On Semi NCP1251 20W低成本离线开关电源解决方案
This Design Note describes a very simple, low cost, yet high performance off-line flyback power supply using ON Semiconductor’s NCP1251B controller (TSOP6 package), NDD04N60 D-Pak Mosfet, and the NCP431 programmable zener (SOT23 package).
The flyback design operates in discontinuous conduction mode and uses the conventional optocoupler (U2) feedback scheme for the voltage loop and an auxiliary Vcc winding on the flyback transformer to power the NCP1251. The Design Note provides the complete circuit and transformer design details for 5 volt, 4 amp, and 12 volt, 1.8 amp output models. Other output voltages from 3.3 up to 28 Vdc are easy to implement by modifying the values (or ratings) of R11, R12, D9, C9 and T1’s secondary turns.
Over-current limiting is provided by sensing the peak current in the MOSFET Q1 via R8. Once the 800 mV threshold level on U1’s pin 4 is exceeded the circuit will go a “hiccup” mode until the over-current condition is removed. An optional OVP circuit is implemented via pending on the application, it may be necessary to add a small pi-network ripple filter to the output as shown in the lower section of the schematic below.
#; Input EMI filter for conducted EMI compliance
#; Schottky output rectifier for high efficiency
#; Very low standby (no load) power
#; Current mode control with adjustable output current
#; Small pc board footprint
#; Low cost components