Zhaoming Qian

Design and Analysis of the Droop Control Method for Parallel Inverters Considering the Impact of the Complex Impedance on the Power Sharing

This paper investigates the characteristics of the active and reactive power sharing in a parallel inverters system under different system impedance conditions. The analyses conclude that the conventional droop method cannot achieve efficient power sharing for the case of a system with complex impedance condition. To achieve the proper power balance and minimize the circulating current in the different impedance situations, a novel droop controller that considers the impact of complex impedance is proposed in this paper. This controller can simplify the coupled active and reactive power relationships, which are caused by the complex impedance in the parallel system. In addition, a virtual complex impedance loop is included in the proposed controller to minimize the fundamental and harmonic circulating current that flows in the parallel system. Compared to the other methods, the proposed controller can achieve accurate power sharing, offers efficient dynamic performance, and is more adaptive to different line impedance situations. Simulation and experimental results are presented to prove the validity and the improvements achieved by the proposed controller.

A Robust Control Scheme for Grid-Connected Voltage-Source Inverters

This paper analyzes the stability problem of the grid-connected voltage-source inverter (VSI) with LC filters, which demonstrates that the possible grid-impedance variations have a significant influence on the system stability when conventional proportional-integrator (PI) controller is used for grid current control. As the grid inductive impedance increases, the low-frequency gain and bandwidth of the PI controller have to be decreased to keep the system stable, thus degrading the tracking performance and disturbance rejection capability. To deal with this stability problem, an H∞ controller with explicit robustness in terms of grid-impedance variations is proposed to incorporate the desired tracking performance and the stability margin. By properly selecting the weighting functions, the synthesized H∞ controller exhibits high gains at the vicinity of the line frequency, similar to the traditional proportional-resonant controller; meanwhile, it has enough high-frequency attenuation to keep the control loop stable. An inner inverter-output-current loop with high bandwidth is also designed to get better disturbance rejection capability. The selection of weighting functions, inner inverter-output-current loop design, and system disturbance rejection capability are discussed in detail in this paper. Both simulation and experimental results of the proposed H∞ controller as well as the conventional PI controller are given and compared, which validates the performance of the proposed control scheme.

Three-level LLC series resonant DC/DC converter

Paper presents a three level soft switching LLC series resonant DC/DC converter. ZVS is achieved for each main switch without any auxiliary circuit. Voltage stress of each main switch is half of input voltage. ZCS is achieved for rectifier diodes. Wide input/output range can be achieved under low frequency range because of two-stage resonance. Only one magnetic component is needed in this converter. Efficiency is higher in high line input, so this converter is fit for power products with hold up time requirement. The principle of operation and the characteristics of the new converter are analyzed and verified on a 500 V/spl sim/700 V input 54 V/10 A output experimental prototype, whose efficiency is 94.1% at rating condition.

A New Design Method for High-Power High-Efficiency Switched-Capacitor DC–DC Converters

Switched-capacitor technology is widely used in low power dc-dc converter, especially in power management of the integrated circuit. These circuits have a limitation: high pulse currents will occur at the switching transients, which will reduce the efficiency and cause electromagnetic interference problems. This makes it difficult to use this technology in high-power-level conversion. This paper presents a new design method for dc-dc converter with switched-capacitorldquo technology. The new method can reduce the high pulse current which usually causes serious problem in traditional converters. Therefore, the power level of this new designed converter can be extended to 1 kW or even higher. A 1-kW 42/14-V switched-capacitor converter was designed for 42-V automotive system. The proposed converter has no requirement for magnetic components and can achieve a peak efficiency of 98% and 96 % at full load. The main circuit of the dc-dc converter is analyzed and its control scheme is presented in the paper. The experimental results verified the analysis and demonstrate the advantages.

Analysis and Optimization of LLC Resonant Converter With a Novel Over-Current Protection Circuit

A novel over-current protection (OCP) method for a LLC resonant converter has been proposed in this paper. This method is very attractive for its inherent current limit ability, especially under short-circuit condition. Combined with the frequency increasing method, good current limit features can be achieved. Compared to the conventional LLC resonant converter with frequency increasing method, LLC with the proposed OCP method can reduce the maximum frequency to an acceptable value. Detailed theoretical analysis and optimization design considerations have been presented. An experimental prototype converter based on the proposed method has been built up to verify the theoretical analysis. Experimental results meet the theoretical analysis very well

Maximum boost control of the Z-source inverter

This paper explores control methods for the Z-source inverter and their relationships of voltage boost versus modulation index. A maximum boost control is presented to produce the maximum voltage boost (or voltage gain) under a given modulation index. The control method, relationships of voltage gain versus modulation index, and voltage stress versus voltage gain are analyzed in detail and verified by simulation and experiment.

A magnetic-less DC-DC converter for dual-voltage automotive systems

The automotive industry is moving toward 42 volt to meet the more electric needs. Several dual voltage (42 and 14 volt) architectures have been proposed for the transition and accommodation of 14-volt loads. A DC-DC converter that connects the 42 and 14 volt architectures is one key device in any dual voltage architecture. This paper presents a compact, efficient, magnetic-less bi-directional DC-DC converter for dual voltage (42/14 Volt) automotive systems. The DC-DC converter is based on the generalized multilevel converter topology having the ability to balance battery voltages, emit zero or low electromagnetic interference (EMI), and have low cost by using low-voltage metal oxide field effect transistors (MOSFETs). The main circuit of the DC-DC converter is analyzed and its control scheme is presented in the paper. A self-powered gate drive circuit is developed for the DC-DC converter to reduce costs, signal connections, and circuit complexity. A prototype has been built and experimental results are presented.

Design of a two-stage low-frequency square-wave electronic ballast for HID lamps

This paper presents a two-stage low-frequency square-wave electronic ballast for high-intensity discharge lamps. The ballast consists of a power-factor-correction stage to achieve power-factor correction and an inverter to supply the lamp with low-frequency square-wave current. Due to its low-frequency operation, no acoustic resonance occurs. Ignition, warmup process, and end-of-life detection are analyzed in detail. The presented circuit was verified with simulation and experimental results.

An Improved Repetitive Control Scheme for Grid-Connected Inverter With Frequency-Adaptive Capability

The power quality of grid-connected inverters has drawn a lot of attention with the increased application of distributed power generation systems. The repetitive control technique is widely adopted in these systems, due to its excellent tracking performance and low output total harmonic distortion (THD). However, in an actual system, the ratio of the sampling frequency to the grid frequency cannot always maintain an integer, and then, the resonant frequencies of the repetitive control technique will deviate from the real grid fundamental and harmonic frequencies. This will degrade the performance of the system, particularly when the grid frequency varies. Even if the ratio is a fixed integer, the auxiliary function for stabilization in the conventional repetitive control technique will also increase the steady-state tracking error and THD of the system. In this paper, an improved repetitive control scheme with a special designed finite impulse response (FIR) filter is proposed. The FIR filter cascaded with a traditional delay function can approximate the ideal repetitive control function of any ratio. The proposed scheme varies the FIR filter according to varied grid frequency and maintains its resonant frequencies matching the grid fundamental and harmonic ones. Finally, the simulation and experimental results show that the improved repetitive control scheme can effectively reduce the tracking error and compensate harmonics of the inverter systems.

Novel sampling algorithm for DSP controlled 2 kW PFC converter

This paper proposes a novel sampling algorithm for digital signal processing (DSP) controlled 2 kW power factor correction (PFCs) converters, which can improve switching noise immunity greatly in average-current-control power supplies. Based on the newly developed DSP chip TMS320F240. a 2 kW PFC stage is implemented. The novel sampling algorithm shows great advantages when the converter operates at a frequency above 30 kHz.