Onsemi NCP1251B | Demoboard DN05012/D

Description

This Design Note describes a very simple, lowcost, yet high performance off-line flyback powersupply using ON Semiconductor’s NCP1251Bcontroller (TSOP6 package), NDD04N60 D-PakMosfet, and the NCP431 programmable zener(SOT23 package).The flyback design operates in discontinuousconduction mode and uses the conventionaloptocoupler (U2) feedback scheme for thevoltage loop and an auxiliary Vcc winding on theflyback transformer to power the NCP1251. TheDesign Note provides the complete circuit andtransformer design details for 5 volt, 4 amp, and12 volt, 1.8 amp output models. Other outputvoltages from 3.3 up to 28 Vdc are easy toimplement by modifying the values (or ratings) ofR11, R12, D9, C9 and T1’s secondary turns.Over-current limiting is provided by sensing thepeak current in the MOSFET Q1 via R8. Oncethe 800 mV threshold level on U1’s pin 4 isexceeded the circuit will go a “hiccup” mode untilthe over-current condition is removed. Anoptional OVP circuit is implemented via Z1.Depending on the application, it may benecessary to add a small pi-network ripple filterto the output as shown in the lower section of theschematic below.

Features

 Input EMI filter for conducted EMI compliance Schottky output rectifier for high efficiency Very low standby (no load) power Current mode control with adjustable outputcurrent Small pc board footprint Low cost components

More information

• Datasheet