DN05033/D - 12V, Off-line Power Supply With Primary Side Feedback Design Note

DN05033/D, Design Note describes a very simple, low cost, primary side controlled, off-line flyback power supply using NCP1251B current mode controller (TSOP6 package) and an NDD04N60 D-Pak Mosfet. The design dispenses with the conventional TL431/optocoupler feedback scheme and uses primary side voltage sensing on the NCP1251 Vcc rail which is derived from an aux winding on the transformer. This scheme provides tight line regulation and a load regulation of +/- 5% which is more than adequate for most applications. The acceptable load regulation is achieved by tight coupling (bifilar wound) between the main 12V secondary and the 12V aux winding in the transformer. The voltage sensing circuit is comprised of Z2 and Q2. The zener voltage of Z2 plus the VBE drop of Q2 sets the nominal output voltage