In power supply design, improving energy efficiency is a key objective, and one effective method is the use of synchronous rectification. This technique replaces traditional diode rectifiers with MOSFETs, significantly reducing voltage drop and conduction losses. As a result, it offers higher current capability and improved system efficiency. However, implementing traditional synchronous rectification in LLC resonant converters presents several technical challenges.
First, setting the minimum on-time can be problematic due to varying operating frequencies under different load conditions. If the minimum on-time is set too high for light loads, it may cause delays in the synchronous rectification process, while setting it too low for heavy loads could lead to switching noise and abnormal shutdowns. A self-adjusting minimum on-time feature is essential to address this issue.
Second, stray inductance can cause premature turn-off of the synchronous rectifier, increasing conduction loss. The presence of parasitic inductance can create a positive bias on the drain node when the MOSFET turns off, which may trigger an early shutdown. To counteract this, a self-regulating turn-off threshold voltage is required to maintain optimal performance.
Third, under light load conditions, the resonant capacitor may not charge sufficiently, leading to a capacitive current spike that causes the synchronous rectification current to reverse. This can negatively impact system stability. A self-adjusting delay mechanism helps prevent such issues by ensuring proper timing for the MOSFET turn-on.
To tackle these challenges, ON Semiconductor has introduced the FAN6248, an advanced synchronous rectifier controller optimized for LLC resonant converters. It is ideal for applications such as server and desktop power supplies, large-screen LCD TVs, network and telecom power supplies, high-power density adapters, and high-power LED lighting systems.
The FAN6248 features several key advantages. It includes reverse breakdown protection to ensure reliable operation and a unique self-regulating dead time control that compensates for parasitic inductance. This ensures a constant dead time of 200 ns, independent of load and inductance, minimizing body diode conduction and maximizing efficiency.
At light loads, the FAN6248 detects reverse current and increases the turn-on delay to avoid false triggering and current reversal. Its self-adjusting minimum on-time circuit improves noise immunity, and it supports high-frequency operation up to 700 kHz. Additionally, it provides two gate drivers for synchronous rectification MOSFETs and operates with a low standby current of 350 µA.
The FAN6248 is highly integrated, requiring minimal external components. A typical application circuit involves an LLC controller on the primary side and a simple configuration on the secondary side, including the FAN6248 controller and two external resistors. In noisy environments, two additional capacitors may be needed.
The controller uses a hybrid control algorithm for its synchronous rectification shutdown. It maintains a consistent dead time of 200 ns by adjusting the virtual turn-off threshold based on the previous cycle's information. This approach optimizes energy efficiency and ensures stable operation.
For different application needs, the FAN6248 is available in two versions: HA and HB. The HA version is suitable for larger packages like TO220 or D2PAK, with VTHOFF set at 130 mV or 228 mV. The HB version is designed for smaller packages like PQFN or DPAK, with VTHOFF set at 100 mV or 175 mV.
Testing results show impressive energy efficiency. At full load, 75%, 50%, and 25% loads, the system achieved an average efficiency of 96.29%. This highlights the FAN6248’s effectiveness in enhancing power supply performance.
In summary, the FAN6248 addresses traditional synchronous rectification challenges through innovative features like self-regulating dead time control, reverse breakdown protection, and self-adjusting minimum on-time. These improvements ensure reliable, efficient, and stable operation across a wide range of applications. With a small package size, powerful gate drive capability, and low standby current, the FAN6248 is a versatile solution for modern power supply designs.
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