降壓轉換器效率之分析

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<div class="WordSection1"> <h1>降壓轉換器效率之分析</h1> <h2 id="__Bookmark__0">摘要</h2> 同步降壓轉換電路是被廣泛地使用在提供非隔離電源給需低電壓和高電流之系統晶片。對電源工程師而言，首要的工作是要充分了解同步降壓轉換器的功率損耗，並進而提昇其效率。本應用須知將對降壓轉換器的效率作詳細的分析，以了解同步降壓轉換器中各主要功率損耗的來源。 <h2 id="__Bookmark__1">1. 降壓轉換器的功率損耗分析</h2> 若要設計出最佳性能的轉換器就必須充分了解轉換器的功率損耗。圖一顯示了常用的單相同步降壓轉換器的電路。在同步降壓轉換器電路中，主要的功率損耗分為下列四項： A : 功率半導體的功率損耗 B : 電感的功率損耗 C : 驅動器的功率損耗 D : PCB走線的功率損耗 <a href="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-tw/image001_zh-tw.png" class="fancybox"> <img width="453" height="250" width="453" height="250" alt="Technical Document Image Preview" src="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-tw/image001_zh-tw.png" /></a> 圖一、同步降壓轉換器 <h2 id="__Bookmark__2">2.功率損耗之計算</h2> 現在或未來較先進的微處理器都非常地需要低功耗、高效率的同步降壓轉換器。本應用須知將詳細介紹如何計算在一個典型的同步降壓轉換器中，分別發生在以下各項的功率損耗。計算條件是以轉換器工作在連續導通模式（CCM）、固定開關頻率、固定輸入電壓和固定輸出電壓之下的。 A : 功率半導體的功率損耗 HMOS（高側MOSFET）之功耗有：切換時與導通時之功耗。 LMOS（低側MOSFET）之功耗有：導通時、空滯期和反向恢復電荷之功耗。 HMOS切通時之功耗: <a href="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-tw/image002_zh-tw.png" class="fancybox"> <img width="430" height="226" width="430" height="226" alt="Technical Document Image Preview" src="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-tw/image002_zh-tw.png" /></a> 圖二、HMOS 切通時之示意圖 <a href="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-tw/image003_zh-tw.png" class="fancybox"> <img width="314" height="305" width="314" height="305" alt="Technical Document Image Preview" src="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-tw/image003_zh-tw.png" /></a> 圖三、HMOS切通時之功耗區間 <a href="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-tw/image004_zh-tw.png" class="fancybox"> <img width="319" height="141" width="319" height="141" alt="Technical Document Image Preview" src="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-tw/image004_zh-tw.png" /></a> HMOS 之導通功耗: 高側 MOSFET 之導通功耗是由該 MOSFET 的導通內阻的和它的 RMS電流來決定。 <a href="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-tw/image005_zh-tw.png" class="fancybox"> <img width="441" height="224" width="441" height="224" alt="Technical Document Image Preview" src="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-tw/image005_zh-tw.png" /></a> 圖四、HMOS導通時 <a href="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-tw/image006_zh-tw.png" class="fancybox"> <img width="323" height="247" width="323" height="247" alt="Technical Document Image Preview" src="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-tw/image006_zh-tw.png" /></a> 圖五、HMOS導通時之區間 <a href="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-tw/image007_zh-tw.png" class="fancybox"> <img width="221" height="132" width="221" height="132" alt="Technical Document Image Preview" src="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-tw/image007_zh-tw.png" /></a> LMOS 之導通功耗： <a href="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-tw/image008_zh-tw.png" class="fancybox"> <img width="444" height="230" width="444" height="230" alt="Technical Document Image Preview" src="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-tw/image008_zh-tw.png" /></a> 圖六、LMOS導通時 <a href="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-tw/image009_zh-tw.png" class="fancybox"> <img width="336" height="264" width="336" height="264" alt="Technical Document Image Preview" src="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-tw/image009_zh-tw.png" /></a> 圖七、LMOS導通時之區間 <a href="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-tw/image010_zh-tw.png" class="fancybox"> <img width="250" height="142" width="250" height="142" alt="Technical Document Image Preview" src="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-tw/image010_zh-tw.png" /></a> LMOS 空滯期的自體二極體功耗： 空滯期的功耗是因 LMOS 自體二極體在空滯期導通所引起。 <a href="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-tw/image011_zh-tw.png" class="fancybox"> <img width="450" height="232" width="450" height="232" alt="Technical Document Image Preview" src="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-tw/image011_zh-tw.png" /></a> 圖八、LMOS 自體二極體導通時 <a href="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-tw/image012_zh-tw.png" class="fancybox"> <img width="346" height="289" width="346" height="289" alt="Technical Document Image Preview" src="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-tw/image012_zh-tw.png" /></a>: 圖九、LMOS 自體二極體導通時之區間 <a href="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-tw/image013_zh-tw.png" class="fancybox"> <img width="386" height="83" width="386" height="83" alt="Technical Document Image Preview" src="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-tw/image013_zh-tw.png" /></a> LMOS反向恢復電荷之功耗： <a href="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-tw/image014_zh-tw.png" class="fancybox"> <img width="416" height="254" width="416" height="254" alt="Technical Document Image Preview" src="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-tw/image014_zh-tw.png" /></a> 圖十、LMOS自體二極體反向恢復之區間 <a href="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-tw/image015_zh-tw.png" class="fancybox"> <img width="204" height="37" width="204" height="37" alt="Technical Document Image Preview" src="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-tw/image015_zh-tw.png" /></a> B:電感直流與交流之功耗 電感的直流功耗： <a href="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-tw/image016_zh-tw.png" class="fancybox"> <img width="412" height="215" width="412" height="215" alt="Technical Document Image Preview" src="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-tw/image016_zh-tw.png" /></a> 圖十一、電感電流的路徑 <a href="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-tw/image017_zh-tw.png" class="fancybox"> <img width="334" height="265" width="334" height="265" alt="Technical Document Image Preview" src="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-tw/image017_zh-tw.png" /></a> 圖十二、電感電流導通之區間 <a href="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-tw/image018_zh-tw.png" class="fancybox"> <img width="191" height="114" width="191" height="114" alt="Technical Document Image Preview" src="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-tw/image018_zh-tw.png" /></a> 電感磁芯的功耗： 電感磁芯的功耗主要是因在磁芯材料的交流磁場所引起，功耗大小是和操作頻率與總磁通量有關，且可能會因不同磁性材料而有所不同。 <a href="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-tw/image019_zh-tw.png" class="fancybox"> <img width="292" height="239" width="292" height="239" alt="Technical Document Image Preview" src="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-tw/image019_zh-tw.png" /></a> 圖十三、電感之漣波電流 <a href="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-tw/image020_zh-tw.png" class="fancybox"> <img width="341" height="215" width="341" height="215" alt="Technical Document Image Preview" src="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-tw/image020_zh-tw.png" /></a> 圖十四、磁芯功耗曲線 無論是由計算或量測而得的磁芯功耗，通常都是直接由電感供應商所提供的。若無提供資料，則可用以下公式來計算磁芯功耗： <a href="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-tw/image021_zh-tw.png" class="fancybox"> <img width="131" height="79" width="131" height="79" alt="Technical Document Image Preview" src="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-tw/image021_zh-tw.png" /></a> PL是磁芯功耗（mW）, Fsw : 操作頻率 B : 峰值磁通密度，單位 (Gauss) Ve : 有效磁芯體積 C，X和Y是每種磁芯材料的功耗參數 C:柵極驅動器之功耗： 柵極驅動器之功耗是因 MOSFET 驅動器去充/放電 HMOS 和 LMOS 的總電荷 Qg ，所以柵極驅動器之功耗是和 MOSFET 總柵極電荷、驅動器電壓及 FSW有關。 <a href="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-tw/image022_zh-tw.png" class="fancybox"> <img width="383" height="216" width="383" height="216" alt="Technical Document Image Preview" src="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-tw/image022_zh-tw.png" /></a> 圖十五、驅動器啟動和關閉之路徑 <a href="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-tw/image023_zh-tw.png" class="fancybox"> <img width="342" height="321" width="342" height="321" alt="Technical Document Image Preview" src="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-tw/image023_zh-tw.png" /></a> 圖十六、MOSFET 被導通 <a href="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-tw/image024_zh-tw.png" class="fancybox"> <img width="339" height="359" width="339" height="359" alt="Technical Document Image Preview" src="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-tw/image024_zh-tw.png" /></a> 圖十七、MOSFET被關閉 <a href="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-tw/image025_zh-tw.png" class="fancybox"> <img width="243" height="60" width="243" height="60" alt="Technical Document Image Preview" src="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-tw/image025_zh-tw.png" /></a> D:PCB之功耗： 圖十八導通的情形分別顯示於圖十九（迴路1 HMOS導通）和圖二十（迴路2 LMOS導通），其中也將走線的電阻 Rtr1 ~ Rtr7 等細節模擬進去。 <a href="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-tw/image026_zh-tw.png" class="fancybox"> <img width="327" height="161" width="327" height="161" alt="Technical Document Image Preview" src="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-tw/image026_zh-tw.png" /></a> 圖十八、PCB走線示意圖 <a href="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-tw/image027_zh-tw.png" class="fancybox"> <img width="345" height="184" width="345" height="184" alt="Technical Document Image Preview" src="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-tw/image027_zh-tw.png" /></a> 圖十九、PCB迴路1走線之示意圖 <a href="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-tw/image028_zh-tw.png" class="fancybox"> <img width="347" height="182" width="347" height="182" alt="Technical Document Image Preview" src="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-tw/image028_zh-tw.png" /></a> 圖二十、PCB迴路2走線之示意圖 <a href="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-tw/image029_zh-tw.png" class="fancybox"> <img width="493" height="206" width="493" height="206" alt="Technical Document Image Preview" src="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-tw/image029_zh-tw.png" /></a> <h2 id="__Bookmark__3">3.功率損耗測量值和計算值之比較</h2> 計算降壓轉換器之功率損耗的公式已清楚介紹並列出如上，將藉由實際應用來查驗這些功率損耗方程式是否準確。表一列出了典型的降壓轉換器應用之參數；而圖二十一則顯示效率的測量值與計算值之間的比較。 表一、降壓轉換器之應用參數 <div> <table class="MsoNormalTable" style="border: currentColor; border-collapse: collapse;" border="1" cellspacing="0" cellpadding="0"> <tbody> <tr> <td style="border: 1pt solid #4bacc6; width: 97pt; padding-right: 5pt; padding-left: 5pt;"> IC 型號 </td> <td style="border: 1pt solid #4bacc6; width: 97pt; padding-right: 5pt; padding-left: 5pt;"> RT8120 </td> </tr> <tr> <td style="border: 1pt solid #4bacc6; width: 97pt; padding-right: 5pt; padding-left: 5pt;"> 輸入電壓 </td> <td style="border: 1pt solid #4bacc6; width: 97pt; padding-right: 5pt; padding-left: 5pt;"> 12V </td> </tr> <tr> <td style="border: 1pt solid #4bacc6; width: 97pt; padding-right: 5pt; padding-left: 5pt;"> 輸出電壓 </td> <td style="border: 1pt solid #4bacc6; width: 97pt; padding-right: 5pt; padding-left: 5pt;"> 1.2V </td> </tr> <tr> <td style="border: 1pt solid #4bacc6; width: 97pt; padding-right: 5pt; padding-left: 5pt;"> 開關頻率 </td> <td style="border: 1pt solid #4bacc6; width: 97pt; padding-right: 5pt; padding-left: 5pt;"> 300kHz </td> </tr> <tr> <td style="border: 1pt solid #4bacc6; width: 97pt; padding-right: 5pt; padding-left: 5pt;"> VDD 電壓 </td> <td style="border: 1pt solid #4bacc6; width: 97pt; padding-right: 5pt; padding-left: 5pt;"> 12V </td> </tr> <tr> <td style="border: 1pt solid #4bacc6; width: 97pt; padding-right: 5pt; padding-left: 5pt;"> 電感 </td> <td style="border: 1pt solid #4bacc6; width: 97pt; padding-right: 5pt; padding-left: 5pt;"> 1µH </td> </tr> <tr> <td style="border: 1pt solid #4bacc6; width: 97pt; padding-right: 5pt; padding-left: 5pt;"> DCR </td> <td style="border: 1pt solid #4bacc6; width: 97pt; padding-right: 5pt; padding-left: 5pt;"> 1.2mΩ </td> </tr> <tr> <td style="border: 1pt solid #4bacc6; width: 97pt; padding-right: 5pt; padding-left: 5pt;"> 高側開關HMOS </td> <td style="border: 1pt solid #4bacc6; width: 97pt; padding-right: 5pt; padding-left: 5pt;"> BSC090N03LS </td> </tr> <tr> <td style="border: 1pt solid #4bacc6; width: 97pt; padding-right: 5pt; padding-left: 5pt;"> 低側開關LMOS </td> <td style="border: 1pt solid #4bacc6; width: 97pt; padding-right: 5pt; padding-left: 5pt;"> BSC090N03LS*2 </td> </tr> </tbody> </table> </div> <a href="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-tw/image030_zh-tw.png" class="fancybox"> <img width="451" height="318" width="451" height="318" alt="Technical Document Image Preview" src="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-tw/image030_zh-tw.png" /></a> 圖二十一、效率測量值與計算值之比較 圖二十二分別顯示降壓轉換器中，主要來自於 HMOS，LMOS，電感，驅動器和PCB走線等的功耗。可用此圖來檢視在不同系統負載電流之下，各種功耗的大小。 <a href="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-tw/image031_zh-tw.png" class="fancybox"> <img width="445" height="337" width="445" height="337" alt="Technical Document Image Preview" src="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-tw/image031_zh-tw.png" /></a> 圖二十二、降壓轉換器中的主要功耗 圖二十三顯示降壓轉換器主要功耗更詳細的分解，並以曲線說明各功耗和負載電流 IOUT 的關係。 HMOS：PHSW（切換時之功耗）和PHCOD（導通時之功耗） LMOS：PLCOD（導通時之功耗），PL_DIODE（空滯期的自體二極體功耗）和PRR（反向恢復電荷之功耗） 電感：PL（電感的直流與磁芯功耗） 驅動器：PDRV（柵極驅動器電荷之功耗） PCB : PPCB （PCB走線之功耗） <a href="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-tw/image032_zh-tw.png" class="fancybox"> <img width="429" height="314" width="429" height="314" alt="Technical Document Image Preview" src="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-tw/image032_zh-tw.png" /></a> 圖二十三、降壓轉換器主要功耗更詳細的分解 <h2 id="__Bookmark__4">4.結論</h2> 本應用文件分析了同步降壓轉換器的功率損耗，並詳細分析各功率損耗是如何計算的，計算出的功耗再和實際測量降壓轉換器的功耗作比較。藉著所提供的主要功耗數據，能有效幫助設計者來思考，如何從元件或 PCB 板的佈局來提昇降壓轉換器的效率。 <h2 id="__Bookmark__5">參考資料</h2> <div class="WordSection3"> [1] Leon Chen, “Power Loss Analysis for Synchronous Buck Converter”, Application Engineer Dept data, 2013. [2] Nelson Garcia, “Determining Inductor Power Losses”, Coil craft Document 486, 2005. <table border="1" cellspacing="0" cellpadding="0" style="width: 637.5pt; border-collapse: collapse; border: medium none;" class="3-51"> <tbody> <tr style="height: 17pt;"> <td style="border-width: 1pt 1px 1px 1pt; border-style: solid; border-color: #009eb8; background-color: #009eb8; background-image: url(e); background-repeat: repeat; background-attachment: scroll; background-clip: border-box; background-origin: padding-box; background-size: auto auto; padding-right: 5pt; padding-left: 5pt; height: 17pt;" colspan="2"> 相關資源 </td> </tr> <tr> <td style="border-width: 1pt 1px 1px 1pt; border-style: solid; border-color: #009eb8; background-color: #ffffff; background-image: url(e); background-repeat: repeat; background-attachment: scroll; background-clip: border-box; background-origin: padding-box; background-size: auto auto; padding-right: 5pt; padding-left: 5pt; height: 17pt;"> 立錡科技電子報 </td> <td style="border-width: 1pt 1px 1px 1pt; border-style: solid; border-color: #009eb8; background-color: #ffffff; background-image: url(e); background-repeat: repeat; background-attachment: scroll; background-clip: border-box; background-origin: padding-box; background-size: auto auto; padding-right: 5pt; padding-left: 5pt; height: 17pt;"> <a class="link" href="http://www.richtek.com/My%20Richtek/Newsletters" target="_blank">訂閱立錡科技電子報</a> </td> </tr> <tr style="height: 17pt;"> <td style="border-width: 1pt 1px 1px 1pt; border-style: solid; border-color: #009eb8; background-color: #ffffff; background-image: url(e); background-repeat: repeat; background-attachment: scroll; background-clip: border-box; background-origin: padding-box; background-size: auto auto; padding-right: 5pt; padding-left: 5pt; height: 17pt;"> 檔案下載 </td> <td style="border-width: 1pt 1px 1px 1pt; border-style: solid; border-color: #009eb8; background-color: #ffffff; background-image: url(e); background-repeat: repeat; background-attachment: scroll; background-clip: border-box; background-origin: padding-box; background-size: auto auto; padding-right: 5pt; padding-left: 5pt; height: 17pt;"> <a class="link" href="http://www.richtek.com/Design%20Support/Technical%20Document/~/media/AN%20PDF/AN005_TW.ashx">PDF 下載</a> </td> </tr> </tbody> </table> </div></div>

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