Transformers have long been essential components in power supply systems, aiming to reduce size, increase power density, and achieve modular design. Although high-frequency conversion technology has been introduced to eliminate the need for bulky power-line frequency transformers, it still requires a high-frequency transformer with a ferrite core. While the volume of such a ferrite-core transformer is smaller than that of a traditional one, it still falls short of meeting modern modular requirements. It remains too large, generates significant heat, and has considerable leakage inductance. In recent years, researchers and engineers have focused on overcoming these challenges. The development of high-frequency flat-panel transformers has marked a major advancement in transformer technology. These transformers are not only much smaller but also exhibit very low internal temperature rise, minimal leakage inductance, and efficiency as high as 99.6%. Additionally, their cost is about half that of conventional transformers of the same power rating. They are suitable for various converter topologies, including single-ended forward, flyback, half-bridge, full-bridge, and push-pull circuits, particularly ideal for low-voltage, high-current applications. As a result, they are widely used in modern computer power supplies.
One of the main issues with conventional high-frequency transformers is leakage inductance. In an ideal transformer, all flux from the primary winding should pass through the secondary without loss or leakage. However, in reality, some flux remains uncoupled, forming an inductance within the windings. This "leakage" inductance stores energy that is not transferred to the secondary circuit, leading to electromagnetic interference (EMI). The challenge lies in balancing insulation requirements with the need for tight coupling to minimize leakage. When the transformer is turned off, the stored energy in the leakage inductance is released, creating noise spikes. The amplitude of these spikes is proportional to the product of the leakage inductance and the rate of change of current: |Uspike| = Lleak × di/dt. As operating frequency increases, the current change rate rises, worsening the effect of leakage inductance. This can lead to voltage spikes that may damage components. To mitigate this, buffer networks are often added, but they increase losses and reduce overall efficiency.
Another issue is inter-winding capacitance. In multi-layer windings, there is a potential difference between layers, resulting in parasitic capacitance. At high frequencies, this capacitance charges and discharges rapidly, causing additional losses. The higher the frequency, the more frequent these cycles become, increasing total losses.
Skin effect and proximity effect also contribute to inefficiencies. Skin effect causes current to concentrate near the surface of conductors at high frequencies, increasing resistance. Proximity effect further complicates this by causing uneven current distribution due to magnetic fields from adjacent conductors. Together, these effects raise losses and reduce efficiency.
Additionally, conventional transformers tend to develop hot spots in the core when operating at high frequencies. To manage this, the magnetic flux density must be reduced, which increases the size of the transformer. This limits its use in high-power-density applications. For example, in a low-output-voltage converter, the turns ratio can be as high as 32:1, requiring multiple layers of windings. This leads to increased leakage inductance, inter-winding capacitance, and severe skin and proximity effects.
In contrast, flat-panel transformers offer significant improvements. They typically use a single-turn or few-turn primary winding with multiple cores, each paired with a single-turn secondary winding. This configuration results in tighter coupling, significantly reducing leakage inductance. For instance, a 30A flat-panel transformer has a leakage inductance of just 2.0nH, making it ideal for fast-switching circuits with minimal losses and reduced stress on other components.
Flat-panel transformers also exhibit superior frequency characteristics, capable of operating efficiently between 100kHz and 500kHz. Their design allows them to be directly mounted on the bottom plate, enhancing heat dissipation. With a larger surface area, they avoid the problem of localized hot spots, ensuring better thermal management and reliability. Overall, flat-panel transformers represent a significant leap forward in power electronics, offering improved performance, compactness, and efficiency compared to conventional designs.
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