摘要：  本文介绍了NCP1251主要特性，方框图，典型应用电路以及20-25W低成本离线电源主要特性，指标和设计电路图与相应元器件数值.

关键字：  PWM控制器，  电源电压，  固定频率

On Semi公司的NCP1251是高度集成的PWM控制器，电源电压高达28V，固定频率65kHz或100kHz电流模式控制，内部有可调的超功率保护(OPP)电路，内部有4ms的软起动，+300 mA /−500 mA 源/沉驱动能力，220VAC时的待机功耗小于100mW，EPS 2.0 兼容，主要用在TV，STB和打印机的AC/DC转换器，笔记本和上网本的离线适配器.本文介绍了NCP1251主要特性，方框图，典型应用电路以及20-25W低成本离线电源主要特性，指标和设计电路图与相应元器件数值.

The NCP1251 is a highly integrated PWM controller capable of delivering a rugged and high performance offline power supply in a tiny TSOP−6 package. With a supply range up to 28 V， the controller hosts a jittered 65 kHz or 100 kHz switching circuitry operated in peak current mode control. When the power on the secondary side starts to decrease， the controller automatically folds back its switching frequency down to a minimum level of 26 kHz. As the power further goes down， the part enters skip cycle while limiting the peak current.

Over Power Protection (OPP) is a difficult exercise especially when no−load standby requirements drive the converter specifications. The ON proprietary integrated OPP lets you harness the maximum delivered power without affecting your standby performance simply via two external resistors. A latched Over Voltage Protection (OVP) is combined on the same pin. For ease of implementation， a latched OVP also monitors the VCC line. They offer an efficient protection in case of optocoupler destruction or adverse open loop operation.

Finally， a timer−based short−circuit protection offers the best protection scheme， letting you precisely select the protection trip point irrespective of a loose coupling between the auxiliary and the power windings.

NCP1251主要特性:

Fixed−Frequency 65 or 100 kHz Current−Mode Control Operation

Internal and Adjustable Over Power Protection (OPP) Circuit

Frequency Foldback Down to 26 kHz and Skip−Cycle in Light Load Conditions

Internal Ramp Compensation

Internal Fixed 4 ms Soft−Start

100 ms Timer−Based Auto−Recovery Short−Circuit Protection

Frequency Jittering in Normal and Frequency Foldback Modes

Option for Auto−Recovery or Latched Short−Circuit Protection

OVP Input for Improved Robustness

Up to 28 V VCC Operation

Latched OVP Protection on VCC

+300 mA / −500 mA Source/Sink Drive Capability

Less than 100 mW Standby Power at High Line

EPS 2.0 Compliant

These are Pb−Free Devices

NCP1251典型应用:

ac−dc Converters for TVs， Set−top Boxes and Printers

Offline Adapters for Notebooks and Netbooks

图1.NCP1251方框图

图2.NCP1251典型应用电路图

20-25W低成本离线电源

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 Z1.Depending 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.

20-25W低成本离线电源主要特性:

􀁹 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

20-25W低成本离线电源主要指标：

图3.20-25W低成本离线电源电路图