In power supply design, improving energy efficiency is a key objective, and one common approach is the use of synchronous rectification. This technique replaces traditional diode rectifiers with MOSFETs, which significantly reduces voltage drop and conduction losses, enabling higher current capability and greater system efficiency. However, implementing traditional synchronous rectification in LLC resonant converters presents several technical challenges, such as difficulty in setting the minimum on-time due to varying operating frequencies, premature turn-off caused by stray inductance, and current reversal under light load conditions. To address these issues, ON Semiconductor has introduced the FAN6248, an advanced synchronous rectifier controller optimized for high-efficiency power supplies in applications like servers, desktops, large-screen LCD TVs, network equipment, high-power adapters, and LED lighting.
One major challenge in traditional synchronous rectification is the difficulty in setting a suitable minimum on-time. The operating frequency of the resonant circuit changes depending on the load, making it hard to choose a fixed value that works across all conditions. If the minimum on-time is set too high for light loads, the rectification may be delayed, while if it's too low for heavy loads, switching noise can cause abnormal shutdowns. Therefore, a self-adjusting minimum on-time is essential for reliable performance.
Another issue arises from stray inductance, which can create a positive bias on the drain node when the synchronous rectifier turns off. This can lead to premature turn-off, increasing conduction loss. A self-regulating turn-off threshold voltage helps maintain stable operation by adapting to different package configurations and parasitic effects.
Under light load conditions, the resonant capacitor voltage may not be sufficient, causing the exciting current to charge the capacitor and produce a capacitive spike during switching. This can trigger the synchronous rectifier incorrectly, leading to current reversal. The FAN6248 addresses this by adjusting the turn-on delay, preventing false triggering and ensuring safe operation even at low loads.
To tackle these challenges, the FAN6248 offers several key features. It includes reverse breakdown protection, self-regulating dead time control, and adaptive minimum on-time adjustment. The controller also supports high-frequency operation up to 700 kHz and provides a low operating current of 350 µA in energy-saving mode. Its gate drivers are designed to drive all MOSFETs efficiently, minimizing on-resistance and maximizing energy efficiency.
The FAN6248 uses a hybrid control algorithm to manage the synchronous rectification shutdown process. By monitoring the drain voltage and previous cycle information, it maintains a consistent dead time of 200 ns, independent of load or parasitic inductance. This ensures optimal performance and minimizes body diode conduction.
At light loads, the FAN6248 increases the turn-on delay to prevent false triggering caused by capacitive current spikes. When current reversal is detected, the delay can increase from 80 ns at full load to 380 ns at light load, ensuring stable and safe operation.
The FAN6248 is available in two versions—HA and HB—each tailored for different MOSFET packages. The HA version is suitable for larger packages like TO220 or D2PAK, while the HB version is ideal for smaller packages such as PQFN or DPAK.
Testing showed impressive energy efficiency, with the system achieving an average of 96.29% at 25% load. This demonstrates the FAN6248’s effectiveness in delivering high performance across various load conditions.
In summary, the FAN6248 overcomes traditional challenges in synchronous rectification with its intelligent control algorithms, self-adjusting features, and robust design. It enhances system efficiency, reliability, and stability, making it a top choice for high-performance power supply applications. With its compact package, low power consumption, and powerful gate drive capability, the FAN6248 is well-suited for modern, energy-efficient designs.
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