Synchronous rectifier controller for LLC power - video

May 17, 2017 //By Nick Flaherty
The FAN6248 is an advanced synchronous rectifier (SR) controller optimised for LLC resonant converter topologies.
ON Semiconductor has developed an advanced synchronous rectifier (SR) controller optimised for LLC resonant converter topologies.

The controller requires minimal additional components, delivers high efficiency, eases thermal management, improves overall system reliability and simplifies the design of LLC power supplies. It uses a mixed SR control methodology combining instantaneous drain voltage detection with previous switching cycle information. The separate 100 V 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.

This approach prevents current inversion and avoids SR mis-triggering due to capacitive current spikes, giving highly 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.

With two external resistors and MOSFETs the controller can be used for a simple 800W LLC converter. 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 device 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.

Inbuilt adaptive parasitic inductance compensation also minimizes 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; power supply designers are able to select the most appropriate for their application based upon 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. Furthermore, the 10.5 V high gate drive output from the FAN6248 ensures the ability to drive these very wide range of MOSFETs.

"The FAN6248 comprehensively addresses the needs of modern LLC PSU designers. Innovations