1. College of Electrical Engineering and New Energy China Three Gorges University Yichang 443002 China; 2. State Grid Chongqing Electric Power Company Wanzhou Power Supply Branch Chongqing 404100 China
Abstract：The air-core Tesla transformer, with advantages of compact structure and good adaptability to repetitive operation, facilitates the compact design of pulse drivers. However, due to its coreless structure and insulation constraints of primary and secondary windings, the coupling coefficient(k) is low, resulting in low energy transfer efficiency of the air-core Tesla transformer. To improve its energy transfer efficiency, this paper uses a double-straight-cylinder coaxial structure for the primary and secondary windings instead of the traditional conical structure of the secondary winding. By optimizing the insulation distance between the primary and secondary windings, while ensuring insulation safety, the k of the air-core Tesla transformer is increased, achieving k≥0.85, and the performance requirements of the compact pulse drive source are met: when the primary is charged at 2 kV, the secondary output voltage U2>53 kV, and the energy transfer efficiency η>40%. Analysis of Tesla transformer's resonant charging process shows voltage gain and energy transfer efficiency are related to detuning coefficient α, k, and effective quality factor Qe. Voltage gain increases with increasing α and Qe; η increases with increasing k, and first increases then decreases with α. For the Tesla transformer's target parameters, when Qe=6, α=2, k≥0.85, the voltage gain greater than 1 and η>40%, meeting the design requirements. The detuning coefficient can be adjusted from external circuit parameters, so the focus is on improving the transformer's coupling coefficient. The double-straight-cylinder coaxial structure for the primary and secondary windings achieves a compact design while also improving its coupling coefficient. The coupling coefficient is related to the transformer's aspect ratio (β=l0/r2, where l0 is the height of primary and secondary windings, r2 is the secondary winding radius) and radius ratio (d=r1/r2, where r1 is the primary winding radius). With β fixed, k decreases as d increases. With d fixed, k increases with β, but remains almost unchanged when β>3. Thus, d=1.1 and β=3 were determined, the theoretical value of k is 0.908. Based on the structural parameters of the air-core Tesla transformer, a field-circuit coupling simulation model was established, and the shielding structure at the high-voltage end of its secondary winding was optimized to improve electric field distribution, with a built-in aluminum strip shielding structure, the electric field strength can be reduced below the breakdown threshold of insulating materials. A spiral air-core Tesla transformer with the turns ratio of 7:230 was developed, the Experiments show that its k>0.87, when the primary is charged at 2 kV, the secondary output voltage reaches 55.8 kV, and the η is 44.5%. When the transformer is charged at 2.2 kV, the transformer can operate 100 times continuously at 5 Hz without insulation breakdown. When α is reduced from 2.24 to 1.80, η can be increased to 54.7%, and the transformation ratio is 26.3, which still achieves the output voltage parameters. The designed air-core Tesla transformer has high coupling coefficient and energy transfer efficiency, safe insulation, compact structure, and stable performance. It has been applied to a compact pulse drive source system.
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2026, Vol. 41 
