The Role of High-Frequency Transformers

　　Currently, switch-mode power supplies typically use a half-bridge power conversion circuit, which includes components like a high-frequency transformer and transistors. When the circuit operates, the transistors alternate between conducting states, generating high-frequency pulses at a rate of 100 kHz. These pulses are then stepped down by the high-frequency transformer, ultimately producing a lower-voltage AC output. The exact output voltage level is determined by the turns ratio of each winding in the high-frequency transformer. In most designs, three transformers are employed: the main transformer, the driver transformer, and the auxiliary transformer. Each of these transformers has its own specific specifications and functions, making them all essential to the system's proper operation.

　　Generally, high-frequency transformers can be categorized into different classes based on their operating frequency ranges: 10 kHz–50 kHz, 50 kHz–100 kHz, 100 kHz–500 kHz, 500 kHz–1 MHz, and above 1 MHz. Additionally, the transformer's power transmission is inversely proportional to its operating frequency—higher power transmission corresponds to lower frequencies, and vice versa.

　　Next, let’s explore how to properly select the magnetic core for a switching transformer.

　　For transformers, the magnetic core is extremely important, making its selection critical—choosing the right one ensures the transformer performs optimally.

　　Typically, transformer cores are made from magnetic materials whose primary component is MnZn. However, due to variations in formulation and manufacturing processes, these magnetic materials come in numerous grades, each with distinct characteristic parameters—such as the operating frequency range, initial permeability, specific loss factor, specific temperature coefficient, saturation flux density, Curie temperature, resistivity, and density, among others. In general, core types include models like EI, EE, EC, U, and UF. When selecting a core, it’s essential to base your choice on the highest operating frequency at which the transformer will be used.

　　Where are high-frequency transformers used?

　　A high-frequency transformer is a power transformer designed to operate at frequencies exceeding the intermediate range (10 kHz). It is primarily used as a high-frequency switching power supply transformer in high-frequency switching power supplies, but it can also serve as a high-frequency inverter power supply transformer in applications like high-frequency inverters and high-frequency inverter welding machines. Based on their operating frequency ranges, these transformers are typically categorized into several tiers: 10 kHz–50 kHz, 50 kHz–100 kHz, 100 kHz–500 kHz, 500 kHz–1 MHz, and above 1 MHz. When handling relatively high power levels, IGBTs are commonly employed as the power devices. However, due to the inherent tailing effect of IGBT turn-off currents, their operating frequencies tend to remain relatively low. In contrast, when dealing with lower power applications, MOSFETs are often preferred, allowing for significantly higher operating frequencies.