Input-parallel Output-parallel Research Articles

Optimization of IPOP-connected dissimilar power-rated converters: An equal incremental cost-based approach

As electric vehicles gain prominence, optimizing the efficiency of charging stations has become a critical issue. This paper explores the application of dual active bridge (DAB) converters arranged in an input-parallel-output-parallel (IPOP) configuration, offering a flexible solution for power conversion across varied power-rated loads. While previous research mainly focused on optimizing single DABs and establishing the system using identical converters, this study leverages dissimilar modules, enhancing flexibility and operational efficiency. We introduce a novel IPOP system incorporating diverse modules and corresponding switch regulators. Additionally, an efficiency optimization strategy featuring dynamic load allocation is proposed. Herein, each module or each group of modules is responsible for its corresponding power range. Based on curve-fitted power profiles, an equal incremental cost-based method is employed to formulate the path of power allocation. Moreover, a simulated annealing algorithm determines the optimal power output strategy to accommodate dynamic power flow requirements. Experimental comparisons with the conventional equal-power sharing scheme underscore the proposed method’s effectiveness and superiority.

Model‐based adaptive control of modular DAB converter for EV chargers

AbstractThis paper presents the discrete‐time modelling and control of modular input‐parallel–output‐parallel (IPOP) dual‐active‐bridge (DAB) converters for electric vehicle (EV) charging. The proposed adaptive control system ensures adequate current‐sharing among parallel modules while minimizing DAB current stress by adopting dual phase‐shift modulation. Driven by the growing need for fast EV charging options, the paper highlights the importance of achieving top‐notch control performance, especially with varying load conditions. Specifically, it introduces a discrete‐time model for adjusting controller parameters adaptively, which simplifies the typically cumbersome manual tuning process associated with these systems. The proposed PI formulae are derived to satisfy specifications on the frequency domain as phase margin and the gain crossover frequency of the open loop gain transfer function, ensuring stability and robustness in operation. Moreover, the implementation of these formulae in discrete microcontrollers facilitates seamless PI autotuning for precise current, voltage, or power control. Notably, the proposed control strategy effectively mitigates current overshot issues commonly encountered during module engagement and shedding operations in modular EV chargers. To validate its efficacy, the proposed controller is evaluated through extensive testing and comparisons within the PLECS environment, particularly focusing on a two‐module IPOP‐DAB converter scenario, and including comparisons with classical offline model‐based pole placement methodology. Furthermore, real‐time hardware‐in‐the‐loop experiments are conducted to confirm the feasibility and performance of the proposed controller under realistic EV charging profiles.

Double B-type magnetic integrated transformer based input-parallel output-parallel LLC resonant converter modules

The LLC resonant converter used in high-power situations suffers from the problems of high conduction loss and current stress, which can be solved using input-parallel output-parallel (IPOP)-connected converter modules. However, this leads to a multiple increase in the number of magnetic components, which reduces power density. Magnetic integration technology is an effective way to reduce the volume of converters. Currently, the magnetic integrated transformer based on EE-type cores is widely used to realize miniaturization, and it uses leakage inductance instead of resonant inductance to improve power density. However, leakage inductance is difficult to control, and the external radiated magnetic field will produce serious eddy current loss and electromagnetic interference. This article proposes a novel double B-type magnetic integrated transformer, which can integrate the magnetic components of two LLC resonant converters simultaneously and where the resonant inductances are wound independently. The structure contains four low reluctance branches, which are used as the cores of the transformer and the resonant inductance. The decoupling integration method, which integrates the four components into a single core, has been used to increase core utilization and improve power density. On this basis, the transformer’s high- and low-voltage windings are cross-arranged to reduce the magnetic field intensity in space, further decreasing the loss and electromagnetic interference. Compared with the EE-type magnetic integrated transformer, the volume of the proposed structure is reduced by 5.9%. A 400W experimental prototype is built, and the results verify the validity of the design.

Unified Real-Time Simulation Method for DC/DC Conversion Systems Consisting of Cascaded Dual-Port Submodules

The DC/DC conversion systems are constructed from multiple cascaded dual-port submodules in series and/or parallel at both the input port and output port. The complexity of the series-parallel structures brings great challenges to real-time simulation. This paper proposes a unified real-time simulation method of series-parallel DC/DC conversion systems, which unifies the input-series-output-series (ISOS), input-series-output- parallel (ISOP), input-parallel-output-parallel (IPOP) and input-parallel-output-series (IPOS) structures. First, based on the nodal analysis method, the dual-port equivalent model of each submodule in the series-parallel DC/DC conversion system is obtained. Second, by cascading each submodule, the unified form of the system dual-port equivalent model of the series-parallel DC/DC conversion system is constructed. For the series connection, the port is equivalent to Thevenin's equivalent circuit. For the parallel connection, the port is equivalent to Norton's equivalent circuit. The real-time simulation model of a specific series-parallel DC/DC conversion system can be obtained by combining the input/output port equivalent circuits. Besides, the high-frequency waveforms of the internal circuit can also be obtained through parallel calculation steps. The real-time simulation of the proposed unified model is carried out with a 250ns time-step on Xilinx K-7 Series FPGA, which verifies the accuracy and simulation efficiency of the unified model.

Output Voltage Response Improvement and Ripple Reduction Control for Input-Parallel Output-Parallel High-Power DC Supply

Output Voltage Response Improvement and Ripple Reduction Control for Input-Parallel Output-Parallel High-Power DC Supply

A Simple Sensorless Current Sharing Control for Input-Parallel Output-Parallel Dual Active Bridge Converters

The circuit parameters mismatch of input-parallel output-parallel (IPOP) dual active bridge (DAB) converter modules will cause power imbalances of the system. To solve this problem, in this article, we present a simple sensorless current sharing control based on dual-phase-shift (DPS) modulation method. First, the full process operation mode of IPOP DAB is analyzed. Then, the power balance mechanism of the IPOP DAB converter is discussed, and a sensorless current sharing control method is proposed. The core principle of the proposed method is based on the perturbation algorithm to estimate the parameter mismatches of each DAB cell. Once the parameter mismatches are known, the phase shifts that achieve current sharing are calculated. The implementation scheme based on CAN Bus is given, and sensitivity and stability analysis of the proposed method is conducted. Finally, the simulation and experimental results verify the effectiveness of the proposed control strategy.

Input parallel–output parallel connected modular nonisolated DC‐DC converters with current self‐sharing capability operating in discontinuous conduction mode

AbstractThis paper covers the self‐sharing analysis of dc‐dc nonisolated converters with input parallel–output parallel (IPOP) configuration and operating in discontinuous conduction mode (DCM). The main contribution of the proposed system is its capability of providing self‐sharing of the currents on both sides of each individual converter, without average current sharing control, even in the face of parametric variations. This self‐balance only occurs for DCM. When the addressed converters operate in continuous conduction mode (CCM), the self‐sharing does not occur naturally under parametric differences among them, requiring the use of additional control loops. The use of self‐sharing converters in nonisolated converters simplifies the control system, it makes the modular solution being attractive for many applications, and it increases the power range that the DCM converters may be applied. This paper brings the theoretical study of self‐sharing of the current mechanism to six basic nonisolated converters operating in DCM. The self‐sharing is verified by experimental results, which are obtained from three modules of dc‐dc SEPIC converters. Both converters were designed to operate with 200‐V input voltage, 125‐V output voltage, 1500‐W rated power (500 W each module), and switching frequency at 30 kHz.

Model-Based Predictive Control with Graph Theory Approach Applied to Multilevel Back-to-Back Cascaded H-Bridge Converters

The multilevel back-to-back cascaded H-bridge converter (CHB-B2B) presents a significantly reduced components per level in comparison to other classical back-to-back multilevel topologies. However, this advantage cannot be fulfilled because of the several internal short circuits presented in the CHB-B2B when a conventional PWM modulation is applied. To solve this issue, a powerful math tool known as graph theory emerges as a solution for defining the converter switching matrix to be used with an appropriate control strategy, such as the model-based predictive control (MPC). Therefore, this research paper proposes a MPC with the graph theory approach applied to CHB-B2B which capable of not only eliminating the short circuit stages, but also exploring all the switching states remaining without losing the converter controllability and power quality. To demonstrate the proposed strategy applicability, the MPC with graph theory approach is tested in four different types of SST configurations, input-parallel output-parallel (IPOP), input-parallel output series (IPOS), input-series output-parallel (ISOP), and input-series output series (ISOS), attending distribution grids with different voltage and power levels. Real-time experimental results obtained in a hardware-in-the-loop (HIL) platform demonstrate the proposed strategy’s effectiveness, such as DC-link voltages regulation, multilevel voltage synthesis, and currents with reduced harmonic content.

Hybrid Input Power Balancing Method of Modular Power Converters for High Efficiency, High Reliability, and Enhanced Dynamic Performance

Input parallel output parallel (IPOP) modular power converters have been used to supply high power and high current in various applications. In general, the IPOP modular power converter uses an output power balancing method to equally distribute the amount of output power to each converter. In this article, a hybrid input power balancing method is proposed to obtain high power conversion efficiency, high reliability, and enhanced dynamic performance of the IPOP modular power converters. In the steady state, the proposed method controls input power to be balanced in each converter. A power loss distribution capability of the input power balancing method can improve the entire power conversion efficiency and can reduce the overall operating temperature of the IPOP converters, which can increase lifetime and can improve reliability. During the transient operation, the proposed method makes the IPOP converters under an interleaving mode to tightly regulate the output voltage and to obtain fast dynamic performance. Experimental results with 200-W prototype modular synchronous buck converters verify the performance improvements of the proposed method such as high power conversion efficiency, the power loss distribution capability in the steady-state operation, and the enhanced dynamic performance in the transient operation.

Efficiency Optimization of IPOP DC/DC System for HEV

Input parallel output parallel (IPOP) DC/DC system is widely utilized in many occasions like data center and hybrid electric vehicle (HEV). However, the efficiency characteristics of different converters usually differs from each other, and the conventional control strategy has difficult to realize efficient conversion of overall power of the system. To address this issue, Buck converter is taken as the module and whose efficiency model under current continuous conduction mode (CCM) is presented. Meantime, parameter identification method based on least square (LS) algorithm is proposed as well. In order to realize the efficiency optimization of the whole parallel system, the constraint conditions of each module are analyzed, and the current distribution calculation method based on sequential quadratic programming (SQP) algorithm for the parallel system is introduced. On this basis, a control strategy of proportional current distribution (PCD) is proposed, which not only ensures that the parallel system can always run as efficiently as possible under different loads, but also realizes the flexible switching between the current-sharing mode and current distribution mode. Finally, the simulation and experimental results prove that the proposed method can achieve 4.46% higher efficiency than conventional method within the given power range of each converter, and realize the efficiency optimization of the IPOP DC/DC system to the greatest extent.

Integrated Current Balancing Transformer Based Input-Parallel Output-Parallel LLC Resonant Converter Modules

The input-parallel output-parallel (IPOP) connected converter system allows the use of a low-power converter module for high-power applications. However, mismatches in the IPOP circuit parameters cause uneven current sharing between the constituent modules, which makes the IPOP system unreliable for practical use. Different techniques have been proposed so far; however, they cause an increase in magnetic volume, cost, component count, and complexity of the IPOP system. In this article, a reliable and efficient magnetic integration technique is proposed for the current sharing requirements of the IPOP system based on LLC resonant converter modules. The proposed integrated magnetic current balancing transformer (ICBT) based IPOP LLC resonant converter modules ensures both the input current sharing and output current sharing between constituent LLC modules, under the open-loop condition quite effectively. The proposed ICBT is the resonant inductors of LLC resonant converter modules, coupled to perform the function of current balancing as well as generates enough leakage inductances for resonance operation, simultaneously. Thus, it can reduce the overall magnetic volume and improves power density of the IPOP LLC modules. Moreover, the current sharing performances of the proposed ICBT in both the steady-state and dynamic-state are analyzed based on the magnetic and electrical model. To validate the performance of the proposed ICBT based IPOP two LLC modules, a 2.6-kW hardware prototype was designed, fabricated, and tested successfully.

Modular Isolated DC-DC Converters for Ultra-Fast EV Chargers: A Generalized Modeling and Control Approach

Electric Vehicles (EVs) play a significant role in the reduction of CO2 emissions and other health-threatening air pollutants Accordingly, several research studies are introduced owing to replacing conventional gasoline-powered vehicles with battery-powered EVs. However, the ultra-fast charging (UFC) of the battery pack or the rapid recharging of the battery requires specific demands, including both: the EV battery and the influence on the power grid. In this regard, advanced power electronics technologies are emerging significantly to replace the currently existing gas station infrastructures with the EV charging stations to move from conventional charging (range of hours) to UFC (range of minutes). Among these power electronics conversion systems, the DC-DC conversion stage plays an essential role in supplying energy to the EV via charging the EV’s battery. Accordingly, this paper aims to present possible architectures of connecting multiple Dual Active Bridge (DAB) units as the DC-DC stage of the EV fast charger and study their Small-Signal Modeling (SSM) and their control scheme. These are, namely, Input-Series Output-Series (ISOS), Input-Series Output-Parallel (ISOP), Input-Parallel Output-Parallel (IPOP), and Input-Parallel Output-Series (IPOS). The control scheme for each system is studied through controlling the output filter inductor current such that the current profile is based on Reflex Charging (RC). The main contribution of this paper can be highlighted in providing generalized SSM as well as providing a generalized control approach for the Input-Series Input-Parallel Output-Series Output-Parallel (ISIP-OSOP) connection. The generalized model is verified with three different architectures. The control strategy for each architecture is studied to ensure equal power sharing, where simulation results are provided to elucidate the presented concept considering a three-module ISOS, IPOP, ISOP, and IPOS DC-DC converters.

Efficiency Optimization of Two Dual Active Bridge Converters Operating in Parallel

Growing interest in dc-type energy storage systems (ESS) and dc/dc converters has led to increased power ratings of such devices, which often requires parallel connection. The process of energy efficiency optimization of such modular systems is described in literature mainly in terms of the varying power demand, but the problem is more complex and not analyzed yet in relation to varying energy storage (ES) voltage level and bidirectional energy transfer at the same time. Therefore, this article considers the input parallel output parallel (IPOP) connection of two dual active bridge (DAB) modules as a representative modular converter, and to answer a question of: how to improve (maximize) efficiency of such a system, at different power demands and varying ESS voltage, only by a turn on/off operation of the module without any circuit and control algorithm changes. The proposed solution defines an operating area (enclosing different ES's voltage levels and loading power conditions) showing where one or two modules should operate to improve the overall efficiency. A curve fitting approach and polynomial approximation of the single module's input/output power curves are utilized for the proposed algorithm based on parametric functions. After simulation study, the method is validated with two 1.4 kW SiC-based DAB converters, operating at 100 kHz of switching frequency within a wide (0-270 V) ES's voltage range.

IPOP-Connected FB-ZCS DC–DC Converter Modules for Renewable Energy Integration With Medium-Voltage DC Grids

This paper presents a dc–dc input-parallel output-parallel (IPOP) converter configuration for renewable energy integration with medium-voltage dc grids. A full-bridge zero-current-switching (FB-ZCS) converter with voltage-doubler output is proposed as the fundamental module. To achieve smooth current commutation and ZCS, the leakage inductance of transformer and a resonant capacitor across its secondary are utilized. This resonant capacitor is always charged to its full-rated capacity irrespective of converter loading and requires a dedicated charging interval resulting in duty-cycle loss. For a given leakage inductance, a large resonant capacitor is required for higher current ratings. With a single-centralized converter, these operational and design constraints result in significant duty-cycle loss, restricting the operation range. This paper proposes an IPOP configuration for high-power applications. Phase shedding at reduced loading ensures that individual modules are operated optimally. Input inductance requirement in modules is reduced due to interleaving of the input current. Additionally, voltage stress across converter components and input current ripple are reduced by 50% in the proposed converter modules. Steady-state operation, modeling, current-sharing control, start-up, and shutdown of modules in the IPOP configuration are demonstrated using simulations. Experimental results on a scaled-down prototype are presented to validate its operation.

Decentralized Suppression Strategy of Circulating Currents Among IPOP Single-Phase DC/AC Converters

To increase the power rating and improve the reliability of the single-phase power supply system, the input-parallel output-parallel (IPOP) single-phase dc/ac converters (SDACs) are widely used. However, among the IPOP SDACs, there are complicated circulating currents which will increase the current stress on the switching devices and influence the safe operation of the system. But, the detailed quantitative analysis of the circulating currents is not well developed and the corresponding suppression strategies are not very comprehensive. Focusing on these problems, this paper pays attention to the detailed mathematic model and effective suppression strategy of circulating currents among IPOP SDACs. First, the detailed mathematic model of the circulating currents is established. Based on this model, it is found that there are various types of circulating currents existing in the IPOP SDACs, including circulating currents within the single SDAC and circulating currents among the multiple SDACs. At the same time, the influence factors are also analyzed, and it is first revealed that the asymmetry of impedances in different sides will cause circulating currents with different frequencies components including the dc, fundamental, and second frequencies. Second, after the detailed analysis, a decentralized control method composed of the common mode (CM) and differential mode (DM) control is proposed to suppress the complicated circulating currents. The control method is comprehensive and can suppress circulating currents caused by diverse influence factors. All the theoretical analyses are verified by the real-time hardware-in-loop (HIL) tests.

Circulating Currents Suppression for IPOP Nonisolated DC/DC Converters Based on Modified Topologies

Nonisolated dc/dc converters are widely used in practice because of their low losses and high power density. To accommodate some high-power scenarios and modular applications, these nonisolated converters are usually connected in the input-parallel output-parallel (IPOP) structure. Because there is no galvanic isolation, the ac side and dc side are coupled together, which causes complicated circulating currents in the system. The great circulating currents will endanger the stable and reliable operation of the IPOP nonisolated dc/dc converters system. Focusing on this problem, this paper proposes a novel decentralized circulating currents suppression strategy for IPOP nonisolated dc/dc converters based on the modified topologies, which can still hold the main advantages of nonisolated solutions. First, the complicated circulating currents among the IPOP nonisolated dc/dc converters are analyzed in detail. It is found that various circulating currents exist in the system including the circulating currents within the single converter and the circulating currents among the multiple converters. Then, the limitations of the conventional dc/dc converter topologies are presented and it is proved that these topologies cannot eliminate all the circulating currents. Second, being inspired by the drawbacks of the conventional topologies, the modified topologies with two degrees of freedom modulation are designed. Based on the modified topologies, a corresponding suppression method consisting of two degrees of freedom control is proposed, which can eliminate the different types of circulating currents in a decoupling way. The first degree of freedom control mainly suppresses the circulating currents within the single converter, whereas the droop-based second degree of freedom control mainly suppresses the circulating currents among the multiple converters. Through the proposed solution, the IPOP nonisolated dc/dc converters can operate well with high reliability and scalability. The effectiveness of the proposed solution is validated by the real-time hardware-in-loop tests.

Circulating Currents Suppression Based on Two Degrees of Freedom Control in DC Distribution Networks

In dc distribution networks, the parallel H-bridge dc/dc converters (HBDCs) are widely adopted to convert voltage levels with the higher power rating and reliability, in which the parallel HBDCs are naturally connected in input-parallel output-parallel (IPOP) form. However among IPOP HBDCs, there are complicated circulating currents that will influence the safe and steady operation of dc distribution networks. This paper focuses on the suppression of these circulating currents. First, the detailed mathematic models of circulating currents among IPOP HBDCs are derived. Through the model, it is found that various types of circulating currents exist in the system, including the circulating currents within the single HBDC and the circulating currents among the multiple HBDCs. Hence, the suppression of circulating currents among IPOP HBDCs is a multiobjective control problem. In this paper, it is proven that the conventional one degree of freedom control based on the bipolar modulation cannot eliminate all the circulating currents. Second, a novel two degrees of freedom control method is proposed to suppress all kinds of circulating currents based on the improved modulation way of HBDCs, which consists of two parts. The droop based control is used to suppress the circulating currents among the multiple HBDCs, whereas the common mode control is used to control the circulating currents within the single HBDC. All the theoretical analyses are verified by the real-time hardware-in-loop tests.

Modeling and Analysis of Circulating Currents Among Input-Parallel Output-Parallel Nonisolated Converters

Input-parallel output-parallel (IPOP) nonisolated converters, including dc/dc and dc/ac converters, are effective solutions to increase the power rating and to improve the reliability of the system. For the safe and steady operation of these IPOP nonisolated converters, the circulating currents are the main challenge, whose detailed mathematic model and corresponding characteristics are not well studied. This paper focuses on the modeling and analysis of circulating currents among these IPOP converters by deriving the detailed mathematic model. Through these models, the complicated characteristics of the circulating currents are presented, which shows that there are various types of circulating currents including circulating currents within the single converter and circulating currents among the multiple converters. It is first demonstrated that the circulating currents will cause port degradation of the converter, that is, the positive and negative currents of the converter are unequal, which makes some port-based control methods ineffective and influences the relay protection. Furthermore, the corresponding influence factors, including line resistances, filters, and so on, are analyzed in detail, especially the asymmetry of line resistances will cause large circulating currents even resulting in instability. It is also found that the traditional control methods cannot completely eliminate these circulating currents because of the strong coupling and complexity of IPOP nonisolated converters. All the theoretical analyses are verified by the real-time hardware-in-loop (HIL) tests.

Sensorless parameter estimation and current‐sharing strategy in two‐phase and multiphase IPOP DAB DC–DC converters

A sensorless current-sharing strategy for two-phase and multiphase input-parallel output-parallel (IPOP) DC–DC converters is proposed here. A dual-active-bridge (DAB) DC–DC converter is chosen as the basic DC–DC converter. With this strategy in two-phase IPOP DAB DC–DC converters, the parameter mismatches between phases are estimated by perturbing the duty cycle in one phase and measuring the changes of duty cycles in both phases, then the duty cycles are adjusted to compensate the mismatches, thus achieving current sharing without current sensor. With this strategy in multiphase IPOP DAB DC–DC converters, by perturbing the duty cycles in (N − 1) out of N phases in turn and measuring the changes of duty cycles, respectively, the parameter mismatches among phases are estimated. According to the mismatches, a set of variables, which are proportional to the per-phase output currents, are calculated. Then with a current-sharing regulator, parameter mismatches are compensated, thus achieving current sharing without current sensor. The validity and feasibility of the proposed sensorless current-sharing strategy are verified through both simulation and prototype experimental results.

Advanced Control Techniques to Improve the Efficiency of IPOP Modular QSW-ZVS Converters

Quasi-square wave mode with zero voltage switching (QSW-ZVS) is an operation mode in which the switching losses can be minimized. However, a large inductance current ripple is needed in this mode, which limits the maximum attainable power of the topology. A possible way to increase the power managed by a QSW-ZVS mode converter is to use a modular converter. An input parallel output parallel (IPOP) arrangement, in which the current can be shared among the modules, can increase the total power proportionally to the number of modules. This paper addresses two main proposals. The first one is a master–slave technique to extend the QSW-ZVS mode to an IPOP modular converter, achieving an interleaved solution which minimizes the total input current ripple and assures the current balance among the modules. The second one is a comparison of four different control techniques applied to the IPOP modular converter to improve the overall efficiency at light to medium load: balanced master–slave technique, master–slave with phase-shedding, asymmetrical master–slave, and burst mode (or hysteretic control). These four strategies are theoretically analyzed, experimentally validated and compared using a 150–400 V 2 kW modular IPOP prototype made up of four synchronous boost converter operating in QSW-ZVS mode.