ηSB: estimated internal preload power efficiency of the flyback converter when its output power is zero. This efficiency is calculated by the converter internal preload power dissipated through RPL, divided by total input power of the converter in stand-by condition, PSTBY. An estimation 50% can be used at beginning of a design.
η: overall efficiency of the converter at full rated output power
ηXFMR: power transfer efficiency of the transformer