高效率CLLLC谐振变换器的高频变压器结构优化设计

doi:10.19595/j.cnki.1000-6753.tces.241287

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Abstract Under high-frequency conditions, the transformer winding losses of the CLLLC converter are influenced by eddy current effects and complex structures. The trade-offs between total transformer losses, size, and temperature rise pose significant challenges for the design of electromagnetic structures that require high power density and efficiency. Traditional transformer design methods often overlook the constraints of temperature rise and the impact of complex structures on winding losses, relying solely on the area-product approach to derive performance-compliant parameter values. It is difficult to achieve optimal transformer design. Existing transformer optimization analytical design methods consider temperature rise constraints and analyze winding losses under high-frequency conditions. However, most methods lack sufficient accuracy in predicting transformer losses, neglect the impact of variable AC resistance and fill factors on efficiency optimization, and fail to address the limitations of excitation frequency on the generality of the optimization design. Furthermore, these methods often involve high computational costs. Therefore, this paper proposes an analytical design method to improve the efficiency of CLLLC converters and formulates a closed-form optimization equation for the essential design variables of the transformer.
First, based on the MSE and IGSE methods under non-sinusoidal excitation, an independent flux density core loss model is derived. The response patterns of area-product and winding turns to core losses are analyzed, revealing the intrinsic relationship between the loss model and transformer structural parameters. Second, an approximate correction of the Ferreira model is proposed to improve the accuracy of winding loss predictions by the derivation of the equivalent area model, the orthogonality of eddy current effects, and the linear equivalence of the Kelvin function. Third, an analytical function is established for the error threshold and the approximation conditions of the winding loss model, which mitigates the influence of winding structural parameters on the analytical function. A constraint condition model for the error threshold and excitation frequency is constructed. Finally, combined with the independent flux-density core loss model and the approximately corrected Ferreira winding loss model, a transformer optimization closed-form equation for temperature rise constraints is derived using the Lagrange function. The response patterns of transformer efficiency to the area-product and winding turns are analyzed. The discrete structural parameters of the transformer are further optimized, and a transformer optimization design process is established.
The finite element simulation of the transformer indicates that the simulated winding losses within the frequency range of 1 kHz to 1 MHz closely match the predictions of the approximately corrected Ferreira model. The simulated total transformer loss, optimal winding, and optimal strand number align well with the values predicted by the analytical method.
A 120 W/48 V CLLLC experimental prototype is built based on the optimal transformer design. The soft-switching characteristics of the CLLLC resonant tank are tested under under-resonant, quasi-resonant, and over-resonant conditions across a 20% to 100% load range. The results demonstrate that soft switching can be achieved under all three operating conditions. The output power and efficiency of the three CLLLC designs are measured under fixed frequency (200 kHz) with varying loads and fixed load (full load) with varying frequencies. The experimental results show that the highest optimized transformer loss is 56.72%, and the maximum efficiency improvement of the CLLLC resonant converter is 0.529%. The optimized loss difference agrees with the simulation data and the predictions from the analytical method.
The proposed approximately corrected Ferreira model and the independent flux-density core loss model are verified. A comprehensive evaluation of existing analytical design methods shows that this approach balances transformer optimization design methodologies’ accuracy, optimization, simplicity, and generality.

Key words： High efficiency area product method transformer optimization design optimization equations

Received: 19 July 2024

PACS:

TM46

Corresponding Authors: 明正峰男,1962年生,教授,博士生导师,研究方向为新型电机及其智能控制、电力电子技术及高频电源、电力传动中的变频调速技术和三相PWM软开关变频技术、变频调速装置相关的控制装置及控制系统等。E-mail: zfming@xidian.edu.cn

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	Zhao Yuhu
	Ming Zhengfeng
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Zhao Yuhu,Ming Zhengfeng,Du Chaofan等. Optimized Design of High-Frequency Transformer Structure for High-Efficiency CLLLC Resonant Converters[J]. Transactions of China Electrotechnical Society, 2025, 40(18): 5970-5987.

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