Haibing Hu

Operation Mode Analysis and Peak Gain Approximation of the LLC Resonant Converter

With the advantage of achieving zero voltage switching for a wide input voltage range, the LLC resonant topology has become increasingly popular for use in high power density and high-efficiency power converter applications. However, when the LLC converter is applied to wide input voltage range applications, the widely used fundamental harmonic approximation is incapable of guiding the design due to its inaccuracy. Thus an accurate LLC converter model is desired. In this paper, a generalized mode analysis is presented that provides highly accurate prediction on resonant current and voltage behavior and dc gain characteristic. Also, because operation modes are affected by load, frequency, and gain conditions, the boundaries and distribution of modes are discussed and illustrated. Based on the mode analysis, an approximation method is developed to estimate the peak gain point, which is useful in LLC design. This approximation demonstrates high accuracy within the simulation results. An experimental prototype is built to verify the analysis.

Power decoupling techniques for micro-inverters in PV systems-a review

This paper reviews the power decoupling techniques of micro-inverters used in single-phase, grid-tied PV systems. The power decoupling techniques are categorized into three groups: (1) PV side decoupling; (2) DC link decoupling; and (3) AC side decoupling. Various topologies and techniques are presented, compared, and evaluated against the size of capacitance, efficiency and control complexity. Finally, potential topologies and technologies are pointed out as the best options for power decoupling implementation.

A three-port Photovoltaic (PV) micro-inverter with power decoupling capability

This paper presents a new micro-inverter topology that is intended for single-phase grid-connected PV systems. The features of the proposed topology are: (1) eliminating the double-frequency power ripple using small film capacitor; (2) improving the maximumpower-point tracking (MPPT) performance; (3) using long life-time film capacitors, which will improve the reliability of the inverter; and (4) requiring no additional circuitry to manage the transformer leakage energy.

Efficiency Improvement of Grid-Tied Inverters at Low Input Power Using Pulse-Skipping Control Strategy

A pulse-skipping control strategy is proposed to improve efficiency of grid-tied inverters at light loads. To maximize the efficiency of pulse-skipping operation mode, three key parameters are identified and optimized based on a loss model, which is developed to find the maximal efficiency points using a 3-D search technique. A 200-W prototype inverter was setup to verify the proposed control strategy. The experimental results show that the proposed pulse-skipping control strategy greatly improves the inverters efficiency at light loads and match the simulation results fairly well, thus, validating the proposed optimization method for pulse-skipping operation.

Efficiency improvement of grid-tied inverters at low input power using pulse skipping control strategy

Pulse skipping control strategy is applied to improve efficiency of grid-tied inverter at light load. To maximize the efficiency of pulse skipping operation mode, three key parameters are identified and can be optimized based on a loss model, which is developed to find the maximal efficiency points using three-dimension searching technique. To reduce the potential of pulsating on the power grid, the synchronization of the power pulse to the grid is addressed by varying the DC bus voltage window, namely varying the upper and lower DC voltage limits. A 200W prototype is setup to verify the proposed strategy. The experimental results show that the proposed strategy greatly improves the efficiency at light load and match the simulation results fairly well, thus verifying the validity of the proposed optimization method for pulse skipping.

An integrated three-port inverter for stand-alone PV applications

This paper presents a novel concept of integrated three-port interface for stand-alone photovoltaic applications. The three-port topology interfaces one solar panel input port and one bi-directional battery port to an isolated output port which generates a rectified sinusoid voltage. Then an unfolding circuit can be adopted to generate an ac wave with very high efficiency because it is operated at very low frequency (50/60Hz). Therefore, this proposed structure uses only one switch-mode conversion stage to replace several independent converters and inverters in order to reduce component count and save the cost, making itself a valuable choice for low cost low power standalone PV system. The circuit operation and control architecture of the three-port interface are presented. It can achieve maximum power harvesting for the solar port, battery charge control for the battery port, while keeping a regulated rectified sinusoid output. The experiments of the three-port interface confirm the topology operation and its ability to achieve the multi-functional power management control.

Hybrid ZVS BCM Current Controlled Three-Phase Microinverter

This paper presents a new zero-voltage switching (ZVS) control method that is suitable for low-power applications such as three-phase microinverters. The proposed hybrid control method increases the efficiency and power density of the microinverters and features both reduced number of components and easy digital implementation. ZVS is achieved by controlling the inductor current bidirectional in every switching cycle and results in lower switching losses, higher operating frequency, and reduced size and cost of passive components, especially magnetic cores. A 400 W prototype of a three-phase inverter and its hybrid control system have been designed and tested under different conditions to verify the effectiveness of the controller. Efficiency measurement and comparison of the three different current modulation schemes have been conducted, and the inverter exhibits peak efficiency of 98.4% with fixed reverse current boundary conduction mode modulation.

High efficiency current mode control for three-phase micro-inverters

Three-phase micro-inverters are critical to the success of AC modules in Mega Watt PV farms. A high performance micro-inverter must have high power density, high reliability, and low cost. Boundary Current Mode (BCM), Variable Hysteresis Current Mode (VHCM), and Constant Hysteresis Current Mode (CHCM) are derived from a proposed softswitching current mode control scheme which is based on the general half-bridge three-phase inverter topology. The frequency range and switch losses are compared and discussed. The VHCM has the highest efficiency at over 97.6%, while the CHCM has the narrowest frequency range. A hybrid control platform combining analog and logic units with DSP was designed and built to achieve the high-speed peak current control. A high frequency, high efficiency, and high power density micro-inverter was built for experimentation. The experimental results verify that the proposed control scheme is a promising solution for high performance three-phase micro-inverters.

High Efficiency Dual-Mode Current Modulation Method for Low-Power DC/AC Inverters

Boundary conduction mode (BCM) zero voltage switching (ZVS) current control is a promising soft switching method for microinverter applications. In this letter, different BCM ZVS current control modulation schemes are compared based on power losses breakdown, switching frequency range, and current quality. Compared to continuous conduction mode current control, BCM ZVS control decreases MOSFET switching losses and filter inductor conduction losses but increases MOSFET conduction losses and inductor core losses. Based on the loss analysis, a dual-mode current modulation method combining ZVS and zero current switching schemes is proposed to improve the efficiency of the microinverter. The experimental results show that by using this proposed current modulation scheme, higher efficiency of 0.5% can be achieved with no additional cost for a 400-W three-phase microinverter.

A comparison of soft and hard-switching losses in three phase micro-inverters

The efficiency of a PV grid-tie micro-inverter is of critical concern to its application. Three representative three-phase voltage source micro-inverter topologies have been selected to make loss comparisons: A Hard Switching Inverter (HSI), an Auxiliary Resonant Commutated Pole Inverter (ARCPI), and an Actively Clamped Resonant DC-link Inverter (ACRDCLI). Loss composition and estimation are given in detail for 350W micro-inverters. It is pointed out that the HSI has outstanding advantages in both efficiency and cost. The ARCPI losses are lower than the other two topologies. But the auxiliary circuits are costly and the controlling and driving circuits are complicated. The ACRDCLI has a simple structure but high DC-link losses. If an improved control scheme is used, the DC-link losses of the ACRDCLI can be reduced. This paper compares these advantages and limitations for assessing the selection of soft-switching topologies for micro-inverters.