用于MR16 LED灯的简单、可配置LED驱动器应用设计
来源：21IC 作者：--- 时间：2011-09-28 00:00
http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/ http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/ http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/ http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/ MR16灯属于多面向反 卤素灯与LED的特性 MR16卤素灯凭借其独特的尺寸、可配置性、聚光能力、美观性和实用性，应用于很多零售和消费性应用的创意照明。常见的MR16卤素灯的光通量范围为150流明(lm)至800http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/ lm，光效约为15http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/ lm/W，或15%的发光效率。低效率、产生热量和卤素囊处理问题是这种技术的缺点。另外，典型的卤素灯泡寿命约为2,000小时。还可能由于猛烈的振动导致灯丝断路使灯泡过早损坏。 相比较，LED具有节能环保的优点，耗电相当低，一般LED的工作电压是2至3.6http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/ V。LED是一种冷发光技术，发热量比普通照明光源低很多；LED完全封装在环氧树脂中，灯体内也没有松动部分，不易损坏，在恰当的电流和电压下，LED的使用寿命可达5至10万小时；LED由无毒的材料制成，没有污染。LED目前的不足是价格相对较贵，发热量的处理有一定难度，因此驱动电源是LED灯整体寿命的一个关键要素。 用LEDhttp://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/ MR16灯替代低效MR16卤素灯 安森美半导体的CAT4201是一款能驱动3颗串联LED的300http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/ mAhttp://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/ ·http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/ 电路特点 CAT4201采用降压拓扑结构，不要求输入/输出(I/O)电气隔离，输入电压为12http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/ Vac，输出功率为2.7http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/ W；其他规格包括：输出电压为9.1http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/ V，额定输入功率3.4http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/ W，额定平均电流为300http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/ mA，最大平均电流310http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/ mA，最小平均电流295http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/ mA，典型能效为83％。图1是采用CAT4201的MR16灯电路。 图1：采用CAT4201的MR16灯电路 CAT4201是一款降压型、易于配置的可调光LED驱动器。该器件采用5引脚SOT-23封装；分立元件不到10个(其中大部分是表面贴装元件)，整个电路板可小到足以放入MR16灯的灯座内。VBAT、CTRL和SW引脚的额定电压为40http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/ V，所以可以用足够的电压驱动12个LED。RSET引脚决定输出直流电流；CTRL用于调光信号输入。SW是内部MOSFET的输出引脚。 ·http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/ 工作原理 该应用电路的工作原理并不复杂，是一个简单的降压转换器。在交流应用情况下，输入经整流桥进行整流。VBAT和GND直接连接至大容量电容C1的两个端点，从而降低了电流纹波。在第一开关相位期间，内部MOSFET随线性上升电流对电感器充电，直到MOSFET关闭；在第二相位期间，MOSFET被关断，而储存在电感器中的电流将通过肖特基二极管(D5)放电，同时电流衰减，直至下一个开关周期。输出电容用来减少LED中的电流纹波。RSET引脚的电压被调节在1.2http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/ V，因此连接至RSET的电阻决定RSET电流。RSET电流与CAT4201的恒流输出大致成正比。 ·电路配置 在电路配置方面，VBAT的额定电压为-0.3至+40http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/ V，因此整流桥上的输入电压高达24http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/ Vac也是安全的。要保持较高输入电压水平就需要一个大容量的C1。大幅波动的经整流的AC电流将在双倍AC线路频率条件下把输出电流拉至零，因此减少了输出电流。220http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/ μF的电容对这个设计已经足够了。对于12http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/ Vac输入，C1额定电压应该是25http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/ V。由于需要较低的正向电压，整流桥选用了MBR0520L表面贴装肖特基二极管。 输出电容C2建议至少为4.7http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/ μF，以减少输出纹波。较大容量的C2将有效抑制输出纹波，同时可提高几毫安的输出电流。不过，由于其对总能效的贡献微不足道，且人眼无法感知高频波动，没有必要使用大于10http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/ μF的电容。L的值建议为22http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/ μH，以将开关频率设置在约为150http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/ kHz。LED电流范围为0至350http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/ mA，所以800http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/ mA左右的电感有效额定电流就足够了。 流经整流器的总连续电流始终低于400http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/ mA，即使提供的输出电流为350http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/ mA。因此，对整流器来说0.5http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/ A的连续额定电流就足够了。仔细观察续流二极管D5两端的电压可以发现，尽管有12http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/ Vac输入，也会出现超过20http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/ V的电压尖峰。较高的输入电压(如15http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/ V)进一步加大了二极管的应力，增加了故障风险。所以D5上的额定电压应为30http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/ V或40http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/ V。安森美半导体的肖特基整流器MBR0540可用作D5。 RSET引脚用来配置输出电流值。RSET电压调节在1.2http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/ V。在RSET和GND之间增加一个电阻可以决定RSET电流，该电流与输出恒压为近似的线性关系： 计算得出，对于300http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/ mA输出，IRSEThttp://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/ =http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/ 0.12http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/ mA。RSET电阻值应该是10kΩ。较小的RSET电阻可能会增加输出电流，为了稳定运行建议使用至少8.0http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/ kΩ的电阻。如果输入电压足够高(如15http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/ Vac)，可以设置RSET为8.2http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/ kΩ，因此可保证至少350http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/ mA的输出电流。CTRL经R2从LED的阴极获得电压。R2的值并不重要。 性能及特点 图2：输入电压、输出电流与效率的关系 在根据图1的电路图配置的电路中，Vin=12http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/ Vachttp://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/ 50Hz。由于AC线路输入，LED电流出现了很轻微的低频(AC频率的两倍，2http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/ ×http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/ 50HZ)波动。使用较大容量的C1或较高的Vin将进一步平稳与AC相关的纹波电流。 在根据图1的电路图配置的电路中，启动和断电瞬态如图4所示。 设计紧凑 图5：微型LED驱动板可以嵌入灯座内
从以下图2至图4显示的波形可以了解电路的基本性能及特点。其测试设备包括：Globalhttp://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/ Specialtieshttp://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/ 1506http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/ AC隔离型电压可变AC线路电源；Voltechhttp://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/ PM1000电源分析仪；泰克TDS754D数字荧光示波器；泰克TCP202电流探头；安捷伦34401A万用表。
图3：LED电流波形
图4：启动和断电瞬态
CAT4201驱动器的优势是紧凑的电路和高能效，使得它非常适合替代基于灯丝的灯泡，如射灯应用。这款超小设计可以放进MR16型
由于经过了优化，该设计在元件数量和热性能方面都更臻完善。这款设计一般可在镜面部分使用3个1http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/ Whttp://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/ LED，还可以做出调整，满足照明系统设计人员的要求。在最终的解决方案中，LED电流及电容器的大小都要有所取舍，以达致最理想的能效、精度、尺寸及元件数量。
总结
该设计中提供了使用CAT4201设计驱动3颗LED的MR-16灯所需的各种要素。CAT4201有助于实现MR-16兼容适配器更小的尺寸和更少的元件。
减少电容器和电感值，或不使用CTRL电阻有可能进一步降低成本和缩小PCB尺寸。使用小容量电容器会导致平均输出电流的大幅下降，因此不推荐这样做，但适用于亮度较低的(200至250http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/http://www.hqew.com/file/CMSImages/2011/09/ mA)的应用。选择较小正向电压的高品质LED对实现更大输出电流的设计非常重要。
