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This article introduces a new transformerless ac–dc–ac converter topology suitable for a high-power-density high-frequency single-phase online uninterruptible power supply (UPS) with a common neutral between the input and output ac ports. The proposed converter comprises an input power factor correction rectification (ac–dc) stage which is followed by an inversion (dc–ac) stage. The rectification stage operates in boost mode during the input positive half line cycle and buck–boost mode during the input negative half line cycle, while the inversion stage operates in buck and buck–boost modes in the output positive and negative half line cycles, respectively. The rectification stage utilizes boundary conduction mode control enabling soft-switching and allowing high-frequency operation. The inversion stage is operated in continuous conduction mode, wherein a digital controller regulates the output voltage of the converter across both resistive and reactive loads. The proposed online UPS utilizes a single (nonsplit) dc-bus between the rectification and inversion stages, resulting in a 50% reduction in dc-bus capacitance requirement compared to conventional split-bus online UPS topologies. Additionally, two battery interface solutions are also investigated which provide a trade-off between passive volume and additional switch-count. To verify the performance and control of the proposed online UPS, a GaN-based electrolytic-free 1-kVA prototype online UPS is designed, built, and tested. The prototype ac–dc–ac converter achieves a peak efficiency of 95.2% and maintains a high efficiency of above 92.3% across the full output power range. The electrolytic-free prototype ac–dc–ac converter for the online UPS achieves a power density of 26.4 W/in3. [ABSTRACT FROM AUTHOR]

This paper presents the design and implementation of a large-step-down soft-switched dc–dc converter based on the active bridge technique, which overcomes some of the limitations of the conventional dual active bridge converter. The topology comprises a double-stacked bridge inverter coupled to a reconfigurable rectifier through a special three-winding leakage transformer. This particular combination of stages enables the converter to run in an additional low-power mode that greatly increases light-load efficiency by reducing core loss and extending the zero-voltage switching range. The converter is implemented with a single compact magnetic component, providing power combining, isolation, voltage transformation, and energy transfer inductance. A 175 kHz, 300 W, 380–12 V GaN-based prototype converter achieves 95.9% efficiency at full load, a peak efficiency of 97.0%, an efficiency above 92.7% down to 10% load, and an efficiency above 79.8% down to 3.3% load. [ABSTRACT FROM AUTHOR]

This paper introduces a new analysis approach to modeling resonant converters that can be used to determine closed-form expressions for the exact resonant network waveforms. This approach, based on superposition of step responses to inverter and rectifier imposed steps, can easily be applied to resonant converters with high-order resonant networks, multiple inverters, and/or rectifiers. The usefulness of the proposed analysis approach is demonstrated first by applying it to a series-resonant converter, wherein it is used to determine accurate closed-form expressions for its tank current in different operating modes. These expressions are utilized to draw insights that help identify the converter's operating mode, and determine associated expressions for its output voltage and the switching frequency required to achieve a desired output power. The proposed approach is also applied to the multi-inverter multi-element impedance control network (ICN) resonant converter, wherein it is used to derive closed-form expressions for the converter's currents. These expressions are utilized to evaluate the behavior and optimize the design of an ICN resonant converter. A prototype 200-W 500-kHz ICN resonant converter optimized using the proposed approach is used to validate the modeling results. It is shown that there is an excellent agreement between the experimental and modeled waveforms during steady-state operation as well as during startup transients. [ABSTRACT FROM AUTHOR]

This paper presents a high-power-density high-power-factor GaN-based two-stage electrolytic-free offline LED driver. The LED driver achieves high power density through the use of a hybrid film–ceramic stacked switched capacitor (SSC) energy buffer, and a comprehensive optimization methodology that evaluates various converter topologies and designs and minimizes overall system volume while maintaining high efficiency. Based on this optimization approach, a totem-pole bridgeless boost power factor correction (PFC) converter is selected for the first stage, while a stacked-input isolated LLC dc–dc converter is selected for the second stage. The optimization procedure also minimizes the passive volume of the SSC energy buffer incorporating the volumetric energy density of commercially available capacitors. The optimized SSC energy buffer utilized in the proposed LED driver has 50% less passive volume than a capacitor-only solution. A 600-W prototype LED driver designed using the proposed approach for universal input voltage ${{(90}}-{ 305 \ \text{V}}_{{\text{rms}}})$ and 24-V output voltage is built and tested. The PFC stage achieves a peak efficiency of 98% while the LLC stage achieves a peak efficiency of 95%. The prototyped LED driver achieves a power density of 28 W/in3. [ABSTRACT FROM AUTHOR]

The recently proposed stacked switched capacitor (SSC) energy buffer architecture can extend the lifetime of single-phase ac–dc converters by replacing the electrolytic capacitors needed for twice-line-frequency energy storage with film or ceramic capacitors, while maintaining comparable effective energy density. This paper presents a methodology for further increasing the effective energy density of SSC energy buffers by optimizing the capacitance ratios of the capacitors used in the energy buffer. It is systematically shown that the relative enhancement in effective energy density depends on the required ripple ratio and the number of backbone and supporting capacitors in the energy buffer. The proposed approach can substantially increase the effective energy density of the energy buffer. For example, the effective energy density of a 1-10 enhanced unipolar SSC energy buffer, designed for a 10% ripple ratio, can be increased by 100% as a result of this capacitance ratio optimization. The presented methodology is validated using a 1-2 enhanced unipolar SSC energy buffer designed for an 8-W offline LED driver. [ABSTRACT FROM AUTHOR]

This paper introduces a MultiTrack power conversion architecture that represents a new way of combining switched-capacitor circuits and magnetics. The MultiTrack architecture takes advantages of the distributed power processing concept and a hybrid switched-capacitor/magnetics circuit structure. It reduces the voltage ratings on devices, reduces the voltage regulation stress of the system, improves the component utilization, and reduces the sizes of passive components. This architecture is suitable to dc–dc and grid-interface applications that require both isolation and wide voltage conversion range. An 18–80 V input, 5 V, 15 A output, 800 kHz, 0.93 in $^2$ (1/16 brick equivalent) isolated dc–dc converter has been built and tested to verify the effectiveness of this architecture. By employing the MultiTrack architecture, utilizing GaN switches, and operating at higher frequencies, the prototype converter achieves a power density of 457.3 W/in$^3$ and a peak efficiency of 91.3%. Its power density is 3$\times$ higher than the state-of-the-art commercial converters with comparable efficiency across the wide operation range. [ABSTRACT FROM AUTHOR]

The stacked switched capacitor (SSC) energy buffer is a recently proposed architecture for buffering energy between single-phase alternating and direct current. When used with film capacitors, it can increase the life of grid-interfaced power converters by eliminating limited-life electrolytic capacitors while maintaining comparable energy density. This paper introduces an enhanced version of the bipolar SSC energy buffer that achieves higher effective energy density and round-trip efficiency while maintaining the same bus voltage ripple ratio as the original design. Furthermore, the enhanced buffer uses fewer capacitors and switches than the original design. The enhancement in performance is achieved by modifying the control and switching patterns of the buffer switches. A prototype enhanced SSC energy buffer, designed for a 320-V bus and a 135-W load, has been built and tested. The design methodology and experimental results for the enhanced SSC energy buffer are presented and compared with the original design. This paper also presents a comparison of unipolar and bipolar SSC energy buffers. It is shown that, although bipolar designs are superior in terms of effective energy density at low ripple ratios, unipolar designs can outperform bipolar designs at high ripple ratios. [ABSTRACT FROM PUBLISHER]

Electrolytic capacitors are often used for energy buffering applications, including buffering between single-phase ac and dc. While these capacitors have high energy density compared to film and ceramic capacitors, their life is limited. This paper presents a stacked switched capacitor (SSC) energy buffer architecture and some of its topological embodiments, which when used with longer life film capacitors overcome this limitation while achieving effective energy densities comparable to electrolytic capacitors. The architectural approach is introduced along with design and control techniques. A prototype SSC energy buffer using film capacitors, designed for a 320 V dc bus and able to support a 135 W load, has been built and tested with a power factor correction circuit. It is shown that the SSC energy buffer can successfully replace limited-life electrolytic capacitors with much longer life film capacitors, while maintaining volume and efficiency at a comparable level. [ABSTRACT FROM AUTHOR]