Phase-shift Modulation Research Articles

Proposed asymmetric phase shift modulation strategy to improve zero-voltage-switch range and transmission power range for dual active bridge converter

The dual active bridge (DAB) DC–DC converter is typical in DC transformers to realize the voltage transformation and power transmission, which is determined by modulation. However, in traditional phase shift modulation strategies, the zero-voltage-switch (ZVS) range and the transmission power range of DAB are limited, and the current bias exists in light load. Therefore, a proposed asymmetric phase shift modulation strategy (PAPSM) to realize full ZVS range, larger transmission power, and non-DC bias in high frequency link (HFL) for DAB is introduced in this paper. In PAPSM, the modulation signals in adjacent periods are symmetric and in the opposite direction, and the current characteristics in the adjacent periods are still consistent. Thus, the equivalent period of DAB in PAPSM is expended, and the current bias accumulation in light load is eliminated in the equivalent period, without any extra operation. Then, the operating principles of DAB in PAPSM are introduced, to analyze the ZVS range, power delivery range, and maximum transmission power in detail. Compared with traditional phase shift modulation strategies, PAPSM can realize full ZVS range, wider power delivery range, and larger maximum transmission power by adjusting an outer phase shift ratio and an asymmetric duty ratio, and the DC bias in HFL can be eliminated. Finally, a DAB experimental setup is established based on RT-LAB, proving the theoretical analysis.

Fault detection and localization method for modular multilevel converters in offshore DC wind turbines

The offshore DC wind turbine, consisting of direct-drive permanent magnet synchronous generator (PMSG) and modular multilevel converter (MMC), has become a promising candidate for the large-capacity wind energy conversion system. Submodule (SM) failure detection and localization are crucial for reliable operation of the MMC. However, existing techniques mainly focus on the rated operation condition, which may not be suitable for the MMC in wind turbine due to varying wind speed. Moreover, proper injection of second-order harmonic circulating current (SHCC) is desirable to reduce capacitor voltage ripple and power loss, which may also affect the accuracy of fault detection. This paper proposes a fault-tolerant strategy immune to variable operation conditions, as well as SHCC injection, for the MMC with phase-shifted carrier modulation. Firstly, the inherent switching harmonics of circulating current with SM failure are analyzed, based on which the fault detection method is developed. Then, the localization approach is presented by measuring the difference of SM capacitor voltages. Compared with existing techniques, the proposed strategy requires neither estimator nor additional sensor, which is robust, simple, and economical. Finally, both simulation and experimental results demonstrate the effectiveness of the proposed method, which is not influenced by SHCC and varying wind speed.

PSO based universal phase shift modulation scheme for DAB converter to eliminate backflow power in energy storage applications

SummaryIn battery energy storage system (BESS) applications, the life of the battery depends on the quality of the charging/discharging current. However, the dual active bridge (DAB) converter, which interfaces BESS with the DC link, suffers from a backflow current in all modulation schemes, degrading the battery's performance. Hence, this paper proposes a particle swarm optimization (PSO) based universal phase shift (UPS) modulation scheme to improve the battery's life. In the proposed modulation scheme, the phase shift ratio of DAB to eliminate the backflow power is estimated using PSO. Also, it is observed that there is an improvement in the zero voltage switching (ZVS) range of the DAB. Hence, the overall efficiency of the DAB converter is increased over a wide operating region. The operating principle of the proposed modulation scheme, steady state analysis of power transmission, and backflow power are reported in this paper. The theoretical performance of the proposed modulation scheme is validated in a 2 kW experimental setup.

The Effects of PWM With High dv/dt on Partial Discharge and Lifetime of Medium-Frequency Transformer for Medium-Voltage (MV) Solid State Transformer Applications

Medium-frequency (MF) transformer in a phase-shift modulated dc solid state transformer (SST) is prone to partial discharge (PD) that impacts the system reliability and lifetime. Tightly packed MF transformer windings carrying PWM voltages create favorable conditions for PD inception. A peak potential condition is created between adjacent posterior windings as a result of phase-shifted modulation. This along with fast alternating electric fields incepts PD at lower than expected operating voltages. Early inception of PD lead to premature transformer failure leading to unanticipated DC SST down-time. The effects of PWM voltages containing high dv/dt switching transients on PD and lifetime characteristics of MF transformer in a dc SST is not yet understood, which is investigated in this paper. A non-intrusive PD detection strategy for MF PWM ac voltage is presented using high speed optical sensors. PD characterization is performed on Kapton polyimide insulated copper foil windings for a foil-type MF transformer. Winding samples are stress tested with steady state PWM voltages upto 2 kV and frequencies upto 50 kHz with high dv/dt upto 60 V/ns while maintaining the transformers geometric profile. Accelerated dielectric lifetime characterization is performed and novel lifetime prediction models are derived containing both frequency and dv/dt dependent variables.

A Simple DC-Offset Eliminating Method of the Series-Inductance Current for the DAB DC–DC Converter

Based on the phase-shift modulation, the dual-active-bridge (DAB) dc–dc converter always features the natural self-volt-second balance performance even during the transient process. Besides, different phase-shift ratios usually result in different initial series-inductance currents. Therefore, when the phase-shift ratio is suddenly changed for dealing with the change of required transferred power, a dc offset in the series-inductance current will emerge. Especially for the high-power condition, this dc offset lasts for a long time because of the high-current value but low damping conduction resistor. Then, the magnetic saturation phenomenon may emerge in the transformer, resulting in the potential failure of the converter system. To address this issue, a simple dc-offset eliminating method is proposed for the DAB dc–dc converter. Different from most of the existing methods, the proposed one is independent from the phase-shift modulation method. Therefore, the proposed dc-offset eliminating method can be easily embedded in different phase-shift modulation methods, such as single-phase-shift, double-phase-shift, and triple-phase-shift methods. Finally, the experimental results are provided to verify the effectiveness of the proposed dc-offset eliminating method for the DAB dc–dc converter.

Flattop Modulation Technique for a Low-Device Count Single-Stage AC-DC DAB Converter to Improve Its Overall Efficiency

This article proposes a flattop modulation technique for a single-stage, single-phase bidirectional ac-dc dual-active-bridge (DAB) converter for achieving a flattopped DAB inductor current. The flattopped DAB inductor current has a low root mean square (rms) value, and it improves the soft-switching performance of the converter. The novelty of the proposed technique is that it guarantees flattopped DAB inductor current throughout the ac line cycle and has direct control over the ac current. In this technique, sinusoidal pulse width modulation (PWM) is performed at the ac side bridge for direct ac current control. Sinusoidal phase shift modulation is performed at the dc side to generate a bridge output voltage with a pulsewidth identical to the ac side bridge. The high-frequency (HF) transformer turns ratio is appropriately selected to match the voltages at the DAB inductor’s ends. Thereby, flattop DAB inductor current is achieved throughout the line cycle. The proposed modulation technique ensures a regulated dc bus at the ac side and provides a ripple-free ac current by interleaving. The operation of the converter under the flattop modulation technique is analyzed in detail, and parameter design guideline is provided. Experimental results from a 2-kW converter prototype support the theoretical claims.

An SVPWM Algorithm for a Novel Multilevel Rectifier with DC-Side Capacitor Voltage Balance

The recently proposed novel unidirectional multilevel rectifier, a three single-phase star-connected multilevel rectifier, has the characteristic of having a large number of DC-side capacitors and a complex capacitor voltage balancing control structure under conventional carrier-based phase-shift sine wave pulse-width modulation (SPWM). Hence, a space vector pulse-width modulation (SVPWM) algorithm for the novel multilevel rectifier is proposed in this paper, which can quickly balance the capacitor voltage without an additional voltage balancing control structure. Firstly, it divides the space vectors of the rectifier, then it determines the two basis voltage vectors that synthesize the output reference voltage. After that, based on the analysis of the relationship between switching states and the charge–discharge of capacitors, the final action sequences of redundant vectors are determined according to the principle of keeping the capacitor charge–discharge time consistent. Thus, the capacitor voltages can be automatically balanced without an additional voltage balancing control structure. Finally, simulation and experimental results validated the feasibility and effectiveness of the proposed SVPWM algorithm. The results also show improvements in current quality, capacitor voltage balance and the fluctuation of the neutral point voltage on the DC-link, allowing for further reduction in the overall volume and cost of the rectifier.

Discrete-Time Sliding Mode Current Control for a Seven-Level Cascade H-Bridge Converter

This paper deals with the implementation and performance analysis of discrete-time sliding mode (DTSM) current control applied to a seven-level cascade H-bridge converter to track three-phase reference currents for a reactive load. The converter output voltages are synthesized using a modulation scheme based on phase-shifted carrier modulation. Simulation and experimental tests have been added to demonstrate the performance of the proposed controller. At the same time, the effectiveness of the DTSM is verified under transient and steady-state conditions, respectively, by measuring the total harmonic distortion and the mean square error.

An Improved Topology of Isolated Bidirectional Resonant DC-DC Converter Based on Wide Bandgap Transistors for Electric Vehicle Onboard Chargers

This article proposes an improved topology for an isolated bidirectional resonant DC-DC converter for electric vehicle (EV) onboard chargers. As opposed to the conventional capacitor-inductor-inductor-inductor-capacitor (CLLLC) resonant converter, the proposed converter’s resonant circuit is composed of a capacitor-inductor-inductor-inductor (CLLL) structure, whose inductances, except the capacitor, can be fully integrated with the leakage and mutual inductances of the high-frequency transformer (HF). Therefore, this offers a smaller size, lower costs, minimal power loss, and eventually higher efficiency. Again, the proposed converter design is based on wide bandgap (WBG) transistor switches that operate at MHz-level switching frequency to achieve high power density, high efficiency, and high compactness. A discrete-time proportional integral derivative (PID) controller has been designed using the phase-shifted pulse width modulation (PSPWM) technique to assure closed-loop control of the proposed CLLL converter. The PID controller parameters have been optimized using both the genetic algorithm (GA) and particle swarm optimization (PSO) algorithm and a comparative analysis has been presented between the two algorithms. To achieve fast switching with very little switching loss, the converter is simulated with several wide bandgap (WBG) switching devices. A performance comparison with conventional Si-based switching devices is also provided. A precise power loss model of the semiconductor switches has been devised from the manufacturer’s datasheet to achieve a perfect thermal design for the converter. A 5 kW CLLL converter with an input range of 400–460 V direct current (DC) and an output range of 530–610 V DC, and a switching frequency of 1 MHz has been designed and investigated under various loading scenarios. Gallium nitride (GaN) switching device-based designs achieved the highest levels of efficiency among the switching devices. The efficiency of this device is 97.40 percent in charging mode and 96.67 percent in discharging mode.

A Highly Integrated Hybrid DC–DC Converter With nH-Scale IPD Inductors

This article presents a new integrated step-down dc–dc hybrid converter that uses only nano-Henry scale inductors at 2–5-MHz switching frequency. Since it is derived from a Dickson-star switched capacitor (SC) converter, the proposed converter inherits the benefit of low voltage stress on switches while enjoying an efficient fine regulation by phase shift, similar to a dual active bridge (DAB) converter. The converter is optimized, designed, and fabricated in 1.7 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times $ </tex-math></inline-formula> 1.9 mm area of a 130-nm bipolar-CMOS-DMOS (BCD) process. The active die is flip-chipped on a 6.5 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times $ </tex-math></inline-formula> 6.5 mm package substrate together with power capacitors and two 10-nH integrated passive device (IPD) inductors for demonstration, illustrating the feasibility of passive components’ integration, resulting in a peak efficiency of 91.2% and a peak power density of 1.36 W/mm3 from 9.6–12-V input to 2.15–3.3-V output. Another demonstration is constructed on the same package substrate but with discrete air-core inductors. It achieves a peak efficiency of 92.4% and a peak power density of 0.62 W/mm3, while delivering a max power of 7.5 W. To achieve the performance, a detailed loss analysis and a unique optimization methodology for the converter, together with the design of key sub-blocks, including gate drivers (GDs), phase shift modulator (PSM), and ramp generator (RG), are provided in this article.

A Load Adaptive Hybrid DPS Control for DAB to Secure Minimum Current Stress and Full ZVS Operation Over Wide Load and Voltage Conversion Ratio

Wide variations in voltage conversion ratio and loading are the most important parameters to consider when designing off-board EV chargers to achieve high power density and efficiency. Dual phase shift (DPS) modulation provides a better way for dual active bridge (DAB) DC-DC converter operation when load side parameter variation is utmost important. To accomplish this, conventional DPS control is modified to ensure zero voltage switching (ZVS) across the entire load range despite wide variations in voltage conversion ratio. The designed control provides ZVS of all FETs as compared to the conventional DPS control over full loading range. For this, magnetizing inductance of HF isolation transformer is utilized and designed to shape the secondary side current for getting ZVS for both active legs. The light loading performance of the converter is improved with the bidirectional conduction angle control. The hybrid utilization of DPS control is incorporated with minimum peak current stress considerations. A Karush-Kuhn-Tucker (KKT) method based on Lagrange's multipliers is used for mathematical derivation of optimized solution of both control parameters. Performance of designed control is verified using experimental results for wide variation in battery terminal voltage. For validation, a 1250 W laboratory prototype is developed. A comparison of ZVS regions among the conventional and designed modulation technique, shows the effectiveness under wide voltage and loading scenarios.

SOC Balancing Control Method for Cascaded Energy Storage System under Extreme Working Conditions

To solve the problem of an unbalanced state of charge (SOC) between the in-phase sub-modules of the cascaded H-bridge energy storage system, this paper proposed a method based on carrier phase-shifting pulse-width modulation. Different from the traditional in-phase balancing method which is achieved by adding components, by setting the parameter related to the SOC difference to adjust the amplitude of the modulation wave, the new method could solve the problem that the traditional method is not applicable under extreme SOC conditions. The new in-phase SOC balancing method could expand the scenario, and the feasibility of the method is verified by simulation.

A Novel Phase-Shift Pulsewidth Modulation Method for Light-Load Bidirectional Resonant Converter

For the bidirectional full-bridge <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CLLC</i> resonant converter, soft switching of primary side and secondary side switches and bidirectional power transmission are its obvious advantages. However, in the pulse frequency modulation control under light-load conditions, the converter will have the disadvantages of output voltage imbalance and low transmission efficiency. In order to improve the range of voltage regulation and efficiency of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CLLC</i> resonant converter under light-load conditions, a novel phase-shift pulsewidth modulation method is proposed in this article. There is low voltage gain regulation and low operation losses can be achieved under light load. Considering the parasitic parameters and dead time of the switches, the voltage gain of the proposed control method under light load is precise research by using the time domain analysis method. Finally, an experimental prototype of a bidirectional full-bridge <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CLLC</i> resonant converter with 200 V input voltage, 200 V output voltage, and a rated power of 800 W is designed. According to the simulation and experimental results, it can be shown that the proposed control method is effective under light-load operation.

Autonomous Input Voltage Sharing Control and Triple Phase Shift Modulation Method for ISOP-DAB Converter in DC Microgrid: A Multiagent Deep Reinforcement Learning-Based Method

This article proposes a multiagent (MA) deep reinforcement learning (DRL) based autonomous input voltage sharing (IVS) control and triple phase shift modulation method for input-series output-parallel (ISOP) dual active bridge (DAB) converters to solve the three challenges: the uncertainties of the dc microgrid, the power balance problem, and the current stress minimization of the converter. Specifically, the control and modulation problem of the ISOP-DAB converter is formed as a Markov game with several DRL agents. Subsequently, the MA twin-delayed deep deterministic policy gradient (MA-TD3) algorithm is applied to train the DRL agents in an offline manner. After the training process, the multiple agents can provide online control decisions for the ISOP-DAB converter to balance the IVS, and minimize the current stress among different submodules. Without accurate model information, the proposed method can adaptively obtain the optimal modulation variable combinations in a stochastic and uncertain environment. Simulation and experimental results verify the effectiveness of the proposed MA-TD3-based algorithm.

Common DC-Link Capacitor Harmonic Current Minimization for Cascaded Converters by Optimized Phase-Shift Modulation

This paper investigates the influence of a constant carrier phase shift on the DC-link capacitor harmonic current of cascaded converters used in fuel-cell and mild-hybrid electric vehicles. In these applications, a DC-DC converter can be adopted between the battery and the motor drive inverter in a cascaded structure, where the two converters share the same DC-link. Since the DC-link capacitor of such a system represents a critical component, the optimization of the converter operation to limit the current stress and extend the lifetime of the capacitor is an primary objective. This paper proposes the use of a carrier phase shift between the modulations of the two converters in order to minimize the harmonic current of the DC-link capacitor. By harmonic analysis, an optimal carrier phase shift can be derived depending on the converter configuration. Analytical results are presented and validated by hardware-in-the-loop experiments. The findings show that the pulse width modulation carrier phase shift between the interleaved boost converter and the voltage source motor drive inverter has a significant influence on the DC-link capacitor current and thus on its lifetime. A case study with two-cell and three-cell interleaved boost converters shows a possible DC-link capacitor lifetime extension of up to 390%.

Modeling and Analysis of Hybrid Dual Active Bridge Converter to Optimize Efficiency Over Whole Operating Range

To improve the efficiency of hybrid dual active bridge (HDAB) converter with a half-bridge structure on the primary side and a full-bridge structure on the secondary side over the entire operating range, this article proposes a novel four-degree-of-freedom (4-DOFs) modulation scheme by taking full advantage of the respective structural characteristics of the half bridge and full bridge. First, a universal steady-state model of the HDAB converter with the 4-DOF modulation scheme is established in the frequency domain, revealing the relationship between modulation DOFs and steady-state quantities. Then, the optimal four-DOF (O4-DOF) modulation scheme is obtained for minimizing the inductor root-mean-square (rms) current with the help of the particle swarm optimization (PSO) algorithm. Compared to the conventional phase shift modulation (PSM) scheme, the proposed O4-DOFs modulation scheme greatly improves the converter efficiency. Finally, the correctness of the theoretical analysis is verified by building an experimental platform with an input voltage of 400 V, an output voltage of 100–200 V, and an output power of 1 kW.

A Variable Frequency Phase-Shift Modulation Constant Power Control Strategy for LCC Resonant Capacitor Charging Power Supply

This article develops the constant power control technology of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LCC</i> resonant capacitor charging power supply (CCPS) to accelerate the charging speed and use the full capacity of the device at the same time. For the constant power control of the CCPS, variable frequency modulation works in a wide frequency range with large reactive power, and phase-shift modulation (PSM) cannot work with a wide gain range. Therefore, a variable frequency (VF)-PSM constant power control strategy is proposed in this article, which achieves high utilization of transformer and high efficiency of power supply system. Based on state plane analysis, the analytical expressions in different operating modes are derived for the ZVS boundary and the multidimensional relationship between power, output voltage, frequency, and phase-shift angle. Accordingly, a control trajectory of the phase-shift angle and frequency in gain-power plane is calculated. Thus, the problems caused by using the current with large harmonic components to calculate the output power in real time are avoided and the control is simple and accurate. Finally, a 500 W, 1 kV charging experiment was conducted to verify the effectiveness of the VF-PSM constant power control strategy and the feasibility of phase-shift angle and frequency trajectory control in gain-power plane.

A Combinational Level-Shifted and Phase-Shifted PWM Technique for Symmetrical Power Distribution in CHB Inverters

In cascaded H-bridge (CHB) multilevel inverters (MLIs), phase-shifted pulse width modulation (PSPWM) and level-shifted pulse width modulation (LSPWM) are the two prominent techniques to attain multilevel and better performance. But both of them have certain drawbacks like in PSPWM the switching loss of all the semiconductors is high, and in LSPWM the power-sharing between the cascaded modules is not identical. These two drawbacks have been addressed in this article by proposing a novel modulation technique, named the phase opposed disposed phase-shifted PWM (PODPSPWM). In this technique, the level-shifted carriers have been rotated in such a way that there should not be any active power loss and simultaneously the carriers are phase-shifted as per the PSPWM technique. A detailed mathematical analysis of switching loss for all three methods has been presented. Moreover, the power-sharing and harmonic spectrum of the proposed PODPSPWM over other PWMs have been discussed. This novel modulation technique has been simulated in the MATLAB/Simulink software to validate the superiority of the operation. The 1.5-kW experimental prototype is developed to validate the performance of the proposed PODPSPWM modulation.

A High Power Density Wide Range DC–DC Converter for Universal Electric Vehicle Charging

This article presents a single stage isolated bidirectional DC–DC converter comprising of LCL-T resonant network for universal electric vehicle (EV) charging. Fixed frequency operation along with phase-shift control enables easier design of passive components, while achieving high efficiency across wide output voltage range. Furthermore, to increase the switching frequency and consequently the power density, gallium nitride (GaN) switches are explored in the proposed solution. In order to facilitate use of commercially available 650 V GaN transistors, a multi-level inverter and a reconfigurable rectifier architecture is utilized along with LCL-T resonant network, accommodating an input voltage of 800 V, generated from an universal three phase ac input supply, and an output voltage ranging from 150 V to 950 V catering to large variation of battery voltages from different vehicle manufacturers. The constant input voltage-to-output current gain property of LCL-T network, along with an efficient phase-shift modulation proposed in this paper enable soft switching of all transistors with minimum circulating currents over constant current (CC), constant power (CP), and constant voltage (CV) modes of battery charging. Experimental results are provided for a single-phase DC–DC converter prototype utilizing 650 V GaN transistors operating from an 800 V DC bus and providing a very wide output voltage from 150 V to 950 V at 6.6 kW maximum power at a constant switching frequency of 500 kHz. The prototype achieves two peak efficiency points of 98.2% and maintains <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$&gt;$</tex-math></inline-formula> 97% efficiency across the entire output voltage range at a power density of 120 W/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> (7.3 kW/L).

Selective Virtual Synthetic Vector Embedding for Full-Range Current Harmonic Suppression of the DC Collector

With the rapid development of rene wable energy (e.g., wind and photovoltaic energy) and energy storage, the dc collector has become an effective conversion link of distributed energy access to medium-voltage power distribution grids. However, considering the inconsistency of dc voltage and the duty cycle of each submodule, the traditional carrier phase-shifting modulation is difficult to achieve the excellent harmonic characteristic in the dc collector, thereby introducing difficulties in the design of passive filters. To solve this problem, this article proposes a selective virtual synthetic vector embedding algorithm to realize full-range current harmonic suppression in the dc collector system. In this scheme, the switching voltage vectors outputted by each submodule are sorted, and the voltage vectors are selectively extracted and synthesized to calculate the embedded virtual voltage vector. Afterward, the voltage vector of each submodule is rotated and calculated online from the perspective of space geometry, and the optimal carrier phase combination in variable-angle carrier phase-shift modulation is obtained. Finally, the experimental results show that the proposed control algorithm can not only be used in the dc collector with any number of modules but also achieve fast and effective harmonic suppression in the whole operating range.