Advanced SR controller for LLC power supply designs

An advanced synchronous rectifier (SR) controller from ON Semiconductor has been optimised for LLC resonant converter topologies.

The FAN6248 can be used for modern, high performance, power supply units where high levels of reliability and efficiency are required in a small space. Typical applications include server and desktop computing, gaming, large screen LCD TV and OLED TV, networking, telecom and LED lighting applications.

The device incorporates an advanced, mixed, SR control methodology combining instantaneous drain voltage detection with previous switching cycle information. The separate 100V rated sense inputs sense the drain and source voltages across the two SR MOSFETs, allowing for any asymmetries or poor coupling in the secondary winding.

With a 10.5V high gate drive output, the FAN6248 prevents current inversion and avoids SR mis-triggering due to capacitive current spikes. The anti-shoot-through control within the controller is said to enhance the reliability of the PSU and prevents the simultaneous turn on of the two SR MOSFETs.

Above resonance operation is achieved without the introduction of voltage spikes. At no load, the FAN6248 enters green mode and disables itself. During light load conditions, the controller pulses within green mode giving power delivery as required by efficiency standards such as 80 PLUS, DoE VI, and CoC Tier 2.

Inbuilt adaptive parasitic inductance compensation also minimises body diode conduction in the SR MOSFETs due to its package’s stray inductance.

There are four devices within the series; the range is suitable for use with MOSFETs in TO-220, D2PAK, DPAK or PQFN package styles.

Design support is available from ON Semiconductor, including a 240W evaluation board incorporating a complete LLC design.

Peggy Lee