Yinglai Xia

High Performance ZVT with Bus Clamping Modulation Technique for Single Phase Full Bridge Inverters

This paper proposes a topology based on bus clamping modulation and zero-voltage-transition (ZVT) technique to realize zero-voltage-switching (ZVS) for all the main switches of the full bridge inverters, and inherent ZVS and/or ZCS for the auxiliary switches. The advantages of the strategy include significant reduction in the turn-on loss of the ZVT auxiliary switches which typically account for a major part of the total loss in other ZVT circuits, and reduction in the voltage ratings of auxiliary switches. The modulation scheme and the commutation stages are analyzed in detail. Finally, a 1kW, 500 kHz switching frequency inverter of the proposed topology using SiC MOSFETs has been built to validate the theoretical analysis. The ZVT with bus clamping modulation technique of fixed timing and adaptive timing schemes are implemented in DSP TMS320F28335 resulting in full ZVS for the main switches in the full bridge inverter. The proposed scheme can save up to 33 % of the switching loss compared with no ZVT case.

Optimal variable switching frequency scheme to reduce combined switching loss and inductor core loss of single phase grid connected inverter

Reducing the power loss of a converter without increasing volume or cost in hardware has always been a much sought-after, but challenging goal. This paper discusses a scheme to improve the system overall efficiency while still meeting a given THD requirement by implementing variable instantaneous switching frequency method without any hardware changes. This paper aims at minimizing the combined switching loss and inductor core loss by including different characteristics of the two types of losses relative to the instantaneous switching frequency in the optimization process. A 1kW H-bridge inverter using SiC MOSFETs and powdered iron core has been built to validate the theoretical analysis. Both constant switching frequency scheme (20 kHz) and optimal variable frequency scheme with simple code modification have been implemented in the experimental prototype using DSP TMS320f28335. The optimal variable switching frequency scheme shows a significant saving of 27.6% on the combined switching loss and inductor core loss compared with constant switching frequency scheme.

A high performance T-type single phase double grounded transformer-less photovoltaic inverter with active power decoupling

Transformer-less PV inverters are gaining widespread applications with lower cost, reduced footprint, and improved efficiency. This paper proposes a topology that can eliminate the common mode leakage current which is a major challenge in transformer-less PV inverters. In addition, an active power decoupling strategy is implemented in this topology instead of using large energy storage element for double line frequency power decoupling, thus achieving a smaller volume. A constant input voltage with negligible double line frequency ripple component ensuring high MPPT efficiency is achieved in this topology. Compared to the previously proposed topology, a T-type branch is added as an improvement to fully take advantage of the inherent three level structure resulting in much reduced switching loss and inductor current ripple. A 1 kW, 100 kHz single-phase prototype with 200 V DC input and 120 V/60 Hz AC output using SiC MOSFETs has been built to validate the theoretical analysis. The control strategy and modulation scheme are implemented in DSP TMS320F28335 resulting in 24% reduction in the total loss and 50% reduction in the inductor current ripple.

A GaN based doubly grounded, reduced capacitance transformer-less split phase photovoltaic inverter with active power decoupling

Transformer-less PV inverters are gaining widespread applications with lower cost, reduced footprint, and improved efficiency. This paper proposes a topology that can eliminate the common mode leakage current which is a major challenge in transformer-less PV inverters. In addition, an active power decoupling strategy is implemented in this topology instead of using large energy storage element for double line frequency power decoupling which is a common problem in single phase inverters / rectifiers, thus achieving a smaller volume and replacing electrolytic capacitors with film capacitors to increase reliability. A constant input voltage with negligible double line frequency ripple component less than 2% ensuring high MPPT efficiency is achieved in this topology through control strategy. Compared with previous topology, the 120 Hz inductor current ripple in the boost stage is reduced significantly resulting in the decrease of the RMS current value by 14% and the peak current value by 41%. Also the RMS current in the input capacitor is decreased by 91%. The dc link capacitor volume is also decreased with the increase of the voltage level. Finally, a 1 kW, 100 kHz single-phase prototype with 200 V DC nominal input and 120 V/60 Hz AC output in split phase configuration using GaN FETs has been built to validate the theoretical analysis. The control strategy and modulation scheme are implemented in DSP TMS320F28335.

A single phase transformerless string inverter with large voltage swing of half-bridge capacitors for active power decoupling

The transformerless single phase inverters are becoming common due to its advantages of reduced volume, lower cost, and higher efficiency but it has two implementation challenges-high frequency capacitive ground current and decoupling of double line frequency power. This paper proposes an optimized power decoupling topology for a single phase string inverter which addresses both the challenges as well as minimizes capacitance required to decouple the ripple power. Unlike conventional power decoupling techniques in full bridge converters, the proposed technique does not significantly increase the voltage stress on the devices. The combination of a half-bridge inverter and a buck boost converter ensure the complete elimination of high frequency capacitive coupled ground currents. The proposed technique requires only 40 μF/kW at 600 V for power decoupling. A closed loop controller design for the converter is detailed and the experimental results at 1 kW, 120 V, 60 Hz output for closed loop operation are provided.

Adaptive Dc link voltage control scheme for single phase inverters with dynamic power decoupling

Power density and efficiency are important metrics for power converters in many applications including renewable energy interface. Combined with the desire to replace the bulky, unreliable electrolytic dc link capacitors with film or ceramic capacitors due to reliability concerns, this has led to the trend of allowing increasingly higher double line frequency dc link voltage ripple. This paper discusses a scheme to improve the system overall efficiency by implementing an adaptive dc link voltage control scheme under different operating conditions for a recently introduced class of single phase inverter topologies that operate with very large dc link voltage ripple. The relationship between the minimum necessary dc link voltage for different input voltage, apparent power and power factor are analyzed and the optimal dc link voltage average value are derived analytically. The analysis and benefits of the proposed scheme are validated on a single-phase inverter hardware prototype employing SiC MOSFETs and DSP28335 for control implementation. The scheme can decrease the dc link voltage average value by 24.1% while decreasing the total loss of the inverter by around 20% compared to a design that does not use the adaptive dc link approach.

Influence of bi-directional power flow on impedance and stability of cascaded systems

This paper presents an impedance based stability criterion of cascaded active converter systems with bi-directional power flow from the perspective of individual subsystems. Influence of bi-directional power flow on the impedance of the subsystems and the stability of the cascaded system are studied. Though bi-directional power flow does not change the stability criterion, it changes the impedance (mostly the phase) of the susbsystems. Unstable critical frequencies caused by impedance interaction are predicted by the proposed impedance criterion in bi-directional power flow situation. Simulation results of a cascaded system comprising of a dual active bridge (DAB) dc-dc stage and a H bridge dc-ac stage are presented as an example to verify the analysis of the influence of bi-directional power flow. The small signal impedance on the dc side of the single phase H bridge is calculated based on power balance analysis. Controller improvement approaches to eliminate impedance interaction between subsystems are discussed.

Inductor feedback ZVT based, low THD single phase full bridge inverter with hybrid modulation technique

This paper proposes a topology based on zero-voltage-transition (ZVT) technique to realize zero-voltage-switching (ZVS) for all the main switches of the full bridge inverter, and inherent zero-current-switching (ZCS) for the auxiliary switches. The advantages of the strategy include the provision to implement zero state modulation schemes such as unipolar or hybrid PWM scheme in the full bridge inverters to decrease the inductor current THD, naturally adaptive auxiliary inductor current and elimination of need for large balancing capacitors. The modulation scheme and the commutation stages are analyzed in detail. The whole inverter including the auxiliary ZVT branch is modelled. Finally a 1 kW, 400 kHz switching frequency inverter of the proposed topology using SiC MOSFETs has been built to validate the theoretical analysis. The ZVT with hybrid modulation technique is implemented in DSP TMS320F28335 resulting in full ZVS for the main switches in the full bridge inverter. The proposed scheme can save up to 32 % of the switching loss compared with no ZVT case.

Comprehensive comparison of THD and common mode leakage current of bipolar, unipolar and hybrid modulation schemes for single phase grid connected full bridge inverters

This paper presents a comprehensive analysis and comparison among the three commonly used PWM schemes for single phase full bridge inverters namely, bipolar, unipolar and hybrid modulation schemes. The inductor current ripple and THD are analyzed using time domain analysis methods and the performance of the three modulation schemes are compared in detail for a range of modulation indices. An interesting result is that in order to achieve the same THD, compared with unipolar PWM, the average switching frequency (considering all the switches) of hybrid PWM is same but the average switching frequency of bipolar PWM needs to be around 3.6 times, when the modulation ratio is between 0.6 and 1. This results in the corresponding increase in the device switching losses. The common mode voltage between the negative terminals of the input and the grid, and the resulting leakage current are also analyzed and compared among the three schemes. Finally, a SiC MOSFETs based 1 kW full bridge inverter using the three modulation schemes has been built to validate the theoretical analysis. The inductor current ripple, THD and common mode leakage current analysis are verified through experiment.