ISM-Band Frequency Transformer Modeling for Isolated High-Power Conversions

In the last few years, the switching frequency of power electronic converters have been extended to the several MHz range. Industrial, scientific, and medical (ISM) frequency bands are specified in this range for power electronic applications. Most converter circuits, such as resonance and soft-switching circuits, require an accurate modeling of the circuit components, such as high-frequency inductors and transformers. This article focuses on the evaluation of a practically simple but accurate high-frequency transformer model. At first, a three-capacitor lumped model is chosen due to its simplicity, as well as the physics-oriented concept. Furthermore, the extraction of the parameters is straightforward. It is found that splitting the leakage inductances into two halves based on the winding capacitance would make the model more accurate for modeling high-frequency characteristics. The article analyzes the model and provides a closed-form transfer function for the input impedance and the voltage gain. To verify the model, different kinds of transformer hardware are fabricated, and their input impedances are measured from 1 to 30 MHz, which includes three ISM bands. The measurements are then compared with the impedance function developed from the model. The model is shown matching well with the measurement as compared to the conventional models. Finally, the model is validated experimentally by designing an isolated 13.56 MHz Class-E power conversion circuit, which includes a transformer with soft switching. The soft switching is designed based on the conventional, three-capacitor model and the proposed model, respectively. The proposed model provides a more accurate design for the inverter, assuring soft switching and optimum performance of the circuit.