Three-Dimensional Temperature Calculation and Optimization Design Method for High Power High-Frequency Transformer

Temperature rise calculation is a complex problem in modeling high-power high-frequency transformers (HFT). An accurate and fast thermal model is significant for the optimal design and stable operation of HFT. At present, lumped parameter thermal network model is usually built through dimensionality reduction, and its calculation precision is easily affected by structural parameters. It is difficult to achieve accurate hot spot prediction in optimizing a wide parameter range. A three-dimensional thermal model of HFT considering anisotropic thermal conductivity is constructed based on the finite difference method (FDM). The discretization error of the three-dimensional thermal model is quantitatively analyzed, which is mainly affected by the loss density, thermal conductivity, and size of the finite difference element. In order to minimize the number of finite differential elements while ensuring the calculation accuracy of the thermal model, three-dimensional sizes of the differential element are adjusted actively according to the discretization error expression. For HTF with nanocrystalline core and litz wire winding, the element sizes in the direction along the width, height of winding and the thickness of core are the key parameters affecting the accuracy of temperature rise calculation. The parameter adaptability of the proposed thermal model is verified in detail with finite element simulation, including structural parameters, loss density, and heat dissipation conditions. The results show that the maximum error of the model is less than 10% in a wide range of parameters. However, the error of the traditional lumped parameter thermal network model is significantly affected by the structural parameters, and the error varies in the range of 10%～80% within the same parameter variation range. Based on the proposed thermal model, the efficiency-power density optimal design of a 10 kHz 150 kW transformer was carried out with the parameter scanning method. The upper limit of temperature rise was set to 100 K, and the differential element sizes were adjusted according to the error of less than 5 K. The optimization design program calculated 500 000 design points within 371 s with parallel computing, and the average time of three-dimensional thermal model calculation for a single design point was 2.8 ms. The temperature rise calculation results of the design points on the optimal design boundary were verified, and the error was less than （下转第 5016 页）the relationship between power transmission characteristics, switch current stress and the magnetic components parameters. The design method of magnetic components parameters is proposed by comprehensively considering the influence of forward and flyback transmission power ratio on the electrical performance of the conversion topology. For the forward-flyback converter, the forward inductor can only work in DCM. Compared with the minimum excitation inductor current in one cycle with zero, its working mode can be divided into Magnetizing current continuous conduction mode (MCCM) and Magnetizing current Discontinuous Conduction Mode (MDCM). Its power transmission characteristics and its relationship with a load resistance RL under different modes show that both the forward power PFW and flyback power PFB increase with the decrease of the RL when the conversion topology works in the MDCM. When the RL is reduced to make the conversion topology enter the MCCM, the PFW increases to the maximum value and no longer increases with the decrease of the RL, while the PFB still increases with the decrease of the RL. The critical excitation inductance Lmc corresponding to PFW=PFB is defined by analyzing the influence of excitation inductance on the power transmission and distribution characteristics of the conversion topology. For the given output power, the PFW increases, and the PFB decreases with the increase of excitation inductance. When the excitation inductor equals Lmc, and the conversion topology works in MDCM, the PFW equals PFB. The PFB increases, and the PFW decreases with the increase of the transformer turn ratio. Regarding the influence of power transmission distribution on the switch current stress, for the given output power, the switch current stress decreases with the decrease of PFW. Therefore, reducing the PFW can reduce the switch current stress. However, reducing the PFW will increase the PFB, while increasing the PFB requires increasing the transformer air gap to reduce the magnetizing inductance, which will increase the transformer loss. Therefore, reducing the PFW is not beneficial to improving the efficiency of the conversion topology. Regarding the influence of the distribution of PFW and PFB on the electrical performance of the conversion topology, reducing the PFW is beneficial to reducing the current stress of the switch, and reducing the PFB can improve the efficiency of the conversion topology. Therefore, to improve the efficiency of the conversion topology and not cause the current stress of the switch to be too high, a component parameter design method is proposed to guarantee that PFW=PFB when the output power reaches the maximum within the given load range. Experimental results verify the correctness of the theoretical analysis and the feasibility of the proposed design method. The proposed design method for component parameters can develop a high-performance forward-flyback conversion topology and promote its popularization and application. © 2023 Chinese Machine Press. All rights reserved.