-will be discussed. Both computer simulation and experimental prototypes validate both of the uniform voltage distribution power converter architectures.     

The uni-polarity phase-shifted controlled voltage mode AC-AC converters with high frequency AC link

A circuit configuration and circuit topological family of voltage mode ac-ac converters with high frequency ac links, which are based on forward converters, and a uni-polarity phase-shifted control strategy are proposed and deeply investigated. These kinds of converters consist of a input cycloconverter, a high frequency transformer, an output cycloconverter, input and output filters, and can transfer an unsteady ac voltage with a high harmonic into steady same-frequency ac sinusoidal voltage with a low harmonic. By using uni-polarity phase-shifted control strategy, output filtering inductance current is naturally commutated, and zero voltage switching of the output cycloconverter is realized. The converters' averaging model, the output characteristic curve, and design criteria for the key circuit parameters are given. The theoretical analysis and test result of 1kVA 220V /spl plusmn/ 10% 50Hzac/110V 50Hz ac prototype have shown that the converters have such advantages as high frequency electrical isolation, simple topology, two-stage power conversion (LFAC/HFAC/LFAC), bidirectional power flow, uni-polarity synchronized pulsewidth modulation waveform, high efficiency, high power density, high steady precision, low total harmonic distortion of the output voltage, strong adaptability to various loads, high line power factor, and low audio noise etc.