Design and analysis of bidirectional three level T-type LLC resonant isolated DC-DC converter

The rapid development of renewable energy sources, battery storage systems, direct current (DC) grids, and electric vehicles has increased interest in bidirectional DC-DC converters. These converters typically function as an interface facilitating power transfer between a high-voltage DC bus, often fed by renewable energy sources, and a low-voltage DC bus to which energy storage devices are connected. In most of these applications, bidirectional power flow between the two DC buses is required, with a wide voltage conversion ratio and high efficiency at light loads, which remains a challenging aspect in designing and modulating bidirectional DC-DC converters. In the field of bidirectional DC-DC converters, the use of three-level topologies is increasingly prevalent. Compared to two-level topologies, these three-level topologies maintain lower voltage stress on the switching elements, thereby reducing losses during switching. Three-level topologies are categorized mainly into two types: I-type and T-type. The switch combination of the T-type topology offers higher reliability compared to the I-type topology. The primary reason for this is the absence of two additional diodes in the T-type topology, which are present in the I-type. This absence shortens the current path in the T-type, reducing conduction losses. Furthermore, the T-type topology requires fewer switching elements than the I-type topology, making it more cost-effective. In this study, the three-level T-type topology is employed due to its lower cost, higher reliability, and reduced losses compared to two-level topologies and three-level I-type topology. Bidirectional isolated LLC resonant DC-DC converters, particularly in terms of buck-boost capability, narrow switching frequency variation range, and wide Zero Voltage Switching (ZVS) operating range, outperform series resonant converters. However, when operating in reverse power flow, this topology still functions as a traditional series resonant converter and lacks symmetric operation. To address this lack of symmetric operation, a bidirectional CLLC resonant converter is proposed. However, the added resonant tank increases the converter's cost and size and reduces the voltage conversion ratio range compared to the traditional LLC resonant converter. In this study, the LLC resonant converter topology is preferred due to its desired wide ZVS operating range and high voltage conversion ratio, as well as the advantages of low cost and compact size. In this thesis, a bidirectional three-level T-type LLC resonant DC-DC converter has been designed and implemented using phase-shift modulation (PSM) for bidirectional power flow, variable frequency modulation (VFM) to achieve soft switching over a wide voltage conversion ratio range and under light loads, and pulse-width modulation (PWM) to reduce reactive circulating current. This work combines three modulation methods, where the duty cycle of the switches in the vertical legs, the phase shift ratio between primary and secondary switches, and the switching frequency are varied depending on the voltage conversion ratio and load. The increase in the number of controlled parameters is referred to in the literature as an increase in the number of degrees of freedom (DOF). This increase significantly enhances the converter's performance; it reduces the root mean square (RMS) current, decreases the reverse circulating power flow, and extends the soft-switching range. Consequently, these improvements increase the overall efficiency and reliability of the converter while reducing switching and conduction losses. However, each increase in the number of DOFs also increases control complexity and computational effort. In the proposed topology of this study, due to its half-bridge structure, it can be controlled with a maximum of four DOF modulation methods. In the four DOF modulation approach, complex calculations are required since the duty cycles of the switches in the vertical legs on the primary and secondary sides of the transformer are different. However, for practicality and ease of processing in this study, the duty cycles in the vertical legs have been set to be equal. Unlike classic 3-DOF control methods, the proposed 3-DOF modulation in this study; developed using parameters such as switching frequency, phase shift ratio, and duty cycle in the vertical legs, and applied for the first time in a bidirectional DC-DC converter. Thanks to the proposed modulation method, the LLC resonant converter can operate with high efficiency in the ZVS region under low load conditions and low voltage conversion ratios, unlike the classic single-phase-shift (SPS) method. This modulation method is achieved by increasing the switching frequency as the load decreases, with the intention of remaining in the soft-switching region between 30% load and full load. The phase shift ratio and the duty cycles of the switches in the vertical legs have been determined based on the changes in the switching frequency. The proposed LLC resonant topology operates as an LLC resonant circuit in forward power flow and as a series resonant circuit in reverse power flow. Therefore, the thesis presents theoretical analyses related to both LLC and series resonant circuit calculations for the converter operating in continuous current mode. In this study, by choosing a magnetizing inductance value higher than the impedance value in the series resonant tank, it has been ensured that the RMS values of the circuit currents at the primary and secondary ends of the transformer are close to each other. This approach has allowed the control parameters to take similar values in both forward and reverse power flow operations. However, if the designer chooses a lower value for the magnetizing inductance, separate theoretical calculations will be required for reverse power flow. The thesis provides detailed theoretical calculations and ZVS analyses for both forward and reverse power flows. The theoretical analyses of bidirectional isolated DC-DC converters are performed using time domain analysis (TDA) and frequency domain analysis (FDA) methods. In the TDA method, state equations are formulated using equivalent circuit models for each operating mode. This method requires more complex calculations than FDA, especially in cases with a high number of degrees of freedom (DOF) and when considering parameters like the junction capacitance of switching elements and dead time. On the other hand, FDA involves the use of Fourier transform for high-frequency voltage and current analysis. A simplified form of FDA, known as fundamental harmonic analysis (FHA), considers only the first harmonic component. However, this approach becomes challenging in accurately determining control parameter values under real application conditions, especially with an increase in the number of DOFs. Therefore, control parameter values obtained through FHA can significantly differ from expected results in practical applications. In this study, a simplified frequency analysis containing normalized values was preferred for its simplicity and accuracy. For the first time in the literature, this study applies simplified frequency domain analysis encompassing all harmonics in bidirectional converters, operating the proposed converter efficiently within the determined ZVS region. The Newton-Raphson method was used to derive control parameters from the equations found in frequency domain analysis. Considering that no significant changes in control parameters occurred beyond the 99th harmonic, the analysis included up to the first 99 harmonics. Theoretically calculated control parameters were compared with experimental results, showing a remarkably low error rate. Formulas for current, voltage, and power were derived for the proposed design, along with analysis of control parameters and the ZVS operating region. For a more accurate determination of the ZVS operating region, a detailed soft-switching analysis was performed, incorporating the MOSFET's junction capacitance and dead time parameters. The proposed 3-DOF control method with LLC resonant converter topology was tested and validated on a prototype capable of 400V input voltage and a variable output voltage range of 180-400V, with a maximum power capacity of 2kW. In the experimental study, parameters such as switching frequency and duty cycle for forward and reverse power flow were derived from equations produced in the theoretical analysis of the thesis. The degree value of the phase shift ratio obtained from both theoretical and experimental studies was compared, and the error rate was calculated. Despite various factors like temperature and parasitic circuit elements in the experimental study, the error rate was found to be below 3.5%. In efficiency analysis of the converter, an efficiency increase of 3.87% was achieved under 30% load conditions compared to a classic SPS-modulated T-type converter. Under full load, this increase was measured at 1.1% compared to the same type of converter. The obtained efficiency values were compared with other bidirectional resonant converters with similar modulation and structures in the literature. It was observed that the applied topology and control method resulted in an efficiency improvement of approximately 2-4% when compared with other bidirectional resonant converters at full load and 40% load conditions with a voltage conversion ratio M=1. The thesis also includes switching and conduction loss analyses for different voltage conversion ratios and load conditions. Additionally, the circuit's performance at low voltage conversion ratios and power levels was tested. The proposed approach with the 3-DOF modulation method combined with the LLC topology enabled high-efficiency operation over a wide voltage conversion ratio range. At a voltage conversion ratio of 0.45, the minimum efficiency measured for 405W output power was 94.15%. The highest efficiency, measured in reverse power flow at 50% load condition, was 97.4%.