Synchronous rectifiers swap out Schottkys

September 24, 2014 // By Graham Prophet
iW671 is a synchronous rectifier designed to enable small, higher power adapters for mobile devices: it replaces the secondary-side Schottky diode used in conventional power supplies with a more efficient MOSFET and works with Dialog’s iW1786 or iW1787 primary-side controllers to deliver efficiency greater than 88%. This enables the higher power density needed for more powerful, small form-factor adapters without exceeding thermal limits.

When replacing the Schottky diode with a synchronous rectifier, the voltage dropped across the switching MOSFET is less than the forward voltage of the Schottky diode, thereby reducing power loss. However, conventional synchronous rectifiers still require a Schottky diode to be used in parallel as they need to turn off when approaching the zero crossing to eliminate the risk of shoot-through, which can overheat and potentially destroy the power supply. The iW671 uses Dialog's digital control technology to eliminate the need for the parallel Schottky diode. Synchronous rectification also leads to an efficiency improvement of approximately 4% at 5V, resulting in the high, greater than 88%, full-load efficiency for higher power operation in a small form-factor without overheating the power supply.

The iW671 is designed to work for 15W or 50W output power, ultra-low no-load standby power consumption of under 15 mW (iW1786) and under 30 mW (iW1787) and high efficiency, enabling chargers and adapters to meet stringent global energy standards including the final 2014 U.S. DOE(1) and European CoC version 5, tier 2(2) regulations.

Dynamic load response (DLR) is determined by the speed at which the system wakes up and responds to changes in the power load. Typical adapter and charger design approaches achieve low standby power and high efficiency at the expense of slow wake up (i.e. slow DLR performance). The iW671 enables ultra-fast DLR for quicker recovery from low-power standby mode by integrating an adaptive voltage position monitor that detects the flyback converter output voltage undershoot.

Dialog: iW671 Product Summary , iW1786 Product Summary , iW1787 Product Summary