This reference document describes a built--and--tested,
GreenPoint? solution for a 40 W (40 W nominal power,
50 W peak power) set--top box (STB) power supply. This
document presents the results of various secondary
rectification and regulation techniques thatwere used to find
the highest practical efficiency scheme for a four--output,
40 watt set top box power supply.
The power supply design is built around
ON Semiconductor’s NCP1308 Current--Mode controller,
on the primary side, using free running quasi--resonance
operation. The secondary side offers four outputs (+5 V,
--5 V, 3.3 V and 12 V). The +5 V output is the main channel
with the closed PWMloop while the 3.3 Voutput is derived
by using the NCP1587 in a buck (step--down) DC--DC
topology with synchronous rectification. The 12Voutput is
derived from+5 V by stacking the 12 V secondary winding onto the 5 V winding.