High-frequency transformers play a crucial role in power electronic systems, performing core functions of voltage transformation and energy transfer. Their efficiency optimization directly impacts the overall system energy efficiency. Iron loss and copper loss are the main sources of loss in high-frequency transformers, and balancing these two losses is key to improving efficiency. Iron loss originates from the energy consumption of the magnetic core in an alternating magnetic field, including hysteresis loss and eddy current loss; copper loss is caused by the resistance heating of the winding coils and is proportional to the square of the effective current. When iron loss and copper loss reach a dynamic balance, the high-frequency transformer can maximize its efficiency. This balance requires the combined efforts of material selection, structural design, and process control.
The choice of magnetic core material is fundamental to balancing iron loss and copper loss. In high-frequency applications, ferrite materials are the mainstream choice due to their low-loss characteristics, exhibiting excellent performance in both hysteresis and eddy current loss at high frequencies. For higher frequencies or higher power densities, nanocrystalline and amorphous alloy materials can further reduce iron loss, but a trade-off between cost and processing difficulty is necessary. For example, nanocrystalline materials can maintain low losses at frequencies above 100kHz, but require special annealing processes to control the magnetic domain structure; amorphous alloys have even lower losses, but their core forming process is complex, making them suitable for applications with extremely high efficiency requirements. Material selection must consider constraints such as frequency range, power density, and cost budget to ensure iron losses remain within a reasonable range.
Winding design is a core aspect of balancing copper losses. The skin effect and proximity effect of high-frequency currents significantly increase the effective resistance of the windings, leading to a surge in copper losses. Using multi-strand stranded wire (Litz wire) can increase the conductor surface area and weaken the skin effect; flat wire structures reduce the number of layers by increasing the width-to-thickness ratio, reducing the uneven current distribution caused by the proximity effect. Furthermore, the winding layout needs to be optimized to reduce leakage flux, for example, by using a "sandwich winding" method (primary-secondary-primary) or segmented winding to ensure tight coupling between the primary and secondary windings and reduce magnetic field leakage. These measures can effectively reduce the effective resistance of the windings, thereby suppressing the growth of copper losses.
Controlling the operating magnetic flux density is a key parameter for balancing iron losses and copper losses. Excessive magnetic flux density leads to core saturation, causing a sharp increase in hysteresis losses; conversely, insufficient flux density necessitates increasing the number of turns or core cross-sectional area, resulting in increased copper losses. During design, the optimal operating point must be determined based on the B-H curve of the core material. Typically, a flux density of 1/3 to 1/2 of the saturation value is chosen to balance iron and copper losses. For example, the saturation flux density of ferrite materials is approximately 0.3-0.5T; the operating flux density should be controlled within the range of 0.1-0.2T during design to avoid entering the high-loss region.
Frequency selection has a dual impact on the balance between iron and copper losses. Eddy current losses in iron are proportional to the square of the frequency, while the degree to which copper losses are affected by frequency depends on the winding structure and the severity of the skin effect. Increasing the frequency can reduce the core size and the number of turns, thereby reducing copper losses, but it will significantly increase iron losses. Therefore, frequency selection must consider the comprehensive performance of the core material and winding design to find the optimal solution between iron and copper losses. For example, ferrite materials achieve good loss balance in the 50kHz to 500kHz frequency range, while nanocrystalline materials can extend this range to over 1MHz.
Heat dissipation design is a long-term condition for ensuring the balance between iron and copper losses. During high-frequency transformer operation, both iron and copper losses are converted into heat. Poor heat dissipation leads to increased core and winding temperatures, further exacerbating losses. Optimizing the core structure (e.g., using a low-thermal-resistance PQ-type core), increasing the heat dissipation area, and employing forced air cooling or liquid cooling can effectively control temperature rise and maintain loss balance. For example, in a confined environment, thermal grease can be used to conduct heat from the core to the heatsink; in an open environment, fans can be used to accelerate airflow and improve heat dissipation efficiency.
Balancing iron and copper losses is the core task of high-frequency transformer efficiency optimization. Through a comprehensive approach of material selection, winding design, flux density control, frequency optimization, and heat dissipation management, a dynamic balance between iron and copper losses can be achieved in specific application scenarios, thereby maximizing the efficiency of the high-frequency transformer. This balancing process, which requires taking into account performance, cost, and reliability, is one of the core challenges in high-frequency transformer design.
