【经验】开关电源直流侧电流纹波估算与电解电容选型
开关电源直流侧往往需放置电容来滤波，以减少开关电源谐波对前级直流侧（电池、母线等）产生影响。电解电容手册中一般会给出额定纹波电流值（Rated ripple current）。若选择的电容额定纹波电流值过大，则会造成成本甚至体积浪费；而选择过小会造成电容无法承受而损坏。在实际项目中我们往往无法精确获得直流侧阻抗信息以供精密计算，因此需要进行一些估算，帮助我们选择适当的电容额定纹波电流值。
本文以一Buck-Boost电路为例分享如何根据电路进行纹波估算从而合理选型。本文所选电容为世强代理厂牌Nippon Chemi-Con（俗称“黑金刚”）的一款高纹波铝电解电容EKZN800ELL820MH20D。主电路如下图所示。
图1 主电路图
工作中直流侧电流与流经Q1的电流i相同，它随着Q1的导通、关断而变化。i波形如下图所示。
图2 i波形图
在实际考虑问题时，上图可简化看做方波电流源。
图3 i简化波形图
因此，主电路直流侧可按如下电路图建模分析。
图4 主电路直流侧建模
到这里，我们可以按照直流侧等效阻抗(jwL+R)和直流侧滤波电容1/jwC分流来分析，但我们无从得知直流侧具体的阻抗。所以这里我们做第二个简化——假设方波电流中所有交流量都由电容吸收，而只有直流分量流经直流侧。
于是我们可以进行如下计算。（本电路额定功率为250W。左侧为48V直流母线，右侧为电池40V~58V。记Q1占空比为D）
方波幅值
有效值为
假设电流中所有交流量都由电容吸收，则
故电容上各次谐波分量总有效值为
故D取最小值0.83时，电容纹波有效值最大，为2.33A。故每个电容上纹波电流为1.16A。
考虑到实际上不会全部纹波电流都由电容吸收，所以所选Nippon Chemi-Con的EKZN800ELL820MH20D铝电解电容的抗纹波能力1.04A是满足的。
若仍有担心，可再放一颗薄膜电容，如图1中C8，薄膜电容具有很好的吸收高频能力。以下是实验中电感电流5A（额定功率下运行）测得的直流侧电流纹波为880mA。有兴趣的话还可以根据电感电流纹波和直流侧纹波反推出电容吸收的纹波，并且可以推出直流侧的阻抗信息。
图5 实验波形图
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黑金刚铝电解电容选型表
提供黑金刚（Nippon Chemi-Con）铝电解电容产品选型。安装方式分为贴片型（SMD），引线型（Radial lead），基板自立型（牛角电容）；常规品范围在-55℃~105℃，1000小时~20000小时寿命保证，额定电压范围在4V～600V；尺寸范围最小规格：贴片型电容4mm*5.2mm，容量范围从1uF~120000uF。
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产品型号
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品类
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系列
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直径(mm)
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长度(mm)
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引脚类型
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电压(V)
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容值(uF)
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电容容差
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最小工作温度(℃)
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最大工作温度(℃)
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MOQ/SPQ(pcs)
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EKHE401VSN241MQ25Z
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牛角型铝电解电容器
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KHE Series
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25.4mm
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25mm
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Snap-in
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400V
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240uF
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±20%(M)
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-40℃
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105℃
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200
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选型表 - Nippon Chemi-Con 立即选型
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电子商城
品牌：Nippon Chemi-Con
品类：Aluminum Electrolytic Capacitors
价格：
现货： 200,000
品牌：Nippon Chemi-Con
品类：Aluminum Electrolytic Capacitor
价格：
现货： 200,000
现货市场
服务
整体外形尺寸小至0.6*0.3*0.3mm (DFN0603)，工作电压范围覆盖2.5V~36V，电容值低至0.2pF，浪涌能力最高可达240安培，静电等级可达空气放电、接触放电±30KV。提供免费浪涌测试仪、静电测试仪测试。
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可定制OGS一片式电容屏，G+G电容屏，G+F电容屏，G+F+F电容屏，尺寸范围：1”~21”，单层产品厚度：0.55mm~2mm，可按客户要求在产品上附加防反射，防指纹，多色丝印，钻孔磨边，2.5D导圆角等工艺。
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