Bipolar dc microgrids have been increasingly adopted in different applications due to their versatility in scaling to higher power levels while maintaining the same safety requirements as lower voltage unipolar ones. On the other hand, power imbalance, commonly faced in bipolar systems, can lead to undesirable voltage deviations that may trigger protection devices, cause corrosion, and increase risks of electric shock. To address this issue, several voltage balancing solutions have been proposed, including self-balancing dc-dc converters, and others active balancers. Both approaches are different in terms of hardware implementation, efficiency, and complexity. In this paper, a two-switch buck/boost voltage balancer and a self-balanced current-fed dual active bridge (CF-DAB) converter are experimentally compared for applications in bipolar dc microgrids. Experimental results are presented using a 5-kW prototype. The results demonstrate that both solutions offer different opportunities for system optimization. However, the processed rms current analysis shows the self-balanced solution providing advantages in terms of efficiency, while the conventional buck/boost balancer offers a more cost-effective solution.

This paper presents an alternative approach for efficient DC power distribution and voltage regulation in data centers. A switched-capacitor (SC) resonant DC-DC converter with differential power processing (DPP) is proposed, enabling servers to be electrically connected in series. This configuration inherently steps down the voltage from a higher-voltage DC bus. Voltage regulation for each server is achieved using SC half-bridge cells that process only the differential (mismatch) power between all servers, while the bulk power is transferred without active processing. It results in notably improved global system efficiency compared to conventional architectures. Additional features of the proposed converter include its simplicity due to open-loop operation, soft-switching capability, and tolerance to hot-swapping of servers. Here, the concept is demonstrated and validated through circuit simulations and experimental results considering a series-stacked server rack with four 12 V servers powered from a 48 V DC bus, delivering up to 600 W.

Series-resonant converters (SRCs) have gained attention for many applications, including power supply units, data centers, and smart transformers, due to their high and very flat efficiency curve. However, ideally, SRCs operate without voltage regulation, as implementing phase shift or frequency control can penalize the converter efficiency. This presents a significant drawback for certain applications such as dc buildings, nanogrids, and dc microgrids, where such converters must operate under droop control, resulting in a variable output voltage. To address this limitation and enable SRCs to operate under variable output voltage, this paper proposes a bidirectional partial power converter as a post regulator stage (P3R). The proposed converter is based on a current-fed topology, featuring fully controlled switches to allow bidirectional power flow and the capability to step-up or down the output voltage. Experimental results, showcasing the converter waveforms for a 5-kW prototype, are presented to evaluate the converter's viability for this application.

The current source inverters (CSI) are considered more reliable than their voltage source counterparts due to the inherent current-limiting feature. This is particularly interesting for many applications, such as battery energy storage, where there is a possibility of very high short-circuit currents. In addition, CSI can provide an inherent voltage boost capability, allowing integration of extra-low voltage batteries with 230 Vac loads for standalone applications. For the past few years, CSI has not garnered much attention due to the unavailability of switches with reverse voltage blocking capability. However, with the emergence of new technologies based on wide bandgap materials and monolithic devices, CSI can potentially provide effective solutions for such applications. In this paper, a CSI is proposed as an interface between a ±48 V battery bank and a 230 Vacload. To minimize the forward voltage-drop across the switches of the current source inverter, silicon carbide MOSFETs-based four-quadrant switches are employed. In addition, a unipolar carrier-based modulation is proposed and implemented. This modulation technique facilitates easier control of overlap-time and minimizes its effect on the power quality output of the inverter. To validate this proposal, a 700 W prototype was built, and laboratory tested.

Dual-active bridge (DAB) converters have gained significant attention in recent years, due to their high efficiency and excellent performance in medium and high-power applications. However, their performance is limited when a wide power and voltage range are required. One application example is the dc microgrid when DAB converters must operate with droop control to enable power balance and dc bus voltage regulation. When operating under droop control, the DAB converter efficiency can be penalized at low power, reducing the overall system efficiency. Being an open topic and not explained in current literature, this paper proposes for the first time to include the droop characteristic in the DAB converter analysis. The DAB converter efficiency is analyzed considering different factors, such as soft-switching conditions, circulating current, and dead-band insertion on droop control curves. Including the droop control as a new factor in the DAB converter design, a 5-kW prototype was built, and laboratory tested, evaluating different efficiency curves according to the selected droop curves.

This paper compares the holdup time for different active-front end (AFE) dc grid-forming converters. AFE converters are required to perform an interface between the utility ac grid and dc microgrids in homes, offices, and other small-scale prosumer electrical installations (PEI). According to the current standards NPR 9090 and IEC 61000-4-11, AFE converters must provide isolation between ac and dc parts and ensure a minimum holdup time for the dc output side. Considering both requirements, two possible solutions are addressed and compared: a conventional dc-ac converter with a low-frequency isolation transformer and a two-stage ac-dc converter that includes a dc-dc stage with high-frequency isolation. To perform a comparative analysis of each solution, different scenarios are described and simulated in PSIM software.