<h1>Practical Tips How to Reduce EMI in Buck Converters</h1> <br> <p class=bady>AN076</p> <br> <p class=bady>Mitigating electromagnetic interference (EMI) is a common issue in switch-mode buck converters. EMI is usually caused by high-frequency current flow. This application note discusses EMI problem that arises from input currents and proposes a solution and different methods for EMI reduction. Towards the end, a practical guide on constructing simple EMI measurement tools is introduced, as well as insightful advice on how to effectively utilize these tools for testing purposes.</p> <br><br> <h2 id="__Bookmark__0">1 The Cause of EMI and its Solution</h2> <p class=MsoNormal>EMI in switch-mode buck converters is mostly caused by high frequency currents flowing in circuit loops.</p> <br> <div class="tb-system"> <table class=MsoTableGrid border=0 cellspacing=0 cellpadding=0 style='border-collapse:collapse;border:none'> <tr> <td width=337 style='width:252.7pt;padding:0cm 5.4pt 0cm 5.4pt'> <p class=MsoCaption><a class="fancybox" href="/content/dam/richtek-eds/resources/application-notes/an076/html-images-en/image001_en.jpg" ><img width=305 height=166 id="Picture 3" src="/content/dam/richtek-eds/resources/application-notes/an076/html-images-en/image001_en.jpg" alt="Buck current loops1.JPG"></a></p> </td> <td width=343 style='width:257.5pt;padding:0cm 5.4pt 0cm 5.4pt'> <p class=MsoNormal><a class="fancybox" href="/content/dam/richtek-eds/resources/application-notes/an076/html-images-en/image002_en.jpg" ><img width=337 height=174 id="Picture 13" src="/content/dam/richtek-eds/resources/application-notes/an076/html-images-en/image002_en.jpg" alt="Technical Document Image Preview"></a></p> </td> </tr> </table> </div> <p class=bady>Figure 1</p> <br> <p class=bady>The input current I1 has very high dI/dt, and can generate a lot of EMI over a broad spectrum. Area A1 as shown in Figure 1 should be made as small as possible. Cin should be placed as close as possible between IC VIN pin and GND pin, see Figure 2.</p> <p class=bady><a class="fancybox" href="/content/dam/richtek-eds/resources/application-notes/an076/html-images-en/image003_en.jpg" ><img width=509 height=189 id="Picture 19" src="/content/dam/richtek-eds/resources/application-notes/an076/html-images-en/image003_en.jpg" alt="一張含有 方案, 圖表, 螢幕擷取畫面, 電路 的圖片&#10;&#10;自動產生的描述"></a></p> <p class=MsoNormal>Figure 2</p> <br><br> <h2 id="__Bookmark__1">2 Other Ways to Reduce EMI in Buck Converters</h2> <div class="tb-system"> <table class=MsoTableGrid border=0 cellspacing=0 cellpadding=0 style='border-collapse:collapse;border:none'> <tr> <td width=347 style='width:260.45pt;padding:0cm 5.4pt 0cm 5.4pt'> <p class=MsoNoSpacing><a class="fancybox" href="/content/dam/richtek-eds/resources/application-notes/an076/html-images-en/image004_en.jpg" ><img width=322 height=175 id="Picture 22" src="/content/dam/richtek-eds/resources/application-notes/an076/html-images-en/image004_en.jpg" alt="一張含有 文字, 圖表, 方案, 行 的圖片&#10;&#10;自動產生的描述"></a></p> <p class=MsoCaption>Figure 3</p> </td> <td valign=top style='width:350pt; padding:0cm 5.4pt 0cm 5.4pt'> <p class=MsoNormal>Add a resistor Rboot in series with Cboot as shown in Figure 3.</p> <p class=MsoNormal>Rboot reduces the switch-on drive current of Q1, which increases the rise time of switch waveform, which reduces the higher harmonics of the switching current. The value of Rboot depends on the size of the high side MOSFET. For most applications around 5 ~ 10&#8486; is used. For smaller (higher Rdson) MOSFETs, larger Rboot values are allowed. Note that the slower MOSFET switch-on will increase switch losses and reduce efficiency.</p> </td> </tr> </table> </div> <br> <div class="tb-system"> <table class=MsoTableGrid border=0 cellspacing=0 cellpadding=0 style='border-collapse:collapse;border:none'> <tr> <td width=347 style='width:260.4pt;padding:0cm 5.4pt 0cm 5.4pt'> <p class=MsoNoSpacing><a name="_Toc60820261"><a class="fancybox" href="/content/dam/richtek-eds/resources/application-notes/an076/html-images-en/image005_en.jpg" ><img width=327 height=207 src="/content/dam/richtek-eds/resources/application-notes/an076/html-images-en/image005_en.jpg" alt="一張含有 文字, 圖表, 方案, 工程製圖 的圖片&#10;&#10;自動產生的描述"></a></a></p> <p class=MsoCaption>Figure 4</p> </td> <td valign=top style='width:350pt; padding:0cm 5.4pt 0cm 5.4pt'> <p class=MsoNormal>Add an RC snubber as close as possible between switch node and power ground.</p> <p class=MsoNormal>Rs will damp to the parasitic resonant LC circuit which consists of MOSFET capacitance and the parasitic inductance of the switching loop as shown in Figure 4. The optimal value for Rs depends on the total switch node capacitance and parasitic inductance. Rs normally ranges from 2.2&#8486; to 10&#8486;. </p> <p class=MsoNormal>The series capacitor Cs is chosen 3 ~ 4 times larger than the circuit parasitic capacitance. 470pF ~ 1nF is usually sufficient. </p> <p class=MsoNormal>After placing the RC snubber, be sure to check the total power loss of the circuit : The converter efficiency will drop, especially at high switching frequency and high input voltages.</p> </td> </tr> </table> </div> <br> <div class="tb-system"> <table class=MsoTableGrid border=0 cellspacing=0 cellpadding=0 style='border-collapse:collapse;border:none'> <tr> <td width=347 style='width:260.4pt;padding:0cm 5.4pt 0cm 5.4pt'> <p class=MsoNoSpacing><a class="fancybox" href="/content/dam/richtek-eds/resources/application-notes/an076/html-images-en/image006_en.jpg" ><img width=311 height=221 id="Picture 8" src="/content/dam/richtek-eds/resources/application-notes/an076/html-images-en/image006_en.jpg" alt="snubber RL.JPG"></a></p> <p class=MsoCaption>Figure 5</p> </td> <td valign=top style='width:350pt; padding:0cm 5.4pt 0cm 5.4pt'> <p class=MsoNormal>Add an RL snubber in series with the resonant circuit as shown in Figure 5. This will add a small amount of series resistance in the resonant circuit, sufficient to provide some damping. Ls can be a very small HF bead like BLM15AX100SN1 or BLM15PG100SN1 and must have sufficient current rating for the input RMS current. Rs normally ranges from 2.2&#8486; ~ 4.7&#8486;. </p> <p class=MsoNormal>The RL snubber must be placed close to the power stage input node, keeping the input loop small. One drawback of the RL snubber is that it creates impedance Rs in the switching loop the high frequency region. During the very fast switch transitions, the switch current pulse will create a short voltage glitch across Ls//Rs, resulting in a small voltage glitch on the power stage input node. Be sure to check the voltage glitch on the IC VIN node during maximum load switching when the RL snubber is added.</p> </td> </tr> </table> </div> <br> <p class=MsoNormal><a class="fancybox" href="/content/dam/richtek-eds/resources/application-notes/an076/html-images-en/image007_en.jpg" ><img width=700 id="Picture 5" src="/content/dam/richtek-eds/resources/application-notes/an076/html-images-en/image007_en.jpg" alt="Buck current loops3.JPG"></a></p> <p class=Figure>Figure 6</p> <br> <p class=MsoNormal>Input filtering is very important to reduce EMI. To reduce the voltage drop across Cin, use low ESR MLCC types and multiple capacitors of different size like 2x10&micro;F 1206 and one 22n ~ 100nF 0402 or 0603 size type close to the buck IC. To reduce the noise in the input loop, it is highly recommended to add extra L-C filtering in the input line. When using pure inductance for L2, it may be necessary to add the electrolytic capacitor C3 to damp any input supply ringing and ensure stable input supply.</p> <br><br> <h2 id="__Bookmark__2">3 Simple EMI Measuring Tools You Can Build Yourself</h2> <p class=MsoNormal>You can do near-field EMI measurements on the PCB by using a small loop antenna. A small electrically shielded loop antenna can be easily made yourself using a length of thin 50&#8486; coaxial cable: see Figure 7.</p> <br> <p class=MsoNormal><a class="fancybox" href="/content/dam/richtek-eds/resources/application-notes/an076/html-images-en/image008_en.jpg" ><img width=597 height=126 id="Picture 9" src="/content/dam/richtek-eds/resources/application-notes/an076/html-images-en/image008_en.jpg" alt="loop tool.JPG"></a></p> <p class=Figure>Figure 7. High Frequency Current Loop</p> <br> <p class=MsoNormal>The loop antenna can be connected to a spectrum analyzer and by moving the loop over the PCB of your application, you can see which areas emit a lot of high frequency magnetic field. You can also connect the loop antenna to an oscilloscope (terminated in 50&#8486;) and the oscilloscope will show switching noise levels in certain areas of the PCB. By keeping the loop at a fixed distance and place, and making some changes to circuits/PCB loops, you can check whether radiated noise levels will increase or reduce. </p> <p class=MsoNormal>The high frequency current in the converter input wiring is a good indication of radiated EMI. You can make a high frequency current probe by placing a couple of windings on an EMI ferrite core: these will form a high frequency current transformer. Construction is similar to the loop tool, but now the loop has 3 turns around the ferrite core. See Figure 8.</p> <br> <div class="tb-system"> <table class=MsoTableGrid border=0 cellspacing=0 cellpadding=0 style='border-collapse:collapse;border:none'> <tr> <td width=359 valign=top style='width:268.95pt;padding:0cm 5.4pt 0cm 5.4pt'> <p class=MsoNormal><a class="fancybox" href="/content/dam/richtek-eds/resources/application-notes/an076/html-images-en/image009_en.jpg" ><img width=358 height=133 id="Picture 10" src="/content/dam/richtek-eds/resources/application-notes/an076/html-images-en/image009_en.jpg" alt="tool transformer.JPG"></a></p> </td> <td width=322 valign=top style='width:241.25pt;padding:0cm 5.4pt 0cm 5.4pt'> <p class=MsoNormal><a class="fancybox" href="/content/dam/richtek-eds/resources/application-notes/an076/html-images-en/image010_en.jpg" ><img width=311 height=130 id="Picture 14" src="/content/dam/richtek-eds/resources/application-notes/an076/html-images-en/image010_en.jpg" alt="Technical Document Image Preview"></a></p> </td> </tr> </table> </div> <p class=Figure>Figure 8. High Frequency Current Probe Construction</p> <br> <p class=MsoNormal>The high frequency current in cables can now be measured by passing a cable through the ferrite core. The current transformer output can be connected to spectrum analyzer or oscilloscope (terminated in 50&#8486;). </p> <p class=MsoNormal>To avoid common mode current flowing from the tested device to the measurement equipment, it is recommended to add a common mode choke in the cable: this can be done by placing a clip-on EMI core with a couple of windings in the cable to the analyzer. The input common mode measurement is shown in Figure 9.</p> <br> <p class=MsoNormal><a class="fancybox" href="/content/dam/richtek-eds/resources/application-notes/an076/html-images-en/image011_en.jpg" ><img width=551 height=221 id="Picture 7" src="/content/dam/richtek-eds/resources/application-notes/an076/html-images-en/image011_en.jpg" alt="一張含有 文字, 圖表, 行, Rectangle 的圖片&#10;&#10;自動產生的描述"></a></p> <p class=MsoNormal>Figure 9</p> <br> <p class=MsoNormal>For more information, please read the full application note <a href="http://www.richtek.com/Design%20Support/Technical%20Document/AN045?utm_source=DesignTip&amp;utm_medium=PDF&amp;utm_campaign=DT001" class="link">Reducing EMI in buck converters</a></p> <br><br><br> <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; padding-right: 5pt; padding-left: 5pt; height: 17pt;" colspan="2"> <span style="color: #ffffff;">Next Steps</span> </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; 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; padding-right: 5pt; padding-left: 5pt; height: 17pt;"> <a class="link" href="http://www.richtek.com/My%20Richtek/Newsletters" target="_blank">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; 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; padding-right: 5pt; padding-left: 5pt; height: 17pt;"> <a class="link" href="~/media/5F00C8F6B70D48F8B28E781C7F35D831.ashx">Download PDF</a> </td> </tr> </tbody> </table> <br><br><br>

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