順向式變壓器設計原理

順向式變壓器設計原理FORWARDTRANSFORMERDESIGNTHEORY概述工作原理設計方法設計實例小結第一節概述順向式FORWARD轉換器又稱單端正激式或“BUCK“式轉換器因其在原邊繞組接通電源VIN的同時繞組把能量傳遞到輸出端故而得名FORWARD變換器中的變壓器是一個純粹的隔離變壓器因此,在副邊輸出端須附加儲能電感器L,用以儲存及傳送能量FORWARD變壓器之轉換功率通常在50500W之間其優點有1正激式變壓器通常使用無氣隙的CORE,電感值L較高,原副邊繞組之峰值電流較小LI因而銅損較小2開關管TR的峰值電流較低開關損耗小3适用于低壓大電流功率較大的場合第二節工作原理正激變換器的主回路如圖1當開關管TR導通時原邊繞組NP有電流IP流過,因副邊繞組NS与NP有相同的同銘端故副邊繞組通過D2把能量傳遞到輸出端當TR關斷時續流二极管D3導通釋放電感L中的能量給負載在TRTON時,變壓器原邊電流IPIMILOAD其中磁化電流IM是無法傳送到副邊的能量在TRTOFF期間此磁能無法被泄放,磁化能量將引起較高的反壓加在TR之CE极間而損壞TR另一方面磁化能量的存在將使變壓器CORE趨于飽和,產生很大的集電极電流IC,使TR損壞為解決上述問題,通常在變壓器中設置一消磁繞組NR,將磁化能量反饋到電源輸入端當TRTON時,儲能電感L內的電流將直線增加,如下式所示DIL/DTVSVO/L而TR集電极電流ICIP可用下式表示ICIPILOADIMIL/NTSDMAXVIN/L式中N初級與次級之匝數比NP/NSIL輸出電感電流,即輸出負載電流AIM磁化電流ATS工作周期TS1/FSSDMAX最大導通占空比DMAXTON/TSL輸出電感器之電感值UHVIN輸入直流電壓V順向式變壓器設計原理FORWARDTRANSFORMERDESIGNTHEORYFORWARDTRANSFORMERDESIGNPRINCIPLE1/10LISCOCT變壓器磁化電流可由下式求得IMVINTON/LMVINTSDMAX/LM因為VOUTDMAXVIN/NUEND/DTNAEDB/DTD/DTLDI/DT而VINNVOUT/DMAX所以IM0TVINDT/LNTSVOUT/LM則IC之關系式可改寫為ICIPIL/NNTSVOUT/LM若忽略磁化電流部分,原邊峰值電流IC為ICIPIL/N2POUT/VINDMAX式中ILIO負截電流APOUT輸出功率POUTVOIOW設80DMAX04則IC62POUT/VIN當TR導通時間結束時,副邊峰值電流IS為ISILTONVSVOVF/2LVF二极管正向壓降在能量轉換過程中,次級電流對磁芯起去磁作用,初級電流僅有很小一部分用來磁化磁芯依據變壓器原理,次級在初級有反射電流ISISNSIS/NPIS/N則NPISNSIS如果激磁電感LM為常數,激磁電流IM線性增長,并等于原邊電流與反射電流之差IMVINTON/LMIPISIPISNS/NP磁化電流在導通時間結束時達到最大,當TRTOFF時,副邊感應電勢反向,二級體D2截止IS0,TON期間存儲在磁場中的激磁能量ERLI2M/2在TOFF時應有釋放通路,且須保持與儲能時間相同因為當正負伏秒值相同時IM方才等于零,如此,复位時間TR為TRVINTON/ERNRTON/NP式中NR為消磁繞組圈數因為NRNP則TRTON,所以DMAX需低于50第三節FORWARD變壓器設計方法一FORWARDTRANSFOTMER設計時之考慮因素1鐵芯飽和問題選用飽和磁通密度BS盡量高,剩余磁通BR盡量低的CORE,使其能承受大的磁場也就是大的電流,實現小體積大功率2電壓的準位性在多路輸出變壓器中,各繞組的伏特秒盡量保證一致,各繞組之電流密度應保持一致,使損耗有相同值BA9B5981EA78BC04FE4FF4E4E85C4B3A2/10LISCOCT3傳輸功率應考量在額定輸出功率下應留有一定余量,通常功率余量不應小于104電流容量有足夠的電流容量,以減小耗損5工作頻率將決定CORE的B和導線直徑6磁化電流IM應使磁化電流盡可能低,激磁電感盡量大所以需用高磁導率的CORE7損耗PPPFEPCUA銅損PCU包括低頻損耗和高頻損耗,低頻損耗很容易計算,也比較容易解決,通過增大導體截面積減小RDC即可降低損耗線圈的高頻損耗因涉及渦流損耗趨膚效應,鄰近效應等問題很難精確確定PCUI2RMSRHFRHF高頻時導體的有效阻抗從上式可見有效電流IRMS正比于PCU,而IRMSIPPD即PCU正比于D,反比于VIN在VIN最低時PCU最大B鐵損PFE又包括磁滯損和渦流損磁滯損正比于頻率和磁感應擺幅B渦流損與每匝伏特數和占空度D有關,而与頻率無關VINNPD/DT即VIN/NPD/DT可見渦流損耗与磁通變化率成正比8溫升變壓器損耗使得線圈與磁芯溫度升高,溫升又使損耗盡一步增加,如此惡性循環將導致變壓器損壞因此,設計時必須限制溫升在一個可接受的範圍變壓器溫升循環圖如圖2溫升對CORE之功率損失特性圖參照各廠商之DATABOOK9漏電感在實際變壓器中因磁通的不完全耦合而產生漏磁通轉換成漏電感形式存在變壓器中,漏電感LK之關係式LKUOURAN2/102上式中LK漏電感銅窗之排線寬度CMA兩繞組間之剖面積CMUR1相對磁導率UO4107N匝數因漏感是一個限制電流IP通過的阻抗所以它將影響變壓器的電壓準位特性同時漏電感所存能量在TROFF時將釋放,產生尖峰電壓,造成元件損壞和電磁干擾,采用吸收電路後將使效率降低,因此在設計變壓器時,應於CORE選擇繞組結構,工藝工法上設法減小漏感10分布電容或稱雜散電容分布電容的存在在電源轉換過程中,會傳輸繞組間的共模雜訊,增加原副邊的漏電流在通信變壓器中,雜散電容影響信號的頻率響應高頻變壓器中的雜散電容包括ACWTOCOREBCWTOWCCLAYETOLAYEDC匝間等因降低雜散電容与減小漏感相互矛盾故設計時須根據用途權衡利弊做取舍BA9B5981EA78BC04FE4FF4E4E85C4B3A3/10LISCOCT溫度升高T235P/APORTRTHPBMAX下降BMBSBR下降B/HLM下降LMKN2IM上升IMVINTON/LMIP上升IPIMILOADB上升,鐵損上升I2R增大,PCU上升BH04NI/LE特性評估PPCUPFE至CORE達溫升太高而飽和圖2變壓器溫升循環圖BA9B5981EA78BC04FE4FF4E4E85C4B3A4/10LISCOCT二變壓器設計流程決定CORE材質高BS,低損耗,低成本決定CORE規格APPS104/2BFJKU設計圈數比NVINDMAX/VOVF初次級圈數設計NPVINTON/BAENSNP/N計算線徑DW4AW/溫升計算T235P/AP決定繞線結構綜合考慮LK,C分布,易制性特性評估效率,頻寬,電壓準位EMI第四節FORWARDTRANSFORMER設計實例一設計步驟STEP0SPECVINFSVOIODTPO限制STEP1選擇CORE材質決定BSTEP2計算CORE之AP值,確定CORE型號規格STEP3計算NPNSSTEP4計算線徑DW,估算銅窗占有率STEP5估算損耗溫升STEP6結構設計STEP7樣品制作STEP8性能評估DQBA9B5981EA78BC04FE4FF4E4E85C4B3A5/10LISCOCT三設計舉例STEP0取得相關規格SPEC例155WPCPOWERMAINXFMRSPECINPUTAC180265V50HZOUTPUTDC5V15A33V12A12V42A68,FS100KHZ電路接線圖如圖3風冷散熱STEP1選擇CORE材質決定B功率變壓器所用功率鐵芯應選擇高I低損高BS材料目前因軟磁鐵氧體具備以上要求而被得以廣泛應用在此選用TDK之PC40材質其相關參數PCV410KW/M3100KHZ正弦波I230025BS390MTBR55MT100因FORWARD電路之磁芯為單向磁化,要使CORE不飽和,磁芯中磁通密度最大變化量為BBSBR故PC40材之B39055335MT但實際應用中由於有高溫效應,瞬變情況等引起BS,BR的變化,使B動態範圍變小而出現飽和,因此,設計時必須留一些安全空間,通常選擇75BSBR,用以限制飽和,此方法可使PFE略小於PCUB選得過小會使匝數增加,PCU增大,產品体積變大,但B選得過高,則PFE將增加且易飽和PC40材最高可取B300MT此時PFE稍高,可調節電路導通比TON/TSD來解決鐵損問題本例選擇75BMB39055075251MT025TSTEP2確定COREAP值決定CORE規格型號APAWAEPS104/2BFSJKU式中AWCORE之銅窗面積CM2AECORE有效截面積CM2PS變壓器傳遞視在功率WPSPO/PO正激式B磁感應增量TFS變壓器工作頻率HZJ電流密度A根據散熱方式不同可取300600A/CM2KU銅窗占用系數取02100WMAXTOALT155WBA9B5981EA78BC04FE4FF4E4E85C4B3A6/10LISCOCT本例選擇B025TFS100103HZJ400A/CM2AP155068155104/202510010340002096CM4上式中之銅窗占有系數KU是以04AW可用,且原副邊繞組各占用50而定的若副邊繞組數過多或占用率超過可用空間的一半時,可適當調大AP值選擇CORE規格查閱TDKDATABOOK選用COREERL28PC40其參數為AP120CM4AE814MM2AW148MM2VE6143MM3AL252025PT228WSTEP3計算NPNS1計算匝比NNP/NS設DMAX035NNP/NSVI/VOVINMINDMAX/VOVF式中VINMIDCVF二极管正向壓降N209035/51121912CHECKDMAXDMAXNVOVF/VINMIN1251/20903440342計算NPNPVINMINTON/BAE2090341/100103/02581434935TSTONDMAXTSUSAE有效截面積MM2TS1/FSUS3計算NS5VNSNP/N3512292取整為TS4CHECKNP（以NS驗算NPNPNSN31236TS取NP36TS從電路原理圖可知,本例之33V与5V使用同一副邊繞組33V另加磁放大器調整故不再計算NS335計算NS12VNS12VVOVFNPTS/VINMINTON1213610/209346597TS6確定NRNRNP36TSBA9B5981EA78BC04FE4FF4E4E85C4B3A7/10LISCOCT通過本步序NPNS已全部確定須再CHECKB之選擇合理性NP36TSNR36TSNS53TSNS127TSBVINMINDMAXTS/NPAE20930410/36814024TBBRBS02400550295039OKSTEP4計算線徑DW1求初級線徑DWPIPPI/VLPO/DMAXVIN155/068034209321AIPRMSIPD321034187AAWPI/J187/50374MM2DWP4AWP/40374/314069MM07MMOR04522求NR繞組線徑DWRNR36TSLN2ALL362252007524MHIMVINTON/L20934/2410303AAWN03/5006MM2DWN4006/3140276MM028MM3求繞組NS5之線徑DWS5AWS5I/J因33V与5V為同一繞組,功率限制為100W故可近似求得IO100/520AISRMS20034117AAWS5117/5234MM2此導體截面積較大,不適合圓銅線繞制,采用銅片查ERL28BOBBIN幅寬223MM03MM考量扣除擋牆約4MM,則有22418MM之可繞寬度,預留適當空間1MM則234MM217MM014MM選擇6MILS17MM之COPPERFOIL4求繞組NS12之線徑DWS12IRMSIODMAX42034245AAWS122455049MM2DWS124049/314079MM080MMBA9B5981EA78BC04FE4FF4E4E85C4B3A8/10LISCOCT考量FS100KHZ頻率較高估算趨膚效應時之電流穿透深度661/F6611000000209MM此為20之穿透深度100約為025MM則可調整DWS12404MMSTEP5估算溫升,損耗1銅損PCUPCUI2RPCU2I2PRMSNPLPX式中IPRMS原邊有效電流ANP初級匝數LP線圈平均匝長CMERL28之LP488X100時之銅阻/CM上式是以2倍原邊銅損為估算基準,求得之PCUPCU2141236488000066046W2鐵芯損耗PFEPFEPVPEPV單位體積損耗W/CM3VECORE體積CM3PV可從供應商之DATABOOK查得,查TDKDATABOOK可知當B024TFS100KHZ時PV041W/CM,則PFEPVVE0416143252W3PPCUPFE046252298W4溫升TT235P/AP235312/1264估算結果,溫升較高,須降低溫升,可重新選擇B,調整NPNSSTEP6結構設計方法一NR120/401202836TS1LNP220/40450736TS1LSHI30/40S12MILS171TS3LNS540/406786MILS173TS1LNS12520/401094044TS3LBA9B5981EA78BC04FE4FF4E4E85C4B3A9/10LISCOCT方法二NR120/401202836TS1LNP220/40450436TS1LSHI30/40S12MILS171TS3LNS540/406786MILS173TS1LN12520/401090444TS3LSHI60/40S12MILS172MILS181LNP720/404504536TS3L第五節小結1作為一個合格的變壓器設計者,應具備理論与實踐相融匯之實作能力2B之取值對CORE之體積,耗損,工作穩定性都有直接影響3導線之電流密度取值大小受COREAP值限制,決定於散熱方式,同一變壓器中電流密度盡量取同一值,在空間允許之狀況下,可盡量調小電流密度取值4CORE之面積乘積AP由功率容量導出,但實際選取CORE時,不僅是單單考慮CORE之傳遞功率,其規格還必須兼顧有足夠的繞線空間,外圍呎寸限制,可配合BOBBIN之引腳數量,呎寸,形狀等5實際設計之變壓器效率通常不應低於90,必須大於轉換器總效率,溫升應盡可能低6最優化之設計應符合,最小的體積,最低的溫升與成本BA9B5981EA78BC04FE4FF4E4E85C4B3A10/10LISCOCTCORE參數對照表DIMENSIONSMMAPAEAWALLEVEWTPCL100KHZ200MTPT100KHZABCCM4MM2MM2NH/N2MMMM3G100WWATTS幅寬PIN形狀TYPEECCOREEC353C853531739513741843016300210000774065300038002158HEC413C854161951162589412100214002700008930108000060002458HEC523C855222421345598018000311003600001050018800001120028312HEC703C8571734516417828127900639003900001440040100002540041412/34HTYPEEECOREEE05PC40525265195000132635002850012603310016002112768HEE63PC4061285795000153314464050012204040024002276HEE8PC4083403600091700130559000194713900070006194786HEE10/11PC4010255475002871210237085000266030200150016668VEE13PC4013060615005701710333511300030205170027002357410VEE16PC40167248007651920398511400035006720033003185610VHEE19PC40191795500124323005404125000394090000480042968VHEE19/16PC401929814750119122405315135000391088200480041968VHEE20/20/5PC402015105101572310050701460004300134000750051EE22PC4022935575015904100387921800039401610008800618458VEE2329SPC402314760436835801220012500064902320001200116EE25/19PC402549466290312840007820200000487019400091009EE254PC4025496663503173403078732000004870196300100009EE2825PC402812751060852586909810330000577050100026002519610VEE30PC403013151070799510900733546900057706310003200291371012VEE30/30/7PC40301157050745559701248721000066904000002200151EE3528PC4034614393133988480158002600006970591000290029615712VEE40PC404017107220001270017323415000770098100050004217312VEE4133PC4041517127282601570018000420000790012470006400625EE42/21/15PC40422121549484178002780038000097901951000880088EE42/21/20PC4042212206462523500275005000009780230000011600116EE47/39PC40471219631562475292420019640666000906021930001080097EE50PC405021314657343226002537361100095802160000116009421312VEE55/55/21PC405515275207136764354003863471000012300437000023400110150MTEE57/47PC4056572361889713234400282368530001020035100001900085EE60PC40602231569855824700399025670001100027100001350012523812VEE503PC40503256611844712085152642900001049012676006800583282512HEE623/62/6PC40623316130330153011982231000012574192400010200885338512HEE65/32/27PC4065153252730762553500575008000001470078700003990059100MTTYPEEFCOREEF126PC4012764360031113002390810002960385002000173510VEF16PC40161805450080020103982110000376075400390032EF20PC40209956501013335030241570004490150000740069TYPEMATERIAL可配合BOBBINCORE參數對照表DIMENSIONSMMAPAEAWALLEVEWTPCL100KHZ200MTPT100KHZABCCM4MM2MM2NH/N2MMMM3G100WWATTS幅寬PIN形狀TYPEMATERIAL可配合BOBBINEF25PC40250512557202376518045872000005780299000150014EF32PC4032116191506515832078302590007430618000320029TYPEEFDCOREEFD103F310552270008472011605000023701710004500268HEFD123F31256235001871140163870000285032500090004768HEFD153F31575465004701500313578000340051000140006888HEFD203C852010665015503100500013000047001460003500271358HEFD253C9025125910393858006789220000570033000080003816410HEFD303C9030159106028690087362100006800470000120005420112HTYPEEICOREEI125PC401247448500235144016321200002130308001900123510VEI16PC4016012248008391980423711000034606700033003185610HVEI19PC4020135550013052400543614000039609500051004290568HVEI22PC40220145557501606420038242400003930163000980068451月8日VEI25PC40253155567503165410077192140004700192700980079988VEI22/19/6PC40220147575019803700535020000041801550008500648451月8日VEI28PC4028016751060600586006983430000482041450022001659610VEI30PC4020021251070820711100739446900058006440003400311371012VEI33/29/13PC403302375127158541185013379440000675080020041003516612VEI35PC403502425100133431014013159380000671068040036002851571月12日VEI3530PC40350242121659212200136003950006800835000430042EI40PC4040027251165233011480015744486000770011300006000481731月12日VEI50PC40500333514655218230002400861100094002160000115009221312VEI60PC406003585156980242470039686567000109002710000139001252381月12日VEI70PC4070054031637761869800541001050000145001015300051900761100MTTYPEEPCOREEP73C859437565001021070950112000155016500080003346HEP103C8511551760025511302257102500193021500110004568HEP133C851286590004561950234014750024204720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