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| High-Efficiency High Voltage Gain DC-DC Converter Based on Input Parallel and Output Series Connection |
| Sang Xikun, WangYijie, Xu Dianguo |
| Department of Electrical Engineering Harbin Institute of Technology Harbin 150001 China |
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Abstract Nowadays, with the rapid depletion of traditional fossil fuels, the harm to the environment is becoming more prominent. The proliferation of renewable energy resources has solved the energy shortage problem and realized sustainable development. The high step-up DC-DC converters have received widespread attention in renewable energy systems, such as photovoltaic module-integrated converters, fuel cells, and hybrid electric vehicles. In general, the high step-up DC-DC converters can be divided into non-insolated and isolated types. The non-isolated DC-DC converters usually adopt coupled inductors and voltage multiplier cells to realize the boost capability. In contrast, the isolated structures can realize the galvanic isolation and the expected voltage gain by changing the transformer turns ratio, which is more suitable for high step-up applications with high safety performance.
The LLC resonant converter has gained popularity due to the soft switching properties of power devices, constant conversion ratio, and high working efficiency near the resonant working point. However, it will generate large current stress on the primary side of the transformer when a high boost ratio is achieved, causing additional ESR losses of the resonant components and efficiency sacrifice. Thus, the resonant tank is usually located at the secondary side in high step-up applications, and the system voltage gain of LLC is regulated by pulse frequency modulation. The switching frequency change will make the resonant point deviate from the ideal state and generate additional reactive power, which brings difficulties to the design of EMI filters and magnetic components. Besides, the soft switching range of power devices is affected as the switching frequency changes. Although LLC is preferable to operate in the unregulated state, it will lose the voltage regulation capacity. Therefore, the two-stage architecture is a popular solution, where LLC can perform as a DC transformer (DCX), and the system dynamic regulation is achieved by the front or latter DC-DC stage. However, a potential disadvantage is that both stages need to transmit all the system power, which is not conducive to efficiency improvement.
A high step-up DC-DC converter based on input-parallel and output-series (IPOS) connection is proposed in this paper. The upper part of the system is an LLC converter with the secondary resonant tank, which operates as a DCX and realizes a constant voltage conversion ratio. A secondary series resonant active clamp DC-DC converter is adopted in the lower part to perform as a system voltage regulator, and the dynamic voltage regulation of the system can be achieved by pulse width modulation. The soft switching properties of all the power devices can be realized in the proposed converter. DXC processes an amount of the system power, and the DC-DC voltage regulator only handles a small part of the total power, thereby reducing device stress and power losses. Compared with the two-stage solution, the IPOS structure can reduce the power level and component ratings of each part more effectively. The transformer’s leakage inductor can replace the resonant inductor in the proposed converter, and the optimization of the windings arrangement reduces the parasitic capacitance. The working efficiency is further improved through the optimal design of the parameters and the magnetic components for DCX at the rated point. The working principle, parameter design criterion, and planar transformer optimization are introduced in detail. An experimental prototype with an input voltage range of 24 V to 32 V, an output voltage of 400 V, a rated power of 200 W, and a switching frequency of 1 MHz is built to verify the theoretical analysis. The peak test efficiency is up to 96.3%.
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Received: 09 January 2023
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2023, Vol. 38 
