DN05053/D, NCP1129, 12 Watt, Off-line Buck Regulator Design Note

DN05053/D, Design Note describes a simple, low power (15 W or less), constant voltage buck power supply intended for powering electronics for white goods, electrical meters, and industrial equipment where isolation from the AC mains is not required. The efficiency limitations of the offline buck converter are also discussed with emphasis on switching losses caused by the freewheel diode recovery characteristics. The output voltage can be set from 12 to approximately 28 volts (or higher) by using the appropriate zener diode for Z1 and selection of trim resistor R4. The NCP1129 Controller/Mosfet in a DIP 8 package is utilized as the buck switching element. Output regulation is accomplished by utilizing a simple zener/optocoupler sensing scheme. The optocoupler is necessary because the control logic is at a switching node common to the freewheel diode (D3) cathode