Soft-switching Boundary Research Articles

A High-Power Density DC Converter for Medium-Voltage DC Distribution Networks

A DC converter is the core equipment of voltage conversion and power distribution in a DC distribution network. Its operating characteristics have a profound impact on the flexible regulation of distributed resources in an active distribution network. It is challenging for the existing single-stage conversion topology to meet the requirements of distributed renewable energy connected to a multi-voltage level, medium-voltage grid. It is necessary to study the multistage transform power unit topology further, which can satisfy high reliability, high efficiency, and wide input range. This paper proposes a high-power density DC converter for medium-voltage DC networks with wide voltage levels. It adopts Buck-LLC integrated modular composition. The input ends of the high isolation resonant power unit are connected in series to provide high voltage endurance, and the output ends are connected in parallel to meet the high-power demand and achieve high-power transmission efficiency. The proposed series dual Buck-LLC resonant power unit topology can adjust the duty cycle of series dual buck circuits to meet the needs of different levels of medium-voltage DC power grids. The soft switching problem within the wide input range of all switching tubes is solved by introducing auxiliary inductors, thereby improving energy transmission efficiency. The auxiliary circuit and control parameters are optimized based on the research of each switching tube’s soft switching boundary conditions. Finally, an experimental prototype of a 6.25~7 kW power unit is designed and developed to prove the proposed topology’s feasibility and effectiveness. Great breakthroughs have been made both in theoretical research and engineering prototype development.

A High-Gain Multilevel dc–dc Converter for Interfacing Electric Vehicle Battery and Inverter

An isolated multilevel bidirectional dc–dc converter is presented in this article to interface the low-voltage (LV) battery and the high-voltage (HV) propulsion inverter in electric vehicles. The proposed topology achieves a HV step-up ratio in the motoring mode by reconfiguring two split capacitor half-bridge circuits in series/parallel through the symmetrical modulation of a voltage bidirectional, two quadrant switch. This reconfiguration generates a high-frequency multilevel voltage output at the dc-bus side of the inverter, which reduces the output filter volume. The dc-bus voltage is regulated by the duty cycle modulation of the intermediary switch in the motoring mode, simplifying the sensing and control. During regenerative braking, the converter is operated with phase shift modulation between the HV half bridges and the LV full bridge to regulate the battery voltage. The presented modulation strategies achieve zero voltage switching for the full-bridge devices over a wide load range. A comprehensive analysis of the proposed multilevel converter in motoring and braking modes along with the design constraints and soft switching boundary conditions is presented in this article. Detailed experimental results on a 1.5-kW laboratory prototype are presented to verify the design approach and illustrate the converter performance.

Natural Boundary Transition and Inherent Dynamic Control of a Hybrid-Mode-Modulated Dual-Active-Bridge Converter

This article proposes a hybrid-mode modulation strategy for the single-phase dual-active-bridge (DAB) dc–dc converter that realizes soft switching in the whole operating range. Different from existing modulation methods that are usually dependent on complex mathematical optimization processes, the presented article reveals that the simple trapezoidal continuous conduction mode and triangular discontinuous conduction mode inspired by a quasi-single-active-bridge operation can naturally and sequentially extend the soft-switching boundary of the conventional single-phase-shift (SPS) modulation to the full range in both the buck and boost modes. Meanwhile, the transformer rms current is also reduced. Moreover, a fast-dynamic control with inherent seamless mode transitions is realized by introducing the predictive-current SPS modulation. The proposed hybrid-mode modulation strategy with closed-form solutions can be easily implemented in a generalized closed-loop controller, which enables an ultrawide-voltage-range operation of the DAB converter with significantly elevated efficiency and fast transient responses. The effectiveness of the proposed method is validated by comprehensive experimental results from a small-scale DAB prototype.

Variable-Frequency Critical Soft-Switching of Wide-Bandgap Devices for Efficient High-Frequency Nonisolated DC-DC Converters

This paper derives a variable-frequency critical soft-switching control method for nonisolated DC/DC converters using wide-bandgap devices. The critical soft switching control technique under maximum frequency trajectory is introduced to maintain zero voltage switching over a wide range of modulation ratios according to the load variation. The concept prevents turn-on losses that are typically much larger than the turn-off losses in SiC and GaN FETs and the latter can be further reduced by adding external drain-source capacitors. We have derived the boundary conditions for critical soft switching operation. For the reduction of inductor value and volume, a maximum available switching frequency is applied to the converter within the constraints of device requirement and soft switching boundary conditions. We demonstrate experimentally that the proposed concept reduces the power losses in the wide-bandgap devices by a factor of approximately 3, enables an increase of the switching frequency by a factor of about 5, and a decrease of the main inductance by a factor of about 10. Then variable frequency critical soft switching control method is proposed with the constraints to maintain the maximum frequency within soft switching operation. Since our test bench uses off-the-shelf inductors, the inductors are subject to significant high frequency losses. Despite this, the converter efficiency increases by 1%.

Modified Auxiliary-Resonant Commutated Pole Applied in a Three-Phase Dual-Active Bridge DC/DC Converter

The purpose of this study is to present the concept and properties of a dual-active bridge dc–dc converter with a modified auxiliary-resonant commutated (MARC) pole concept, and to discuss preliminary test results of a high-power setup. In this approach, the dual-active bridge based on IGCT is considered. First, the soft-switching boundaries for the converter with lossless snubbers and high switching delays are introduced. Second, the MARC pole is presented to overcome this border to receive full soft-switching capability in the entire operation range of the dc–dc converter. This is of crucial importance since IGCTs will be destroyed in case of a snubber dump. The advantage of the modified circuit is the guaranteed switching under zero-voltage conditions without applying an additional boost current as needed in the classic auxiliary-resonant commutated pole (ARCP) approach. Thus, complexity is reduced and reliability is increased. Measurement results of the MARC are presented from a mega-watt dc–dc converter where the concepts are fully implemented. The results of large-scale systems impressively show how the ARCP reduced the impact of parasitic elements, which would normally affect the switching transitions tremendously.

Minimum-Reactive-Power Scheme of Dual-Active-Bridge DC–DC Converter With Three-Level Modulated Phase-Shift Control

This paper presents a novel reactive-power minimization method under the three-level modulated phase-shift control to improve the efficiency for a wide operation condition. First, in order to unify all operation modes, a mathematic model of the dual-active-bridge converter with the three-level modulated phase-shift control by a harmonic analysis method is obtained. Then, the detailed analysis of the odd-order harmonic components of the active and reactive powers varied with the multiple control dimensions is presented. The soft-switching boundaries with respect to the voltage ratio and unified active power are specified. On this basis, a novel optimal three-level phase-shift (OPS) control strategy for minimizing reactive power is proposed. Experimental results of reactive power and operation efficiency among the proposed OPS method, and other methods are compared to verify the effectiveness of the OPS algorithm.

Voltage balancing control of isolated modular multilevel dc–dc converter for use in dc grids with zero voltage switching

The isolated modular multilevel dc–dc converter (IMMDCC) constructed by two modular multilevel converters (MMCs) and a medium frequency transformer is attractive for medium- and high-voltage dc-grid applications by offering the following features: the modular multilevel structure can handle high voltages with the series connection of lower voltage submodules; the MMC operating manner reduces the current stress of the switches by dividing the ac currents among the phase-legs; using the square-wave modulation scheme can increase the power capacity meanwhile, achieve zero-voltage switching operation. In the square-wave modulation scheme of IMMDCC, the voltage balancing control of the floating capacitors needs further investigation since the unmatched losses and circuit parameters lead to the deviation of the voltages. In this study, a capacitor voltage balancing control strategy that does not rely on the detection of the arm currents is presented by introducing small delay times among the switching pulses of the submodules, meanwhile dv/dt in the ac link is reduced by doing so. Soft-switching boundary analysis imposes constraints on the balancing method to achieve zero-voltage switching of the converter. Experimental results have confirmed the correctness of the theoretical analysis as well as the effectiveness of the proposed control strategy.

Reactive Power and Soft-Switching Capability Analysis of Dual-Active-Bridge DC-DC Converters with Dual-Phase-Shift Control

This paper focuses on a systematical and in-depth analysis of the reactive power and soft-switching regions of Dual Active Bridge (DAB) converters with dual-phase-shift (DPS) control to achieve high efficiency in a wide operating range. The key features of the DPS operating modes are characterized and verified by analytical calculation and experimental tests. The mathematical expressions of the reactive power are derived and the reductions of the reactive power are illustrated with respect to a wide range of output power and voltage conversion ratios. The ZVS soft-switching boundary of the DPS is presented and one more leg with ZVS capability is achieved compared with the CPS control. With the selection of the optimal operating mode, the optimal phase-shift pair is determined by performance indices, which include the minimum peak or rms inductor current. All of the theoretical analysis and optimizations are verified by experimental tests. The experimental results with the DPS demonstrate the efficiency improvement for different load conditions and voltage conversion ratios.

ZVT DC–DC PWM converters with magnetically-coupled auxiliary voltage source: a unifying analysis

A generalised circuit diagram for ZVT PWM converters with magnetically-coupled auxiliary voltage source is proposed. The generalised circuit is presented and analysed; its operation modes and constraints are defined. By means of the proposed generalised topology, published ZVT PWM converters with magnetically coupled auxiliary voltage source can be systematised and better evaluated. Based on the approach presented, the transformer (or coupled inductor) turns ratio N is determined for each topology analysed, guaranteeing a more realistic soft-switching boundary. In addition, a comparative analysis concerning the volume of magnetic ferrite cores is carried out. The comparative analysis addresses some of the most important ZVT converters with magnetically-coupled auxiliary voltage sources. The efficiency performance of these converters is evaluated and experimental results given, obtained from 1 kW–100 kHz laboratory prototypes, which confirm the value of the proposal and theoretical analysis.

Unified design for ZVT PWM converters with resonant auxiliary circuit

Operation modes of zero-voltage transition (ZVT) PWM converters with resonant auxiliary circuit (RAC) are identified and analysed by means of a common topology. A novel state-plane approach is used to determine the soft-switching boundary for this sort of converter. A unified design criterion, based on the analytical definition of the soft-switching boundary, is described in detail. By means of the proposed criterion the time spent on repetitive numerical simulation of a given topology to define its design guidelines based on a set of particular features can be significantly reduced. The theoretical analysis is confirmed by means of experimental results obtained from ZVT-PWM boost converters, which operate at 1 kW, 100 kHz.