Integrated ACF controller incorporates four functions

Ally Winning


Active Clamp Flyback (ACF) Controller for Power Adapters to 100W Provides Industry’s Highest Integration and Operational Efficiency

There is a new trend in the industry where manufacturers no longer include power adapters with their products. As with many trends, this one started, or at least was made popular by Apple who stopped shipping adapters with their iPhones. That lead has been followed by Samsung and other manufacturers. As the majority of consumer goods feature USB-C connectors, which are freely available, it’s no surprise that those manufactures have decided to cut costs.


The result of the trend is that consumers will be forced to buy their own adapters. As batteries in mobile devices are getting larger and consumers demand faster charging, higher levels of power are required from those adapters. It also makes sense for those consumers to choose an adapter that can charge two or more devices at the same time for additional convenience. Developing a power supply with enough density to charge two high-power devices from a single adapter in a form factor that consumers know and like is a real challenge for designers. With that type of power density, unless the design has very high efficiency it will give off a large amount of heat, making the adapter dangerous to touch and shortening the life of the components inside.


Silanna Semiconductor has attempted to meet this challenge with its new active clamp flyback (ACF) controller for power adapters to 100W. The SZ1131 ACF controller is a successor to the company’s SZ1130 which was launched last year. The new ACF controller incorporates four different functions into its package. These are the adaptive digital PWM controller, an active clamp FET, active clamp gate driver and a startup regulator. The high level of integration makes design easier and quicker with the end product requiring less space and external components. An active clamp topology was chosen to reduce the peak voltage, allowing a more aggressive transformer turns ratio and a lower SR FET voltage rating. Th active clamp technology also switches near the ZVS of the main switch, meaning that it is highly efficient due to lower switching losses and has lower EMI thanks to the smaller switching voltage.


This design allows the SZ1131 to provide efficiencies of up to 95% across universal (90Vac – 265Vac) input voltages and varying loads, combined with an ultra-low no-load power of less than 20mW up to 65W, significantly reducing overall energy consumption. The SZ1131 combines the ease-of-design of a simple flyback controller with all the benefits of an ACF design, including recycling the leakage inductance energy of the flyback transformer and limiting the primary FET drain voltage spike during the turn-off events. Employing Silanna’s OptiMode digital control architecture, the SZ1131 adjusts the device’s mode of operation on a cycle-by-cycle basis to maintain high efficiency, low EMI, fast dynamic load regulation, and other key power supply parameters in response to varying line voltage and load.

The device is supplied in a 16-pin SOIC package and compatible with both silicon FETs to reduce costs and wide bandgap FETs to increase performance. The device offers protection against over-temperature, over-voltage, over-current, over-power and short circuit conditions as well as transformer core saturation faults without the need for additional, external components.