Analysis of Buck Converter Efficiency

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<div class="WordSection1"> <h1>Analysis of Buck Converter Efficiency</h1> <h2 id="__Bookmark__0">Abstract</h2> The synchronous buck circuit is wildly used to provide non-isolated power for low voltage and high current supply to system chip. To realize the power loss of synchronous buck converter and to improve efficiency is important for power designer. The application note introduces the analysis of buck converter efficiency and realizes major power component loss in synchronous buck converter. <h2 id="__Bookmark__1">Buck converter power loss analysis</h2> To realize the power loss in converters is important for converter design optimization. Figure1 shows the general single phase synchronous buck converter circuit. The major power losses in synchronous buck converter circuit are listed as bellow : A : Power semiconductor loss B : Inductor loss C : Driver loss D : PCB trace loss <a href="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-cn/image001_zh-cn.png" class="fancybox"><img width="453" height="250" alt="Technical Document Image Preview" src="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-cn/image001_zh-cn.png" /></a> Figure 1. Synchronous buck converter <h2 id="__Bookmark__2">Power loss calculation</h2> Low-power loss and highly efficient synchronous buck converters are in great demand for advanced micro-processors. The application note introduces and provides how to calculate the majority of power losses in a typical synchronous buck converter occur in the following components based on that the converter works in continuous conduction mode (CCM) fixed switching frequency, fixed input voltage and fixed output voltage. A : Power semiconductor loss : HMOS (High-Side MOSFET) summarizes to include : switching on & off and conduction loss. LMOS (Low-Side MOSFET) summarizes to include : conduction, dead-time and reverse recovery charge loss. HMOS switching on loss : <a href="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-cn/image002_zh-cn.png" class="fancybox"><img width="430" height="226" alt="Technical Document Image Preview" src="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-cn/image002_zh-cn.png" /></a> Figure 2. HMOS Driver switching on <a href="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-cn/image003_zh-cn.png" class="fancybox"><img width="314" height="305" alt="Technical Document Image Preview" src="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-cn/image003_zh-cn.png" /></a> Figure 3. HMOS switching on loss area <a href="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-cn/image004_zh-cn.png" class="fancybox"><img width="319" height="141" alt="Technical Document Image Preview" src="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-cn/image004_zh-cn.png" /></a> HMOS conduction loss : The conduction loss of high-side MOSFET is determined by the on-resistances of the MOSFET and the transistor RMS current. <a href="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-cn/image005_zh-cn.png" class="fancybox"><img width="441" height="224" alt="Technical Document Image Preview" src="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-cn/image005_zh-cn.png" /></a> Figure 4. HMOS conduction on <a href="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-cn/image006_zh-cn.png" class="fancybox"><img width="323" height="247" alt="Technical Document Image Preview" src="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-cn/image006_zh-cn.png" /></a> Figure 5. HMOS conduction on period <a href="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-cn/image007_zh-cn.png" class="fancybox"><img width="221" height="132" alt="Technical Document Image Preview" src="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-cn/image007_zh-cn.png" /></a> LMOS conduction loss : <a href="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-cn/image008_zh-cn.png" class="fancybox"><img width="444" height="230" alt="Technical Document Image Preview" src="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-cn/image008_zh-cn.png" /></a> Figure 6. LMOS conduction on <a href="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-cn/image009_zh-cn.png" class="fancybox"><img width="336" height="264" alt="Technical Document Image Preview" src="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-cn/image009_zh-cn.png" /></a> Figure 7. LMOS conduction on period <a href="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-cn/image010_zh-cn.png" class="fancybox"><img width="250" height="142" alt="Technical Document Image Preview" src="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-cn/image010_zh-cn.png" /></a> LMOS dead time body diode loss : Dead-time loss is induced by LMOS body diode conduction during dead-times. <a href="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-cn/image011_zh-cn.png" class="fancybox"><img width="450" height="232" alt="Technical Document Image Preview" src="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-cn/image011_zh-cn.png" /></a> Figure 8. LMOS body diode conduction on <a href="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-cn/image012_zh-cn.png" class="fancybox"><img width="346" height="289" alt="Technical Document Image Preview" src="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-cn/image012_zh-cn.png" /></a>: Figure 9. LMOS body diode conduction on period <a href="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-cn/image013_zh-cn.png" class="fancybox"><img width="386" height="83" alt="Technical Document Image Preview" src="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-cn/image013_zh-cn.png" /></a> LMOS reverse recovery charge loss : <a href="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-cn/image014_zh-cn.png" class="fancybox"><img width="416" height="254" alt="Technical Document Image Preview" src="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-cn/image014_zh-cn.png" /></a> Figure 10. LMOS body diode reverse recovery period <a href="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-cn/image015_zh-cn.png" class="fancybox"><img width="204" height="37" alt="Technical Document Image Preview" src="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-cn/image015_zh-cn.png" /></a> B : Inductor DC & AC loss Inductor DC loss : <a href="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-cn/image016_zh-cn.png" class="fancybox"><img width="412" height="215" alt="Technical Document Image Preview" src="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-cn/image016_zh-cn.png" /></a> Figure 11. Current through inductor path <a href="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-cn/image017_zh-cn.png" class="fancybox"><img width="334" height="265" alt="Technical Document Image Preview" src="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-cn/image017_zh-cn.png" /></a> Figure 12. Inductor current path period <a href="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-cn/image018_zh-cn.png" class="fancybox"><img width="191" height="114" alt="Technical Document Image Preview" src="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-cn/image018_zh-cn.png" /></a> Inductor core loss : Inductor core losses are major caused by an alternating magnetic field in the core material. The losses are a function of the operating frequency and the total magnetic flux swing. The core loss may vary from one magnetic material to another. <a href="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-cn/image019_zh-cn.png" class="fancybox"><img width="292" height="239" alt="Technical Document Image Preview" src="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-cn/image019_zh-cn.png" /></a> Figure 13. Inductor ripple current <a href="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-cn/image020_zh-cn.png" class="fancybox"><img width="341" height="215" alt="Technical Document Image Preview" src="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-cn/image020_zh-cn.png" /></a> Figure 14. Core loss curve The calculated and/or measured core loss is often directly provided by the inductor supplier. If not, a formula can be used to calculate the core loss as bellow : <a href="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-cn/image021_zh-cn.png" class="fancybox"><img width="131" height="79" alt="Technical Document Image Preview" src="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-cn/image021_zh-cn.png" /></a> The PL is the power loss (mW), Fsw : operating frequency B : peak flux desity in Gauss Ve : effective core volume The specific value of C, X and Y are core loss parameters for each material C: Gate driver loss : The gate driver loss is straightforward given by MOSFET driver to charge /discharge total HMOS and LMOS Qg. The gate driver loss is depending on MOSFET total gate charge, driver voltage and Fsw. <a href="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-cn/image022_zh-cn.png" class="fancybox"><img width="383" height="216" alt="Technical Document Image Preview" src="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-cn/image022_zh-cn.png" /></a> Figure 15. Driver turns on and off path <a href="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-cn/image023_zh-cn.png" class="fancybox"><img width="342" height="321" alt="Technical Document Image Preview" src="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-cn/image023_zh-cn.png" /></a> Figure 16. MOSFET driver on <a href="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-cn/image024_zh-cn.png" class="fancybox"><img width="339" height="359" alt="Technical Document Image Preview" src="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-cn/image024_zh-cn.png" /></a> Figure 17. MOSFET driver off <a href="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-cn/image025_zh-cn.png" class="fancybox"><img width="243" height="60" alt="Technical Document Image Preview" src="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-cn/image025_zh-cn.png" /></a> D : PCB loss : Figure 18 could be illustrated as Figure 19 and Figure 20 with Rtr1~Rtr7 with loop1 (HMOS conduction) and loop2 (LMOS conduction) in detail. <a href="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-cn/image026_zh-cn.png" class="fancybox"><img width="327" height="161" alt="Technical Document Image Preview" src="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-cn/image026_zh-cn.png" /></a> Figure 18. PCB trace diagram <a href="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-cn/image027_zh-cn.png" class="fancybox"><img width="345" height="184" alt="Technical Document Image Preview" src="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-cn/image027_zh-cn.png" /></a> Figure 19. PCB loop1 trace <a href="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-cn/image028_zh-cn.png" class="fancybox"><img width="347" height="182" alt="Technical Document Image Preview" src="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-cn/image028_zh-cn.png" /></a> Figure 20. PCB loop2 trace <a href="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-cn/image029_zh-cn.png" class="fancybox"><img width="493" height="206" alt="Technical Document Image Preview" src="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-cn/image029_zh-cn.png" /></a> <h2 id="__Bookmark__3">Power loss measurement and calculation comparison</h2> Although the buck converter power loss calculated equations are well introduced and documented. In order to check the accuracy of these power loss equations, Table1 shows the typical buck converter application parameter and Figure 21 illustrates the efficiency comparison between measurement and calculation. Table 1. Buck converter application parameter <div> <table border="1" cellspacing="0" cellpadding="0" style="border-collapse: collapse; border: medium none;" class="MsoNormalTable"> <tbody> <tr> <td style="width: 97pt; border: 1pt solid #009eb8; padding-right: 5pt; padding-left: 5pt;"> IC </td> <td style="width: 97pt; border: 1pt solid #009eb8; padding-right: 5pt; padding-left: 5pt;"> RT8120 </td> </tr> <tr> <td style="width: 97pt; border: 1pt solid #009eb8; padding-right: 5pt; padding-left: 5pt;"> VIN </td> <td style="width: 97pt; border: 1pt solid #009eb8; padding-right: 5pt; padding-left: 5pt;"> 12V </td> </tr> <tr> <td style="width: 97pt; border: 1pt solid #009eb8; padding-right: 5pt; padding-left: 5pt;"> Vout </td> <td style="width: 97pt; border: 1pt solid #009eb8; padding-right: 5pt; padding-left: 5pt;"> 1.2V </td> </tr> <tr> <td style="width: 97pt; border: 1pt solid #009eb8; padding-right: 5pt; padding-left: 5pt;"> FSW </td> <td style="width: 97pt; border: 1pt solid #009eb8; padding-right: 5pt; padding-left: 5pt;"> 300kHz </td> </tr> <tr> <td style="width: 97pt; border: 1pt solid #009eb8; padding-right: 5pt; padding-left: 5pt;"> VDD </td> <td style="width: 97pt; border: 1pt solid #009eb8; padding-right: 5pt; padding-left: 5pt;"> 12V </td> </tr> <tr> <td style="width: 97pt; border: 1pt solid #009eb8; padding-right: 5pt; padding-left: 5pt;"> L </td> <td style="width: 97pt; border: 1pt solid #009eb8; padding-right: 5pt; padding-left: 5pt;"> 1mH </td> </tr> <tr> <td style="width: 97pt; border: 1pt solid #009eb8; padding-right: 5pt; padding-left: 5pt;"> DCR </td> <td style="width: 97pt; border: 1pt solid #009eb8; padding-right: 5pt; padding-left: 5pt;"> 1.2mW </td> </tr> <tr> <td style="width: 97pt; border: 1pt solid #009eb8; padding-right: 5pt; padding-left: 5pt;"> HMOS </td> <td style="width: 97pt; border: 1pt solid #009eb8; padding-right: 5pt; padding-left: 5pt;"> BSC090N03LS </td> </tr> <tr> <td style="width: 97pt; border: 1pt solid #009eb8; padding-right: 5pt; padding-left: 5pt;"> LMOS </td> <td style="width: 97pt; border: 1pt solid #009eb8; 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-cn/image030_zh-cn.png" class="fancybox"><img width="451" height="318" alt="Technical Document Image Preview" src="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-cn/image030_zh-cn.png" /></a> Figure 21. Measurement and calculation of efficiency comparison Figure 22 shows the key component loss in buck converter including HMOS, LMOS, inductor, driver and PCB trace loss. Readers can check what the major loss contributed in each system loading. <a href="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-cn/image031_zh-cn.png" class="fancybox"><img width="445" height="337" alt="Technical Document Image Preview" src="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-cn/image031_zh-cn.png" /></a> Figure 22. Key component loss in buck converter Figure 23 shows detail component loss in buck converter and illustrates the loss v.s Iout in the curve. HMOS : PHSW (Switching loss) and PHCOD (Conduction loss) LMOS : PLCOD (Conduction loss), PL_DIODE (Dead-time body diode loss) and PRR (Reverse recovery loss) Inductor : PL (Inductor DC & core loss) Driver : PDRV (Gate driver charge loss) PCB : PPCB (PCB trace loss) <a href="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-cn/image032_zh-cn.png" class="fancybox"><img width="429" height="314" alt="Technical Document Image Preview" src="/content/dam/richtek-eds/resources/application-notes/an005/html-images-zh-cn/image032_zh-cn.png" /></a> Figure 23. Detail power loss in buck converter <h2 id="__Bookmark__4">Conclusion</h2> This application document analyzes power loss in synchronous buck converters and presents the detailed calculations for each part of the power loss. The loss calculation also compares with real buck converter measurement and provides the key component loss data to consider how to improve the buck converter efficiency for component and PCB plane consideration. </div> <div class="WordSection2"> <h2 id="__Bookmark__5">Reference</h2> </div> <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" class="3-51" style="width: 637.5pt; border-collapse: collapse; border: medium none;"> <tbody> <tr style="height: 17pt;"> <td colspan="2" 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;"> Next Steps </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;"> Richtek Newsletter </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 target="_blank" href="http://www.richtek.com/My%20Richtek/Newsletters" class="link">Subscribe Richtek Newsletter</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;"> Download </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 href="~/media/AN%20PDF/AN005_EN.ashx" class="link">Download PDF</a> </td> </tr> </tbody> </table> </div>

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