Vesa Väisänen

Variable-Frequency Phase Shift Modulation of a Dual Active Bridge Converter

In this paper, a variable-frequency modulation method for a dual active bridge converter is introduced. The proposed method ensures zero-voltage switching over a wide power range with a minimal circulating current. Unlike previously presented modulation schemes, this modulation method can also be used for half-bridge variants of a traditional dual active bridge converter. The modulation method is given in a closed form, which makes it easy to apply in practice. Further, the phase drift phenomenon is discussed, and a simple phase drift compensation scheme is presented. Finally, a detailed analysis of the proposed modulation method is provided and its feasibility is verified by measurements.

Effects of Switching Asymmetry on an Isolated Full-Bridge Boost Converter

The effects of switching asymmetry and flux walking on an isolated current-fed full-bridge boost converter are presented with two different secondary configurations: a voltage doubler (VD) and a diode bridge (DB). The results show that a drop in the converter efficiency is clearly visible when there are switching asymmetries in the DB configuration, whereas the VD configuration is more tolerant of switching asymmetries because of its inherent capability to oppose flux walking.

AC resistance calculation methods and practical design considerations when using litz wire

This paper reviews and compares two easily applicable methods used to calculate the AC resistance and an optimal strand diameter for a litz wire. The methods can be implemented in mathematical software, such as MATLAB, by using the equations and program listings given in this paper. The practical limitations of the litz wire are discussed based on the results of the methods. The AC to DC resistance ratios are plotted as a function of various strand diameters with respect to skin depth, which allows the approximation of litz wire AC resistance with a small calculation effort. The analysis and results presented in this paper are verified by measurements and FEM simulations with various litz wire configurations.

Galvanic Isolation and Output LC Filter Design for the Low-Voltage DC Customer-End Inverter

In this paper the need for a better design of the galvanic isolation and the output filter inductor in the customer-end inverter (CEI) of the low voltage DC (LVDC) network is discussed. The galvanic isolation can be implemented either with a 50 Hz transformer after the CEI or with an isolated DC-DC converter between the DC network and the CEI. However, the 50 Hz transformer solution adds a significant amount of volume and mass to the system. Based on calorimetric measurements conducted with the current system in which the galvanic isolation is implemented with the former method, the minimum requirements for the isolated DC-DC converter are presented and a comparison is carried out. For the system to be cost efficient the DC-DC converter should result in reduced lifetime cost compared with the transformer solution. In the output filter part a design method for an amorphous core filter inductor based on minimization of the lifetime cost is presented.

A bidirectional current-fed resonant push-pull converter for low voltage, high current applications

In this paper, a bidirectional current-fed resonant push-pull converter is proposed for energy storage applications where high voltage conversion ratio, high power and low current ripple are needed. In this study, theoretical waveforms of bidirectional RPP are shown, analytical analysis is provided and a modulation method for reverse power flow operation is proposed. Finally, theoretical results are verified with a 5kW prototype. The main contribution of this paper is modifying previously presented resonant push-pull topology and its modulation principle to provide bidirectional power flow.

Core and air gap influence on the accuracy of inductor AC winding resistance calculation methods

This paper discusses the accuracy of typical AC resistance calculation methods in gapped inductors. It is shown that despite the additional losses caused by fringing flux induced eddy currents, the calculated resistances can be overestimated considerably due to non-tangential field distribution and reduced winding proximity losses. However, this phenomenon is not so significant with windings having only a few winding layers; in such cases 1D calculation methods can be useful in predicting the winding resistances. It is also demonstrated how the fringing flux induced losses affect the calculated results and how they can be taken into account when using the 1D methods.

Implementation design of the converter-based galvanic isolation for low voltage DC distribution

In this paper the implementation of the galvanic isolation in the low voltage direct current (LVDC) network is discussed. The galvanic isolation can be implemented either with a 50 Hz transformer located at the customer AC output of the customer-end inverter (CEI) or with an isolated DC-DC converter at the input DC network side of the CEI. However, the former solution results in a significant increase of the volume and mass of the system and has a high amount of no-load losses. Therefore, an isolated DC-DC converter is studied in this paper. Based on calorimetric measurements conducted for the passive 50 Hz transformer, the requirements for the DC-DC converter are set. Selection of the converter topology is carried out and the selected topology is studied by simulations and calculations. The results are compared with the 50 Hz transformer.

Life-cycle cost analysis for the customer-end inverter used in low voltage dc distribution

This paper analyses the life-cycle cost of a customer-end inverter that is used in a low voltage DC distribution system. Cost of the losses during the utilization period of the system and the investment cost of the main components is included in the study and the distribution between them is analyzed to determine the dominating cost factors. A sensitivity analysis is carried out to determine how variations in the cost of electricity, the cost of the hardware and the selected switching frequency affect the life-cycle cost.

Requirements for the control system of an SOFC power conversion unit in stationary power generation

In this study, the solid oxide fuel cell (SOFC) is briefly introduced, the most important characteristics of the SOFC in stationary applications are reviewed, and operational limits for power conversion units (PCU) are introduced. This paper aims to review the most important points in designing a controller for an SOFC power conversion unit in stationary applications such as distributed generation (DG). The control principles of DC/DC and DC/AC converters as well as the control of an entire power conversion unit are reviewed. The most important design factors for a PCU control system are proposed based on the review.

Maximum efficiency point tracking algorithm for dual active bridge converters

This paper presents an algorithm for seeking the most suitable operating point and the maximum efficiency in respect of the operating conditions in a dual active bridge converter. The method is the most suitable for half-bridge variants of the dual active bridge since the previously presented modulation methods are derived only for full-bridge configurations. The maximum efficiency point tracking method is based on a perturb-and-observe type tracker and a variable frequency modulation method where the turn-on currents of the primary and secondary bridges can be adjusted by using closed-form expressions.