Adaptive Synchronous Rectification for LLC Resonant Converters

The STEVAL-ISA165V1 from STMicroelectronics is a product evaluation board designed to demonstrate the performance of the SRK2001 synchronous rectification controller. The SRK2001 implements a control scheme specific for secondary-side synchronous rectification in LLC resonant converters that use a transformer with center-tap secondary winding for full-wave rectification. It provides two high-current gate-drive outputs (for driving N-channel power MOSFETs).

Each gate driver is controlled separately and an interlock logic circuit prevents the two synchronous rectifier (SR) MOSFETs from conducting simultaneously. Device operation is based on adaptive algorithms for both turn-on and turn-off of SR MOSFETs. During fast load transitions or during above resonance operation, a further turn-off mechanism is provided, based on a ZCD_OFF comparator that triggers the gate drive circuit for very fast MOSFET turn-off. The board includes two SR MOSFETs (in a TO-220 package) and can be easily implemented in an existing converter as a substitute for rectifier diodes.

The SRK2001 controller on the STEVAL-ISA165V1 implements a control scheme specific for secondary side synchronous rectification in LLC resonant converters that use a transformer with center tap secondary winding for full wave rectification. It provides two high current gate drive outputs, each capable of driving one or more N-channel power MOSFETs. Each gate driver is controlled separately and an interlock logic circuit prevents the two synchronous rectifier MOSFETs from conducting simultaneously.

The control scheme in this IC provides for each synchronous rectifier being switched on as the corresponding half-winding starts conducting and switched off as its current goes to zero. The innovative turn-on logic with adaptive masking time and the adaptive turn-off logic allows maximizing the conduction time of the SR MOSFETs eliminating the need of the parasitic inductance compensation circuit. The low consumption mode of the device allows to meet the most stringent requirement for converter power consumption in light-load/no load conditions.

Key features of the STEVAL-ISA165V1 include: Synchronous rectifier for LLC resonant converter, with adaptive turn-on and turn-off; VCC range of 4.5 V to 32 V; Maximum frequency of 500 kHz; Dual gate drive for N-channel MOSFETs (strd level driver); Synchronous rectification MOSFET type is STP120N4F6 (rated for 40V and 4.3mΩ) in a TO-220 package.