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A High-Efficiency Synchronous Rectifier Flyback for High-Density AC/DC Adapter

A High-Efficiency Synchronous Rectifier Flyback for High-Density AC/DC Adapter

With the fast development of the tablet PC, the increasing load demand requires the adapter to have high power density, high efficiency, and low profile. This application note presents a high efficiency Flyback converter with output synchronous rectification, which can achieve high efficiency within a wide load range to meet the high density requirements. By using Texas Instrument’s UCC28610 green mode controller and the UCC24610 Synchronous Rectifier (SR) controller, a 17W SR Flyback converter reference design, PMP4305, with 5.6V output voltage demonstration board is designed with experimental verification.

In the recent consumer market trend, the tablet Personal Computer (tablet PC) is a hot topic because of its ease of use and wireless network connectivity to support multiple functions for end users. Because of these additional functions, such as wireless for Internet and local network connection or other external accessories, and desired fast Li-battery charging, the tablet PC demands much higher power than before. So the key design challenge for the power supply adaptor of a tablet PC is to achieve higher power rating with the same small form factor, which means the trends for tablet PC power supply adapter are higher density and higher efficiency.

In the small wattage power supplies, the Flyback converter is widely used because of its simplicity and low cost. However, due to the high peak and RMS currents, the MOSFET and output rectifier diode in the Flyback have high conduction losses, which results in its relatively low efficiency. Through power loss analysis on Flyback converters, there are two key power loss factors: the first one is main MOSFET switching loss on the primary side during switch turn- on when the MOSFET has a high drain to source voltage, Vds. And the second is the conduction loss of the secondary side output diode. In order to reduce both switching losses on the primary side and conduction losses on the secondary side, there are some methods of improvement, such as implementing valley switching on the primary side with frequency variation to optimize wide load range efficiency, and using a synchronous rectifier on the secondary side. This application note presents a high efficiency Flyback with variable frequency switching and secondary side synchronous rectification to improve the overall efficiency for high density adapters. Texas Instruments’ UCC28610 is used on the primary side and is a green mode Flyback controller with variable switching frequency in which every switching cycle immediately follows at least one zero crossing detected by the ZCD pin. This method of switching results in ensuring that the converter is always in discontinuous current mode and that switching actually takes place at or near the lowest Vds voltage as it will occur on the downslope of the resonant ring. The UCC24610 is a synchronous controller which ensures the output SR- MOSFET operates as a near-anideal diode to reduce the conduction loss without any current reversing issue.