Ultra-High-Power Density of LLC Transformer Based on Unbalanced-flux Magnetics Approach

Inside the power supplies that charge our devices and run our data centers, a quiet battle has been fought for decades over space. The goal is simple: to squeeze more electrical power into a smaller box. For years, engineers believed the only way to shrink the heavy, bulky components inside these converters was to make them switch electricity on and off incredibly fast, moving into the megahertz range. This high-speed switching allows the magnetic cores—the iron or ceramic hearts of the transformer—to be much smaller. However, this approach has hit a wall. When the switching gets too fast, the magnetic material gets too hot and loses its ability to carry energy efficiently. To keep the device from burning out, engineers have been forced to dial back the magnetic intensity, which ironically cancels out the size savings they were hoping for. The result is a system that is efficient but stubbornly large, with power density, or the amount of power packed into a cubic centimeter, stuck at a level that has barely moved in years.

A researcher at Mid Sweden University has now demonstrated a different path forward, one that ignores the obsession with speed and instead focuses on how the magnetic material itself is shaped. They have built a prototype transformer for a 1-kilowatt power converter that achieves a power density of 230 watts per cubic centimeter. This is more than three times the density found in standard high-frequency designs. They did not achieve this by making the electricity switch faster; they did it by changing the geometry of the magnetic core to allow it to carry a much stronger magnetic field without overheating. The result is a device that is significantly smaller and lighter, yet maintains high efficiency, proving that the key to shrinking power electronics lies not in how fast we switch, but in how we arrange the magnetic material.

The researcher, led by Sobhi Barg, is working with a concept called unbalanced-flux magnetics. To understand the breakthrough, one must first understand the traditional problem. In a standard transformer, the magnetic core is a uniform loop. When electricity flows through the wires wrapped around it, a magnetic field is created. If the field is too strong, the core gets hot and wastes energy. If the field is too weak, the transformer becomes too large to carry the necessary power. For a long time, the solution was to increase the switching speed to the megahertz range, which allowed the use of smaller cores. But as the speed increased, the heat generated by the magnetic material also increased, forcing engineers to weaken the magnetic field to prevent melting. This created a frustrating trade-off where higher speed meant lower capacity.

The new approach rejects the idea that the magnetic field must be uniform throughout the core. Instead, the researcher splits the core into two distinct zones with different jobs. One zone, the active region, is designed to carry the maximum possible magnetic field to transfer energy efficiently. The other zone, the passive region, is designed to handle the heat and losses that inevitably occur. By separating these functions, the team could push the active part of the core to its absolute limit without worrying that the entire device would overheat. It is a bit like a highway where the fast lanes are kept clear for traffic, while the shoulders are widened to absorb any accidents or slowdowns, keeping the main flow moving smoothly. In this design, the "shoulder" is a larger, optimized section of the magnetic core that dissipates heat effectively, allowing the "fast lane" to carry a much heavier load than before.

To test this theory, the researcher constructed a physical prototype of a 1-kilowatt converter, a device capable of delivering enough power to run a small appliance or charge an electric vehicle battery. They used a specific type of circuit known as an LLC converter, which is common in high-efficiency power supplies, and equipped it with a transformer built from two integrated magnetic cores. A critical part of their design was the ratio between the outer section of the core and the central section. In their first attempt at this technology, the ratio was not quite right, limiting the performance. For this second experiment, they adjusted the geometry so that the outer section was about 1.23 times larger than the central section. This specific shape, they found, allowed the heat to escape much more effectively, enabling the transformer to handle the intense magnetic fields required for high power density.

The results were immediate and measurable. The transformer they built occupied a volume of just 4.78 cubic centimeters. At its peak output of 1.1 kilowatts, this tiny component achieved a power density of 230 watts per cubic centimeter. To put this in perspective, typical high-frequency transformers in similar applications usually manage between 55 and 70 watts per cubic centimeter. The new design is roughly three times more dense. Furthermore, the device did not sacrifice efficiency for size. At half power, the converter reached an efficiency of 96.07 percent, meaning very little energy was lost as heat. Even at its full rated power of 1 kilowatt, it maintained an efficiency of 95.3 percent. These numbers are significant because they show that the new design does not just shrink the component; it actually improves how well it works.

The researcher also paid close attention to how the heat behaved. In high-power electronics, heat is the enemy. If a component gets too hot, it fails. The team used thermal cameras to watch the temperature of their transformer as they increased the power from a low 100 watts up to the maximum 1.1 kilowatts. They found that the temperature rose in a predictable, linear fashion, which is a sign of a stable and well-designed system. At the rated power of 1 kilowatt, the transformer reached a temperature of about 83 degrees Celsius. This is a safe operating temperature that can be managed with a simple fan. When they compared this to their previous prototype and other designs in the field, the improvement was clear. The new geometry allowed the heat to dissipate so effectively that the transformer could run cooler and handle more power than any comparable design they had seen.

Another key finding was the behavior of the electrical current flowing through the system. In many high-frequency designs, the current spikes wildly, putting stress on the components and causing inefficiency. The researcher measured the current in their new transformer and found that the peaks were much lower and smoother than in other high-power designs. This smoother flow means less stress on the switches and the magnetic core, which contributes to the high efficiency and reliability of the device. The team also noted that their design worked well at a switching frequency of 350 kilohertz, which is fast but not in the extreme megahertz range that often causes electromagnetic interference. This suggests that the new approach could be used in a wider variety of applications, including those that do not rely on the delicate soft-switching techniques required by megahertz designs.

The implications of this work extend beyond just making a smaller box. The researcher demonstrated that the unbalanced-flux concept is not limited to one specific type of circuit. While their prototype was an LLC converter, the underlying principle of splitting the magnetic core into active and passive zones could be applied to other types of power converters, including those that switch hard rather than soft. This opens the door to shrinking power supplies in a much broader range of technologies, from industrial machinery to consumer electronics. The team emphasized that their success did not come from using exotic new materials or complex manufacturing processes, but from a fundamental rethink of how the magnetic core is shaped. They used standard magnetic materials and standard manufacturing techniques, proving that the breakthrough was in the design philosophy rather than the hardware itself.

Looking ahead, the researcher sees a clear path for further improvement. They plan to investigate whether they can push the power density even higher by increasing the switching frequency while maintaining the high magnetic flux density that makes this design work. They also intend to apply the unbalanced-flux concept to duty-cycle-controlled converters, which are common in many power systems. The goal is to continue shrinking the size of power electronics while keeping them cool and efficient. For now, the prototype stands as a proof of concept: a transformer that is three times denser than the current standard, running at high efficiency, and proving that the limits of power density are not set by the speed of the switch, but by the shape of the magnetic core. The work suggests that the future of compact power electronics may not be about going faster, but about thinking differently about the space inside the box.