This paper proposes an indirect input-series output-parallel (I2SOP) dc-dc full-bridge converter system based on asymmetric PWM (APWM) control strategy for high input-voltage applications. When a short-circuit fault occurs in the high input-voltage dc bus, the proposed I2SOP system can disconnect from the dc bus effectively and avoid the input filter capacitors of constituent modules discharge compared with the traditional ISOP system, facilitating the fault handling and protecting the input filter capacitors. Moreover, with the proposed I2SOP structure, it is easy to achieve redundancy design when one or a few constituent modules fail. The APWM control strategy is introduced to the I2SOP system rather than the classical phase-shifted control strategy for the basic full-bridge module to reduce the voltage stress of power switch and reduce the filter requirement. The characteristics of input voltage sharing and output current sharing of the I2SOP system are also analyzed. A three-module I2SOP system prototype was built and the experimental results verify the correctness and effectiveness of the proposed solution.

A novel hybrid resonant zero-voltage zero-current switching pulsewidth modulation full-bridge converter with capacitive output filter is proposed in this paper, which is suitable for large photovoltaic parks connecting to medium-voltage dc (MVDC) grids. The hybrid resonant converter, with two full-bridge cells sharing a bridge leg and connecting the secondary windings of two transformers in series, operates in discontinuous current mode and can achieve zero-current switching for main power switches and rectifier diodes over the whole load range. Meanwhile, the auxiliary switches with small current rating can realize zero-voltage switching naturally. Hence, the switching loss is reduced, which is vital in MVDC applications. By the trajectory formulas of the resonant current and voltage, the trajectory paths can be achieved, which is helpful to compare the peak and turn-off currents of switches. The design rules of main parameters can be deduced with the trajectory formulas and paths. The influence of the turns ratio of auxiliary transformer and the resonant capacitance on the peak and turn-off currents is discussed in detail. A simulation model is established in the software PLECS and the operation principles of the converter are verified. A 150 V-1500 V/2 kW prototype was built to validate the operation principles and parameters optimization.

The bipolar dc microgrid is attracting attention for its high efficiency and reliability, but the bipolar dc bus is susceptible to unbalanced power. This article proposes a bipolar current-fed dc–dc converter with automatic voltage balance under phase-shift plus pulsewidth control. Combined with the current-fed dual active bridge converter and voltage balancer, the circuit structure is streamlined and fewer devices are required. The proper magnetizing inductance allows the converter to achieve automatic bipolar voltage balancing even under extreme load imbalance, and achieve full-range zero-voltage switching (ZVS) over a wide voltage range. In addition, the coupled inductor is utilized to further reduce the number of magnetic components. Finally, based on the accurate ZVS analysis considering parasitic capacitance, the parameters of the converter are designed, and an experimental prototype is constructed to verify the feasibility of the proposed scheme.

Conventional zero-voltage and zero-current-switching (ZVZCS) full-bridge converters are widely used because they can realize zero-voltage switching (ZVS) for the leading leg and zero-current switching (ZCS) for the lagging leg. However, the large turn-off loss of the leading leg is still unacceptable in high-voltage high-power applications where insulated-gate bipolar transistors (IGBTs) with current tailing are employed. Hence, it is preferable to achieve ZCS for all IGBTs of full-bridge converters. This paper analyzes the ZCS conditions for pulsewidth modulated (PWM) full-bridge converters and proposes a general strategy for achieving ZCS for main switches, including the control strategy and design method of the topology. Based on this theory, a novel ZCS PWM full-bridge converter with simple auxiliary circuit is derived and tested. It can achieve ZCS not only for the main switches of full-bridge converter, but for the auxiliary switch as well, and obtain high efficiency. The operation principle of the proposed ZCS PWM full-bridge converter and the design procedure are presented. The experimental results of a 600 V/2.5 kW prototype are also included to verify the theoretical analysis.

A novel hybrid resonant zero-voltage zero-current switching three-level converter with capacitive output filter is proposed in this paper, which undergoes half of the input voltage for switches and is suitable for offshore wind farms in medium-voltage dc (MVDC) collection systems. The proposed converter, utilizing two transformers with very different power ratings, adopts insulated-gate bipolar transistors for all six power switches and operates in discontinuous current mode and can achieve zero-current switching for the four main switches and rectifier diodes over the whole load range. Meanwhile, the auxiliary switches with small current rating can realize zero-voltage switching naturally. Hence, the switching loss is reduced, which is vital in MVDC applications. By the trajectory formulas of the resonant current and voltage, the trajectory paths can be achieved, which is helpful to compare the peak and turn-off currents of switches. The design rules of main parameters can be deduced with the trajectory formulas and paths. The influence of the turns ratio of auxiliary transformer and the resonant capacitance on the peak and turn-off currents is discussed in detail. The operation principles of the proposed converter are analyzed and verified by the simulation and experimental results.

To maintain the stability and balance of the output voltage of the bipolar system under unbalanced loads, this letter proposes a novel buck converter with bipolar outputs, where only one “switch circuit” is on the input side while two diode rectifiers and two LC filters are on the output side. Instead of two independent filter inductors, one coupled inductor is adopted. As a result, the bipolar output voltages can be balanced autonomously under any load conditions without extra components or voltage-balancing control loop. Meanwhile, the wide voltage regulation range and easy control of the traditional buck converter are reserved. The autonomous voltage-balancing mechanization and the coupling coefficient effect are analyzed. The proposed coupled inductor method can be applied to other buck-type converters, such as the phase-shifted full-bridge (PSFB) converter. Finally, an experimental prototype of the bipolar PSFB converter is built to verify the feasibility and advantage of the proposed converter.