Kemal Kalayci

This paper proposes a bidirectional three-level T-type LLC resonant DC-DC converter operating with phase-shift control, which is simple and effective, variable frequency modulation to perform soft switching in a wide voltage ratio range and light loads, and pulse width modulation to reduce reactive circulating current. In this combined modulation method, three control parameters: switching frequency, the duty cycle of switches in the vertical arms, and phase shift between primary and secondary switches, are changed depending on the voltage conversion ratio and load. The circuit operates as LLC resonant in forward operation and series resonant in reverse operation. Thus, LLC and series resonant analyses of the proposed converter, operated in continuous current mode and buck mode, are presented. A simplified frequency domain analysis, including all harmonics, is used in the theoretical calculations. Thus, the control parameters are accurately calculated for forward and reverse operations. The current, voltage, and power formulas in the working principle of the converter are derived, and an analysis of the control parameters and zero voltage switching region is performed. Soft switching analysis is done by considering the junction capacitor of the MOSFET and the dead time. The theoretical performance of the proposed modulation method has been verified with a 2kW prototype. Soft-switching and loss analyses of the proposed converter have also been investigated under different load conditions and voltage conversion ratios. The proposed LLC resonant topology and modulation method improved significantly more efficiency than the phase-shifted modulated T-type converter under 30-100% load conditions.

Electromagnetic interference is the main problem in high switching power converters like DC-DC converters. One of the solution methods for electromagnetic interference reduction is to design a snubber circuit. The snubber circuit is a practical method to reduce fluctuations caused by parasitic components from the layout and non-ideal characteristics of switching devices. An effective snubber circuit design increases the converter's efficiency and reduces EMI noise. In this paper, electromagnetic interference is reduced using different snubber circuits in a three-level T-type isolated bidirectional DC-DC converter. The converter is simulated in PSIM to see the effectiveness of the noise reduction with the snubber circuits. CISPR 25 standard is used for the compliance assessment in the study.

This study presents a novel three‐level T‐type isolated bidirectional DC–DC converter (3LTT‐IBDC) for bidirectional DC power transfer. Owing to of the T‐type structure of the proposed converter it has less number of switches and thus, lower cost and higher efficiency, as well as easy control are the advantages of the proposed converter compared to three‐level counterparts. Additionally, 3LTT‐IBDC is more reliable than three‐level converters since unequal voltage blocking does not occur in T‐type structure. Moreover, the symmetrical operation of the isolation transformer due to three‐level symmetrical voltage waveform, lower voltage stress of switches and thus, higher efficiency are other advantages of 3LTT‐IBDC compared to two‐level counterparts. Here, the DC voltage gain and power transfer characteristic of the proposed converter for steady‐state operation and the expressions of the leakage inductance are derived. The proposed converter is simulated using PSIM. A 2‐kW prototype is built to verify the theoretical analysis of the converter. Theoretical and experimental results show a good agreement and validate the competency of the presented converter design. The efficiency of the proposed converter and switching transitions of the switches are analysed. The full load and maximum efficiency of the converter are measured as 96.27% and 96.81%, respectively.