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2011 飞兆半导体公司 www.fairchildsemi.com Rev. 1.0.1 ? 5/21/13 AN-9729 使用 100W 路灯照明的半桥 LLC 谐振转换器 LED 应用设计指南 简介 本应用指南介绍用于大功率 LED 照明应用(如户外 照明或路灯照明)的LED 驱动系统,该系统采用半 桥LLC 谐振转换器. 对于传统 PWM DC-DC 转换器 而言,采用非隔离型 DC-DC 转换器控制 LED 的电流 和光强会导致其功率转换效率降低. 而半桥 LLC 谐 振转换器不仅可以控制 LED 电流,而且可以大幅提 升功率转换效率. 此外,整个 LED 驱动系统的成本 和体积也得到了降低. 关于 LED 驱动 如今,LED 照明正在快速取代传统照明光源,如白炽 灯,荧光灯和卤素灯等,因为 LED 照明可降低功耗 . 相对传统照明光源,LED 照明寿命更长(约为荧 光灯和白炽灯的 5~20 倍),且不含毒性物质和有害 紫外线. 包括紧凑型荧光灯 CFL 在内的所有的卤素 灯和荧光灯几乎都含有有害元素汞. LED 的输出光强取决于流经 LED 电流的大小. LED 的特性决定所需电流大小所必需的正向电压. 根据 LED 电压与电流特性的关系,控制 LED 两端的正向 电压会导致不同的输出光强的大小. 因此,大多数 LED 驱动均通过调节电流来进行亮度控制, 改变 LED 电流的大小可直接控制亮度. 关于 LLC 谐振转换器 无源器件的尺寸限制了开关电源功率密度的不断增加 . 采取高频运行,可以大大降低无源器件,如变压 器和滤波器的尺寸. 但是过高的开关损耗势必成为 高频运行的一大障碍. 为了降低开关损耗和容许高 频运行,谐振开关技术已经得到了发展. 这些技术 采用正弦方式处理电力,开关器件能够实现软换流. 使得开关损耗与噪声大为降低[1-7] . 在各种类型的谐振转换器中,最简单和最普遍的谐振 转换器为 LC 串联谐振转换器,其中整流器-负载网络 与LC 谐振网络串联,如? 图1[2-4]所示. 在该电路结构 中,LC谐振网络与负载一起形成分压器. 通过改变驱 动电压Vd的频率,可以改变该谐振网络的阻抗. 输入 电压在谐振网络阻抗与反射负载之间进行分压. 由于 LC 串联谐振转换器等效于一个分压器,因此其直流增 益始终小于 1.在轻载条件下,相比谐振网络的阻抗 ,负载阻抗较大,这样,输入电压几乎全部施加在负 载上. 这使得轻载下很难调节输出. 在空载时,为 了能够调节输出,理论上谐振频率应该为无限大. 图1. 半桥LC串联谐振转换器 为了克服串联谐振变换器的限制,提出了 LLC 谐振 变换器[8-12] . LLC 谐振转换器为一种改进型 LC 串联 谐振转换器,区别是变压器初级绕组并联了一个电感 ,如? 图 2所示. 采用并联电感可以增加初级绕组的 环流,有利于电路运行. 由于这个概念不直观,在 该拓扑首次提出时没有受到足够的重视. 但是,对 于高输入电压的应用场合,与导通损耗相比,变换器 的开关损耗占主导地位,此时该谐振变换器在提升效 率方面就显得十分突出. 在多数实际设计中,该分流电感往往通过变压器励磁 电感来实现. LLC谐振转换器的电路图与LC串联谐振 转换器的电路图十分相似. 唯一的差别在于:励磁 电感的取值不同. LLC 谐振转换器的励磁电感远远 大于 LC 串联谐振转换器的励磁电感 (Lr),LLC 谐 振转换器中的励磁电感为 Lr的3~8 倍,通常通过在 变压器中设置气隙来实现. AN-9729 应用指南 ?