Synchronous rectifier controller optimises LLC-based PSU designs
ct AC-DC power supplies
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 accurately sense the drain and source voltages across the two SR MOSFETs, allowing for any asymmetries or poor coupling in the secondary winding. With this advanced control approach, the FAN6248 prevents current inversion and avoids SR mis-triggering due to capacitive current spikes, giving reliable operation. The anti-shoot-through control within the controller enhances the reliability of the PSU and prevents potentially destructive simultaneous turn on of the two SR MOSFETs.
The FAN6248 and two external resistors and MOSFETs delivers simple LLC converters supporting up to 800W. In particularly noisy environments, adding two capacitors further enhances system stability. The FAN6248 operates reliably and efficiently across a wide range of power levels. 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 very efficient power delivery as required by efficiency standards such as 80 PLUS, DoE VI, and CoC Tier 2.
Adaptive parasitic inductance compensation also minimises body diode conduction in the SR MOSFETs due to its package’s stray inductance, thereby increasing PSU efficiency. There are four devices within the series, selected for switching frequency (either 25 kHz-70 kHz or 60 kHz-700 kHz) and the level of stray inductance associated with the MOSFET. The range is suitable for use with MOSFETs in TO-220, D2PAK, DPAK or PQFN package styles. 10.5V high gate drive output drives a wide range of MOSFETs.
Design support includes a 240 W evaluation board incorporating a complete LLC design. A daughter board is also available to incorporate the FAN6248 into existing designs. In an SOIC-8 package the part is priced at $0.487 (10,000) unit quantities.
ON Semiconductor; www.onsemi.com
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