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On Semi NCP1014能源之星离线8W LED驱动方案
来源:电子设计应用 作者: 2009/12/7 0:00:00 人气:2295
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       On Semi 公司的NCP101x系列自供电单片开关电路,主要用于低待机功耗的离线开关电源(SMPS),集成了固定频率电流模式控制器和700V MOSFET,提供实现可靠低成本电源所需要功能,包括软起动,频率抖动,短路保护,跳频,最大峰值电流设定和动态自供电(不需要辅助线圈).主要用在小功率充电器适配器,USB,家用电器,TV等辅助电源.本文介绍NCP101x系列主要特性,方框图, 6.5W反激转换器和通用火线电压输入的7.0W转换器以及能源之星离线8W LED驱动器参考设计NCP1014LEDGTGEVB电路图和材料清单(BOM).

NCP1014: Self-Supplied Monolithic Switcher for Low Standby-Power Offline SMPS

The NCP101X series integrates a fixed−frequency current−mode controller and a 700 V MOSFET. Housed in a PDIP−7, PDIP−7 Gull Wing, or SOT−223 package, the NCP101X offers everything needed to build a rugged and low−cost power supply, including soft−start, frequency jittering, short−circuit protection, skip−cycle, a maximum peak current setpoint and a Dynamic Self−Supply (no need for an auxiliary winding).

Unlike other monolithic solutions, the NCP101X is quiet by nature: during nominal load operation, the part switches at one of the available frequencies (65 − 100 − 130 kHz). When the current setpoint falls below a given value, e.g. the output power demand diminishes, the IC automatically enters the so−called skip−cycle mode and provides excellent efficiency at light loads. Because this occurs at typically 1/4 of the maximum peak value, no acoustic noise takes place. As a result, standby power is reduced to the minimum without acoustic noise generation.

Short−circuit detection takes place when the feedback signal fades away, e.g. in true short−circuit conditions or in broken Optocoupler cases. External disabling is easily done either simply by pulling the feedback pin down or latching it to ground through an inexpensive SCR for complete latched−off. Finally soft−start and frequency jittering further ease the designer task to quickly develop low−cost and robust offline power supplies.

For improved standby performance, the connection of an auxiliary winding stops the DSS operation and helps to consume less than 100 mW at high line. In this mode, a built−in latched overvoltage protection prevents from lethal voltage runaways in case the Optocoupler would brake. These devices are available in economical 8−pin dual−in−line and 4−pin SOT−223 packages.

NCP1014主要特性:

Built−in 700 V MOSFET with Typical RDSon of 11 ohm and 22 ohm

Large Creepage Distance Between High−Voltage Pins

Current−Mode Fixed Frequency Operation: 65 kHz – 100 kHz − 130 kHz

Skip−Cycle Operation at Low Peak Currents Only: No Acoustic Noise!

Dynamic Self−Supply, No Need for an Auxiliary Winding

Internal 1.0 ms Soft−Start

Latched Overvoltage Protection with Auxiliary Winding Operation

Frequency Jittering for Better EMI Signature

Auto−Recovery Internal Output Short−Circuit Protection

Below 100 mW Standby Power if Auxiliary Winding is Used

Internal Temperature Shutdown

Direct Optocoupler Connection

SPICE Models Available for TRANsient Analysis

Pb−Free Packages are Available

NCP1014典型应用:

Low Power AC/DC Adapters for Chargers

Auxiliary Power Supplies (USB, Appliances,TVs, etc.)


图1.NCP1014简化的内部方框图

图2.采用NCP1012的6.5W反激转换器

图3.典型的通用火线电压输入的7.0W转换器

能源之星离线8W LED驱动器参考设计NCP1014LEDGTGEVB

Offline LED Driver Intended for ENERGY STAR Residential LED Luminaire Applications This reference document describes a built−and−tested, GreenPoint solution for an isolated 8 W constant current LED driver which is intended to support the residential power factor requirements of the DoE ENERGY STAR Standard for Solid State Lighting Luminaires (Version 1.1 − 12/19/08). Some of the typical products in this category include portable desk lamps, under−cabinet lights, and outdoor porch lights.

One of the most common power supply topologies for low power offline LED drivers is an isolated flyback topology. Unfortunately standard design techniques used for these supplies typically result in a power factor in the range of 0.5−0.6. This design note describes why the power factor is low and discusses techniques to improve the power factor.

Finally it illustrates how an existing design was modified to substantially improve the power factor and easily comply with the residential power factor requirements.

Background

The NCP1014LEDGTGEVB evaluation board has been optimized to drive 1−8 high power high brightness LEDs such as the Cree XLAMP XR−E/XP−E, Luxeon Rebel, Seoul Semiconductor Z−POWER®, or OSRAM Golden Dragon. The design is built around the NCP1014, a compact fixed frequency PWM converter which integrates a high voltage power switch with internal current limiting.

Since the converter is limited to a maximum power of approximately 8 W with a universal AC input (90 –265 Vac), the number of LEDs which can be driven is a function of the drive current. Specifically for this design note, the load will be one Cree XLAMP MC−E driven at 630 mA where all LEDs are connected in series. The MC−E is comprised of 4 LEDs mounted in a single package and the maximum rated current per LED is 700 mA. The evaluation board can be modified for other LED drive currents by making slight modifications to the bill of materials.

A typical off−line flyback power converter utilizes a full wave bridge rectifier and substantial bulk capacitance preceding the switching regulator. This configuration is chosen because twice every line cycle the line power reduces and ultimately reaches zero before rising to the next peak.

The bulk capacitor fills in the missing power providing a more constant input to the switching regulator maintaining power flow to the load.

This configuration comes at the expense of poor utilization or power factor of the input line waveform. Line current is drawn in high amplitude narrow pulses near the peaks of the voltage waveform introducing disruptive high frequency harmonics. Passive solutions are well documented but typically introduce many additional components. One approach is the valley−fill type rectifier where a collection of electrolytic capacitors and diodes increases the line frequency conduction angle resulting in improved power factor. In effect, this process charges the series−connected capacitors from the high line voltage at low current and discharges them to the switching regulator at a lower voltage with higher current. A typical application uses two capacitors and three diodes or, for enhanced power factor performance, three capacitors and six diodes.


图5.Valley Fill 电路

图6. NCP1014LEDGTGEVB电路图

材料清单(BOM):
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