Onsemi NCP1251B | Demoboard DN05033/D

Description

This Design Note describes a very simple, low cost, primary side controlled, off-line flyback power supply using ON Semiconductor’s 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’s 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. The load regulation over the usable load range and the nominal output voltage can be further optimized by adjusting the values of R5 and R11, respectively.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 output OVP clamp is implemented via Z1 to limit the maximum no load output voltage.

Features

Primary side voltage sensing – no optocoupler Input EMI filter for conducted EMI complianceSchottky output rectifier for high efficiencyCurrent mode control with adjustable output current limit setpoint

Typical applications

• White Goods, Small Instruments, E-Meters, Industrial Equipment

More information

• Datasheet