Ernesto L. Barrios

Analytical Design Methodology for Litz-Wired High-Frequency Power Transformers

In the last quarter of a century, high-frequency (HF) transformer design has been one of the major concerns to power electronics designers in order to increase converter power densities and efficiencies. Conventional design methodologies are based on iterative processes and rules of thumb founded more on expertise than on theoretical developments. This paper presents an analytical design methodology for litz-wired HF power transformers that provides a deep insight into the transformer design problem making it a powerful tool for converter designers. The most suitable models for the calculation of core and winding losses and the transformer thermal resistance are first selected and then validated with a 5-kW 50-kHz commercial transformer for a photovoltaic application. Based on these models, the design methodology is finally proposed, reducing the design issue to directly solve a five-variable nonlinear optimization problem. The methodology is illustrated with a detailed design in terms of magnetic material, core geometry, and primary and secondary litz-wire sizing. The optimal design achieves a 46.5% power density increase and a higher efficiency of 99.70% when compared with the commercial one.

High-Frequency Power Transformers With Foil Windings: Maximum Interleaving and Optimal Design

Foil conductors and primary and secondary interleaving are normally used to minimize winding losses in high-frequency (HF) transformers used for high-current power applications. However, winding interleaving complicates the transformer assembly, since taps are required to connect the winding sections, and also complicates the transformer design, since it introduces a new tradeoff between minimizing losses and reducing the construction difficulty. This paper presents a novel interleaving technique, named maximum interleaving, that makes it possible to minimize the winding losses as well as the construction difficulty. An analytical design methodology is also proposed in order to obtain free-cooled transformers with a high efficiency, low volume, and, therefore, a high power density. For the purpose of evaluating the advantages of the proposed maximum interleaving technique, the methodology is applied to design a transformer positioned in the 5 kW-50 kHz intermediate HF resonant stage of a commercial PV inverter. The proposed design achieves a transformer power density of 28 W/cm3 with an efficiency of 99.8%. Finally, a prototype of the maximum-interleaved transformer is assembled and validated satisfactorily through experimental tests.

Frequency-based energy management of stand-alone systems: Design of the control parameters for high versatility

Frequency-based energy management strategies have been proposed for stand-alone systems with either centralized or distributed energy storage. This paper proposes a methodology for the design of the control parameters which ensures stability in both type of systems. As a result, the inverter manufacturer can implement one sole strategy, regardless of whether the expected stand-alone system is centralized or distributed. In addition, the battery inverter becomes more versatile since it can be used alone or in parallel with other battery inverters. Simulation results corroborate the theoretical analysis and demonstrate that energy management is carried out with no need of communication cables for a system with 1, 2 or 3 battery inverters with the same firmware.

Optimal DC gapped inductor design including high-frequency effects

DC inductor design has recently drawn the attention of magnetic designers due to the rise in the number of DC-DC power converter applications, such as electric vehicles and renewable distributed generation systems. The design methods found in the literature are highly iterative and make it difficult to assess the design trends and optimize the power converter as a whole. In this paper, the DC gapped inductor design problem is analyzed. A unified inductor model is first developed and a novel inductor design methodology is then proposed. The comprehensive inductor model makes it possible to easily introduce all the phenomena present in the inductor operation in the design process, including winding high-frequency effects and fringing flux in the gap. With this so-called analytical inductor design methodology, the inductor energy density can be maximized while achieving high efficiencies. Finally, an inductor for a fuel cell 2.4 kW-22 kHz boost converter is optimally designed reaching an energy density of 0.61 J/dm3 and an efficiency of 99.04%.

Modelling, improvement and experimental validation of a 50 kHz-5 kVA litz-wired transformer for PV inverters

Modelling of medium-frequency power transformers is analyzed and experimentally validated to characterize these devices along the operating power range. Special attention is paid to the core losses, winding losses and thermal modelling at operating conditions typical of power electronics applications. The performance of a commercial 50 kHz-5 kVA litz-wired transformer for PV resonant converters is characterized and tested by means of a specific measurement procedure. A complete model is selected from the estimated and measured results. Finally, this model is applied to improve the design of the commercial transformer previously analyzed in order to match typical PV requirements such as a high efficiency at different irradiation levels.

Analytical winding loss calculation for high-frequency low-permeability inductors

Traditional winding loss estimation models lead to unacceptable estimation errors when applied to low-permeability inductors. This is due to the highly two-dimensional character of the magnetic field in these inductor windings. In this paper, a new analytical model is proposed which makes it possible to carry out a straightforward and accurate calculation of the winding loss in high-frequency low-permeability inductors.

Simple and robust PLL algorithm for accurate phase tracking under grid disturbances

Phase-locked loops (PLLs) are commonly used for the synchronization of grid connected power converters. However, the standard PLL structures are not able to reject grid disturbances such as voltage harmonics, unbalances or DC components in the measured voltage. Since the response of the power converters depends on the grid phase tracking, it is necessary to ensure an accurate synchronization even under distorted conditions. This paper proposes an enhanced PLL structure, which is able to perform fast synchronization even under weak grid conditions and frequency variations. Due to its low computational burden, the proposed PLL allows its implementation on low cost microprocessors.

Tool for the comparison and selection of power semiconductors in the converter design process

The design of power converters is a complex task since it involves different highly interrelated physic disciplines. One of the main questions to be answered when designing a converter is which topology and semiconductor technology need to be used in order to have an optimal design. A common solution to this problem is to carry out different designs, one for each module, and to compare and select the best design a posteriori. This procedure makes it difficult to carry out a sensitivity analysis of the final characteristics of the converter with respect to the design variables, e.g. the switching frequency. In this paper, a tool that allows the easy comparison of different semiconductor modules as a first step in the converter design process is presented. The proposed tool is based on power loss and thermal models of the semiconductor modules directly extracted from the data provided by the manufacturers. These models are validated by means of the manufacturers power losses calculation software. The tool is used to compare and select the best commercial module for a 3-phase PV inverter application in the 60 kW range. Finally, the optimal ratio between conduction and switching losses is obtained and useful general rules for the correct use of semiconductors are formulated.

High-frequency effects and minimum size design methodology for SMPS transformers with solid round conductors

In terms of volume, transformers are the most important components of switch mode power supplies (SMPS). When moving towards higher frequencies in order to reduce the converter volume, the impact of high-frequency (HF) effects in the windings increases. This paper proposes a transformer design methodology that takes into account these HF effects in solid round windings and allows maximizing the transformers power density. The methodology is developed considering a forward converter case study.