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1、 德州仪器在线技术支持社区 德州仪器在线技术支持社区 1 请加入请加入 TI 微博 微博 德州仪器在线技术支持社区 德州仪器在线技术支持社区 2 作者介绍 Robert Kollman 现任 TI 高级应用经理兼科技委员会的资深委员 他 拥有在电源电子领域超过 30 年的工作经验 并为电源电子设计了从低功 耗 sub watt 到超低功耗 sub megawatt 的磁性元件 工作频率在兆 赫兹范围内 Robert 毕业于得克萨斯 A 2 as a corollary to 1 no master slave operation 3 minimum interconnect 4 no adve
2、rse impact on efficiency 5 good voltage regulation and 6 preserved load dynamics The droop method provides a simple way to meet many of these requirements It works by allowing the power output voltage to sag as a function of load current As shown in Figure 1 paralleled power supplies tend to equaliz
3、e output currents because of this load line Figure 1 Drooping power supply Figure 1 Drooping power supply output voltage enables current output voltage enables current shashare re Click on image to enlarge Click on image to enlarge This figure shows the output voltage versus load characteristics of
4、three power supplies Because of component tolerances the three power supplies have slightly different V I characteristics For a given load condition a horizontal line represents the output voltage when all three supplies are connected in parallel The intersections of the horizontal line with the loa
5、d lines represent the output currents of the individual supplies This method obviously degrades the system voltage regulation There is a trade off between how well the currents balance and the voltage regulation The first step of this trade is to determine the regulator tolerance i e how far does th
6、e worst case design depart from the nominal The important items to establish are reference accuracy over temperature and divider tolerance see Power Tip 18 While how close to nominal you can set your output voltage by picking resistors that affect your accuracy it will not impact the current share Y
7、ou then are ready to pick either your slope or your allowable deviation and calculate the other If you assume that the slope is relatively constant the variables are simply related as Where SPA Set point accuracy in percent D Voltage droop from no load to 德州仪器在线技术支持社区 德州仪器在线技术支持社区 55 full load in pe
8、rcent LE Load extreme or how far the loads could be off in percent In doing the calculations you will find the short coming of this method It takes extreme accuracy in setting the output voltage and significant voltage droop to get a reasonable current share For instance as shown in Figure 1 with 3
9、5 tolerances and 20 voltage droop you can have 35 current mismatch This amount of droop may be acceptable in high voltage systems but will not be in low voltage supplies The first thought about implementing voltage drop might be to put a big resistor in series with the output voltage until you look
10、at the tolerance issues and the resulting losses In our previous example we would loose over 20 percent of the output power in this resistor The next thought is to measure the output current of the power supply amplify and use this measurement to offset the output voltage setting circuit This works
11、for voltage mode control but with current mode control a much simpler method is available By limiting the DC gain of the control loop you have built a synthetic resistor Appendix 1 below after the About the Author box goes through the simple math for calculating the output impedance based on Figure
12、2 The result is that the output impedance of this system is equal to the negative inverse of the compensator gain times the power stage gain Most power supplies contain an integrator in the compensation which results in a very large DC compensator gain By adjusting the DC gain to a specific value a
13、desired droop can be obtained Usually this is quite easy to implement just add a resistor across the error amplifier Figure 2 Voltage droop is easily Figure 2 Voltage droop is easily implemented with current mode implemented with current mode control control Click on image to enlarge Click on image
14、to enlarge Please join us next month when we will discuss tips for hot swap controllers For more information about this and other power solutions visit 德州仪器在线技术支持社区 德州仪器在线技术支持社区 56 电源设计经验谈电源设计经验谈 28 估算热插拔 估算热插拔 MOSFET 的瞬态温升的瞬态温升 第第 1 部部 分分 在本电源设计经验谈以及下一篇的 经验谈中 我们将研究一种估算热插拔 MOSFET 温升的简单方法 热插拔电路用 于将电容
15、输入设备插入通电的电压总线 时限制浪涌电流 这样做的目的是防止总 线电压下降以及连接设备运行中断 通过 使用一个串联组件逐渐延长新连接电容 负载的充电时间 热插拔器件可以完成这 项工作 结果 该串联组件具有巨大的损 耗 并在充电事件发生期间产生温升 大 多数热插拔设备的制造厂商都建议您查 阅安全工作区域 SOA 曲线 以便设备 免受过应力损害 图 1 所示 SOA 曲线 显示了可接受能量区域和设备功耗 其一 般为一个非常保守的估计 MOSFET 的主 要忧虑是其结温不应超出最大额定值 该 曲线以图形的形式向您表明 由于设备散 热电容的存在它可以处理短暂的高功耗 这样可以帮助您开发一个精确的散热
16、模 型 以进行更加保守 现实的估算 在 电源设计经验谈 9 中 我们讨 论了一种电气等效电路 用于估算系统的 散热性能 我们提出在散热与电流 温度 与电压以及散热与电阻之间均存在模拟 电路 在本设计经验谈中 我们将增加散 热与电容之间的模拟电路 如果将热量加 到大量的材料之中 其温升可以根据能量 Q 质量 m 和比热 c 计算得到 即 cm Q T 能量正好是功率随时间变化的积分 dtPQ 然后合并上述两个方程式 我们得到 我们的电容散热模拟 m c 如下 dtP cm T 1 图图 1 MOSFET SOA 1 MOSFET SOA 曲线表明了允许曲线表明了允许 能耗的起始点能耗的起始点 表 1 列出了一些常见材料及其比热 和密度 其或许有助于建模热插拔器件内 部的散热电容 德州仪器在线技术支持社区 德州仪器在线技术支持社区 57 材料 比热 J g oC 密度 g cm3 硅 0 7 2 3 熟铜 0 4 8 铝 0 9 2 7 环氧树脂 1 1 4 表表 1 1 常见材料的物理属性常见材料的物理属性 只需通过估算您建模的各种系统组 件的物理尺寸 便可得到散热电容 散热 能力等于组
