NCP1207+电流型同步整流实现高效电源

AND8127/D

Implementing NCP1207 inQR 24 W AC DC Converterwith Synchronous Rectifier

Prepared by: Petr LidakON Semiconductor

www.51wendang.com

APPLICATION NOTE

Introduction

The NCP1207 is a controller dedicated for driving thecurrent mode free running quasi resonant Flyback offlineconverter.

This converter is designed for consumer products likenotebooks, offline battery chargers, consumer electronics(DVD players, set top boxes, TVs), etc.

The growing interest for EMI pollution reduction,efficiency improvement, and maximum safety has beentaken into account while designing the NCP1207.

By implementing the NCP1207 one can build a powersupply that can meet all those requirements. This can beachieved with help of the following NCP1207 mainfeatures:

Current Mode Control: Cycle by cycle primary

current observation is helping to prevent any significantprimary overcurrent which would cause transformer’score saturation and consequent serious power supplyfailure.

Critical Mode Quasi resonant Operation: Prevents theconverter operation in Continuous Conduction Mode inany input and output condition. It is provided by thezero crossing detection of the auxiliary winding’svoltage.

By addition of the reasonable delay the switch turn oninstant can be shifted to the minimum (valley) of drainvoltage. This improves EMI noise and efficiency. Dynamic Self Supply: Ensures IC proper operation inapplications where the output voltage varies duringoperation like battery chargers. The DSS also suppliesthe IC when the overvoltage event is being latched andconverter operation is stopped.

Overvoltage Protection: By sampling the plateau

voltage on the auxiliary winding, the NCP1207 entersinto latched fault condition whenever the overvoltage isdetected. The controller stays fully latched until theVCC decreases below 4.0 V, e.g. when the user unplugsthe power supply from the mains outlet and re plugs it.The OVP threshold can be adjusted externally.

Over Load Protection: by continuously monitoring the

feedback loop activity, NCP1207 enters hiccup

operation as soon as the power supply is overloaded. Assoon as overload condition disappears, the NCPresumes operation.

The 24 W AC DC Adaptor Board Specification

The adaptor has following maximum and performanceratings.

The schematic diagram of the adaptor can be seen inFigure 1.

Transformer Design

The bulk capacitor voltage than can be calculated:

Vbulk min+VAC min2+180@+255VDC

(eq. 1)

Vbulk max+VAC max2+240@+

339VDC

(eq. 2)

The requested output power is 24 W.

Assuming 87% efficiency the input power is equal to:

P24

PIN+OUT++27.6W

(eq. 3)

The average value of input current at minimum inputvoltage is:

IIN AVG+

V

27.6P++108mA255bulk min

(eq. 4)