原标题：On Semi NCP1236 65W笔记本电脑离线适配器设计方案

　　On Semi公司的NCP1236是新型固定频率电流模式控制器,具有动`态自给电源(DSS),从而简化了辅助电源的设计,可调过功率补偿,VCC工作高达 28V并具有压检测.驱动能力500mA,软起动时间4.0ms,内部具有热关断.主要用在笔记本电脑,LCD和打印机的AC/DC适配器,离线电池充电器,消费类电子电源等.本文介绍了NCP1236主要特性,方框图, 采用NCP1236的反激转换器以及65W笔记本电脑离线适配器参考设计主要特性,电路图,材料清单和PCB元件布局图.

　　The NCP1236 is a new fixed−frequency current−mode controller featuring Dynamic Self−Supply (DSS). This device is pin−to−pin compatible with the previous NCP12xx families.

　　The DSS function greatly simplifies the design of the auxiliary supply and the VCC capacitor by activating the internal startup current source to supply the controller during transients.

　　Due to frequency foldback, the controller exhibits excellent efficiency in light load condition while still achieving very low standby power consumption. Internal frequency jittering, ramp compensation, and a versatile latch input make this controller an excellent candidate for converters where components cost is the key constraints.

　　In addition, the controller includes a new high voltage circuitry that combines a start−up current source and a brown−out detector able to sense the input voltage either from the rectified ac line or the dc filtered bulk voltage. The high voltage sensing circuitry is used for the overpower protection purposes as well. Overpower protection, overload protection, and next protective features increases safety level of the final application.

　　Finally, due to a careful design, the precision of critical parameters is well controlled over the entire temperature range (−40 C to +125 C).

　　NCP1236主要特性:

　　Fixed−Frequency Current−Mode Operation with Built−In Ramp Compensation

　　65 kHz Oscillator Frequency (100 kHz version available upon request)

　　Frequency Foldback then Skip Mode for Maximized Performance in Light Load and Standby Conditions

　　Timer−Based Overload Protection with Latched (option A) or Auto−Recovery (option B) Operation

　　High−voltage Current Source with Brown−Out detection and Dynamic Self−Supply, Simplifying the Design of the VCC Capacitor

　　Frequency Modulation for Softened EMI Signature, including during Frequency Foldback mode

　　Adjustable Overpower Compensation

　　Latch−off Input for Severe Fault Conditions, Allowing Direct Connection of an NTC for Overtemperature Protection (OTP)

　　VCC Operation up to 28 V, with Overvoltage Detection

　　_500 mA Peak Source / Sink Current Drive Capability

　　4.0 ms Soft−Start

　　Internal Thermal Shutdown

　　Pin−to−Pin Compatible with the Existing NCP12xx Series

　　These Devices are Pb−Free, Halogen Free/BFR Free and are RoHS Compliant

　　NCP1236典型应用:

　　AC−DC Adapters for Notebooks, LCD, and Printers

　　Offline Battery Chargers

　　Consumer Electronic Power Supplies

　　Auxiliary/Housekeeping Power Supplies

　　图1.NCP1236简化方框图

　　图2.采用NCP1236的反激转换器

　　65W离线适配器参考设计

　　When designing adapters, the important defining regulations for efficiency and no load

　　power requirements are the ENERGY STAR ® specifications. With the release of the EPS 2.0 standard, the light load input power consumption and the standby power consumption have became more important, reflecting more accurately the actual usage of a laptop adapter which spends a considerable amount of its time with no-load or a minimal load (laptop in sleep mode) attached.

　　When focusing on the light load efficiency of adapter design, the key losses need to be identified. Switching losses play a major role in determining the light load efficiency, and are directly linked to the control methodology. These losses are caused by the energy stored in the sum of all capacitances at the drain node (MOSFET output capacitance, stray capacitance of the transformer and other parasitic capacitances on PCB) together with the gate charge losses associated with driving the MOSFET. These are proportional to the switching frequency, hence, reducing the switching frequency reduces the losses and improves the efficiency. One of the best methods to achieve optimal balance between transformer design and light load efficiency is to have the switching frequency vary as a function of load.

　　This is implemented in the NCP1234/36 family of PWM controllers as a frequency foldback function, thereby lowering the frequency at lighter loads.

　　65W离线适配器主要特性:

　　Current mode control

　　Dynamic self-supply

　　Frequency foldback

　　High voltage sensing

　　Brown-out protection

　　Timer based overload protection

　　Overpower protection

　　Built-in internal slope compensation

　　Latch protection mode

　　Skip mode for light load

　　Frequency modulation for softened EMI

　　65W离线适配器参考设计指标:

　　图3.65W笔记本电脑离线适配器参考设计电路图

　　图4.离线适配器参考设计原型外形图(实际尺寸146.6x50.8mm)

　　65W笔记本电脑离线适配器参考设计材料清单(BOM):