HV-Bus-Assisted Flyback Converter With Only One Additional Active Switch for Leakage- Energy Recycling and Full-Range Soft Switching

In the modern world of electricity, power often travels along a high-voltage highway to reach its destination, while the devices that actually use it—computers, sensors, and controllers—require a much gentler, low-voltage current. To bridge this gap, engineers use specialized circuits that step down the voltage and, crucially, isolate the low-voltage side from the high-voltage side to prevent dangerous shocks. One of the most common tools for this job is a device called a flyback converter. It is a workhorse of the industry, prized for its simplicity and low cost, but it suffers from a persistent flaw. Inside its magnetic heart, a tiny amount of energy gets trapped in a gap called leakage inductance. When the main switch turns off, this trapped energy has nowhere to go but to slam into the switch, creating a violent voltage spike. To protect the circuit, engineers usually add a safety valve that simply burns this excess energy away as heat, wasting power and making the system less efficient.

Researchers at the National Kaohsiung University of Science and Technology in Taiwan have proposed a new way to handle this wasted energy without adding complex parts. They designed a modified version of the flyback converter that taps into the existing high-voltage power line already present in the system to recycle that trapped energy. Instead of letting the energy dissipate as heat, their circuit guides it back to the high-voltage bus, where it can be used again. This approach not only recovers the wasted power but also smooths out the switching process, allowing the main electronic switch to turn on and off with almost no electrical friction. The team built and tested three working models of this new converter, proving that it can operate independently, recycle energy, and run efficiently across a wide range of power levels.

The core of this innovation lies in how the researchers utilized the environment around the converter. In many modern systems, such as those found in electric vehicles or data centers, a high-voltage direct current bus runs alongside a battery. The new converter, which the authors call an HV-bus-assisted flyback converter, connects directly to the battery but uses a single extra component to talk to that high-voltage bus. In its simplest form, this extra component is just a diode, a one-way valve for electricity. When the main switch turns off, this diode opens a path for the trapped leakage energy to flow back up to the high-voltage line instead of burning up. This action clamps the voltage spike, protecting the switch and recovering energy that would otherwise be lost.

However, the researchers realized they could do even better by replacing that simple diode with an actively controlled switch. By turning this switch on and off at precise moments, they could force a small amount of current to flow in the opposite direction through the magnetic component just before the main switch turns on. This reverse current acts like a gentle hand pushing down on a spring, discharging the electrical charge that builds up on the switch. This allows the main switch to turn on when the voltage across it is already zero, a state known as zero-voltage switching. When a switch turns on with zero voltage, it generates almost no heat and creates very little electrical noise, which is a significant improvement over traditional methods where the switch fights against high voltage.

To prove their concept, the team constructed three physical prototypes, each rated at 200 watts, with output voltages of 12, 24, and 48 volts. They connected these modules to a common high-voltage bus set at 400 volts and a battery that varied between 120 and 150 volts. The setup included a separate bidirectional converter to manage power flow between the battery and the high-voltage bus, ensuring that the new flyback modules could operate independently without interfering with the rest of the system. The experiments showed that the modules worked exactly as predicted. When the main switch turned on, the voltage across it was indeed zero, confirming the soft-switching capability. The leakage energy was successfully redirected to the high-voltage bus, and the output voltages remained steady even when the load changed rapidly or the battery voltage fluctuated.

The performance results were measured with precision. When the auxiliary switch was used only as a diode, the converters achieved full-load efficiencies of 87.2 percent for the 12-volt model, 92.6 percent for the 24-volt model, and 95.3 percent for the 48-volt model. When the researchers activated the full control of the auxiliary switch to enable zero-voltage switching, the efficiency improved further. The 12-volt module reached 89.6 percent, the 24-volt module hit 94.2 percent, and the 48-volt module achieved 96.1 percent. These numbers indicate that while the active control adds a small amount of complexity, the reduction in switching losses makes the system more efficient, especially at higher power levels. The researchers noted that the output diode remained the largest source of energy loss in the system, particularly in the lower voltage models where the current is higher, but the overall gains were substantial.

One of the most significant findings was the system's ability to operate independently. The three different voltage modules ran side-by-side on the same power bus, and changing the load on one did not disturb the others. This suggests that the design is highly modular, meaning engineers could easily add more units to a system without needing to redesign the entire circuit or worry about complex interactions between components. The control system, managed by a single microcontroller, successfully regulated the output voltage across all conditions, including when the battery voltage was ramped up from 120 to 150 volts. The transient response, or how quickly the system reacted to sudden changes in power demand, was fast, settling in just a few milliseconds.

The study also compared their design to other existing solutions that attempt to recycle leakage energy or achieve soft switching. Many of those alternatives require multiple extra switches, capacitors, or inductors, which increases the cost, size, and complexity of the circuit. In contrast, the new design achieves its goals with only one additional active switch and one diode, keeping the component count low. While some other converters have shown slightly higher efficiency in specific tests, often by using more complex synchronous rectification techniques that require sophisticated control, this new approach offers a balanced solution. It maintains the simple, low-cost structure that makes flyback converters popular while adding the performance benefits of energy recycling and soft switching.

The researchers concluded that this HV-bus-assisted approach provides a practical, low-cost solution for isolated power conversion in systems where a high-voltage bus and a battery coexist. By recycling leakage energy and enabling soft switching, the design reduces heat generation and electrical noise, which are critical factors for the reliability of industrial power supplies, factory distribution systems, and battery storage setups. The successful testing of the three modules demonstrates that the theory holds up in the real world, offering a path toward more efficient and robust power electronics without the burden of excessive complexity. The work stands as a clear example of how a small, targeted modification to a classic circuit can yield significant improvements in performance and efficiency.