High efficiency σ/Sigma Front-End DC/DC (2007)
To embrace the traits of the LLC resonant converter at the resonant point and achieve the output voltage/current regulations, the σ/Sigma DC/DC architecture is proposed. A tiny regulated converter with only about 2% of total output power is seamlessly mated with the unregulated resonant DC/DC transformer (DCX) which is responsible for about 98% of the total power during both the steady state and transient. The resonant DCX is always operated at the best point during steady state for the highest efficiency with the compelling features of negligible switching loss, simple driving of synchronous rectifier and fast dynamic response. By taking advantage of the ultra-low output impedance of the DCX, the tiny regulated converter is deemed to steer the big unregulated DCX toward the output voltage regulation, soft starting and output current limiting, and current sharing for N 1 redundancy. Thus, the overall efficiency is determined by the DCX while the output regulations are executed by the tiny regulated converter.
Furthermore, at light load condition, tiny converter operates more efficiently than big DCX. Therefore, the major output power will be shifted to the tiny converter by lowing down the switching frequency of the DCX to drastically reduce its quiescent operating power consumption, so that the high light load efficiency can be easily achieved by the tiny converter.