Integrated Optimization Design of LLC Four-Element-Matrix Planar Transformer Considering Loss and Parasitic Parameters
Jiang Menhan1, Wu Qunfang1, Wang Qin1, Sun Zhifeng2, Lü Hui3
1. School of Automation Nanjing University of Aeronautics and Astronautics Nanjing 210016 China; 2. School of Electrical and Information Engineering Hunan University of Technology Zhuzhou 412007 China; 3. State Power Investment Group Zhuhai Hengqin Energy Development Co. Ltd Zhuhai 519000 China
Abstract：LLC converters are widely used in the power stage of battery energy storage converters due to their excellent soft switching performance and low output impedance. Under low voltage and high current conditions, matrix transformers are often used on the secondary side of LLC to reduce current stress. In practical circuits, the volume of matrix transformers accounts for about 25% of the total main power, which seriously restricts the improvement of device power density. This paper proposes an integrated optimization design of LLC four-element-matrix planar transformer considering loss and parasitic parameters. Decoupling the influence of various structural parameters of transformers on parasitic parameters this method achieves efficient operation of transformers and controllable parasitic parameters. Voltage drop and oscillation are effectively suppressed. Firstly, establish an accurate transformer loss model based on the proposed distributed magnetic core loss calculation method. Select the key parameters of the magnetic core that meet the efficiency and volume requirements: the radius of the magnetic core's central pillar r and the total width of the winding c. Afterward, select the winding layer structure. Establish a leakage inductance model based on transformer leakage magnetic field energy, determine the feasibility of leakage inductance integration through PCB thickness constraints, and design leakage inductance values. The experiment shows that the secondary-side Vds voltage oscillation is significantly reduced after integration optimization, reducing the parasitic capacitance value. By matching the resonant inductance to ensure that LLC operates in critical continuous mode at 300 kHz, the magnitude of the transformer's primary leakage inductance before and after integration can be calculated. The resonant inductance before integration is 1.8 μH, indicating the original edge leakage is 0.7 μH. After integrated optimization, the resonant inductance is 2.3 μH, indicating the original edge leakage is 0.2 μH. When operating in reverse, with the same input voltage of 3.2 V, the LLC output voltage rises from 34 V to 35 V. This means that the secondary edge leakage has decreased after integration. After resonance point matching, the secondary leakage inductance can be reduced from 33 nH to 12 nH. The secondary side Vds oscillation caused by parasitic capacitance is close and small, which verifies the control effect of parasitic parameters. This method can achieve the following effects. (1) The proposed distributed magnetic core loss calculation method for planar transformers based on P-B curves eliminates the influence of uneven magnetic density distribution on the accuracy of the magnetic core loss model, achieving accurate modeling of integrated transformer losses. (2) This method provides a judgment method for the feasibility of leakage inductance integration. (3) The prototype parasitic parameters can be controlled by accurately modeling the leakage inductance and parasitic inductance. This method provides theoretical support for designing and optimizing energy storage converters in LLC low-voltage and high-current scenarios.
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