ABSTRACT
LLC current resonant power supply, which is characterized by soft switching, resonance control with a duty ratio of 50% and leakage transformer structure, is suitable for effi ciency improvement, noise reduction and profi le lowering in switching power supply. Fuji Electric has developed the 2nd generation “FA6A00N Series,” which inherits the characteristics of the 1st generation LLC current resonant control IC, “FA5760N,” and is enhanced with lower standby power and improved protective functions. It integrates the world’s fi rst high-precision secondary side over-load protection function while further reducing the standby power by approximately 20%. For the over-current protection function, the delay time can be externally adjusted.

Introduction
Switching power supply products, which are used in various types of electronic equipment, are rapidly being improved in terms of efficiency, noise reduction and low profile to meet the demands for energy efficiency and space saving. An LLC current resonant power supply is characterized by its use of high-efficiency, low-noise soft switching technology and low profile leakage transformer structure. These characteristics facilitate efficiency improvement, noise reduction and profile lowering and make it suitable for use as a power supply of 100 to 500 W, which is a medium capacity range for a switching power supply. The LLC current resonant power supply, however, is prone to a switching shoot-through phenomenon*1 during a start up, heavy load conditions or low input voltage conditions. The power supply has problems including a breakdown of a power metal-oxide-semiconductor field effect transistor (MOSFET) due to this phenomenon and efficiency degradation with a light load due to an excitation current, and these factors limited its scope of application.