電子-電流-外文翻譯-外文文獻(xiàn)-英文文獻(xiàn)

高精度穩(wěn)壓直流電源文摘：目前對于可調(diào)式直流電源的設(shè)計和應(yīng)用現(xiàn)在有很多微妙的，多種多樣的，有趣的問題。探討這些問題（特別是和中發(fā)電機(jī)組有關(guān)），重點(diǎn)是在電路的經(jīng)濟(jì)適用性上，而不是要達(dá)到最好的性能。當(dāng)然，對那些精密程度要求很高的除外。討論的問題包括溫度系數(shù)，短期漂移，熱漂移，瞬態(tài)響應(yīng)變性遙感和開關(guān)preregualtor型機(jī)組及和它的性能特點(diǎn)有關(guān)的的一些科目。介紹從商業(yè)的角度來看供電領(lǐng)域可以得到這樣一個事實(shí)，在相對較低的成本下就可以可以獲得標(biāo)準(zhǔn)類型的0.01%供電調(diào)節(jié)。大部分的供電用戶并不需要這么高的規(guī)格，但是供應(yīng)商不會為了減少客戶這么一點(diǎn)的費(fèi)用而把0.1%改成0.01%。并且電力供應(yīng)的性能還包括其他一些因素，比如說線路和負(fù)載調(diào)解率。本文將討論關(guān)于溫度系數(shù)、短期漂移、熱漂移,和瞬態(tài)的一些內(nèi)容。目前中等功率直流電源通常采用預(yù)穩(wěn)壓來提高功率/體積比和成本，但是只有某些電力供應(yīng)采用這樣的做法。這種技術(shù)的優(yōu)缺點(diǎn)還有待觀察。溫度系數(shù)十年以前，大多數(shù)的商業(yè)電力供應(yīng)為規(guī)定的0.25%到1%。這里將氣體二極管的溫度系數(shù)定位百分之0.01[1]。因此，人們往往會忽視TC（溫度系數(shù)）是比規(guī)定的要小的?，F(xiàn)在參考的TC往往比規(guī)定的要大的多。為了費(fèi)用的減少，后者會有很大的提高，但是這并不是真正的TC。因此，如果成本要保持在一個低的水平，可以采用TC非常低的齊納二極管，安裝上差動放大電路，還要仔細(xì)的分析低TC繞線電阻器。如圖1所示，一個典型的放大器的第一階段，其中CR1是參考齊納二極管，R是輸出電位調(diào)節(jié)器。

圖2等效的齊納參考電路圖2等效的齊納參考電路圖1電源輸入級假設(shè)該階段的輸出是e3,提供額外的差分放大器，在穩(wěn)定狀態(tài)下e3為零，任何參數(shù)的變化都會引起輸出的漂移；對于其他階段來說也是一樣的，其影響是減少了以前所有階段的增益。因此，其他階段的影響將被忽略。以下討論的內(nèi)容涵蓋了對于TC整體的無論是主要的還是次要的影響。R3的影響CR1-R3分支的等效的電路如圖2所示，將齊納替換成了它的等效電壓源E'和內(nèi)部阻抗R2。對于高增益調(diào)節(jié)器，其中R3的變化對差分放大器的輸入來說可以忽略不計，所以前后的變化由R3決定?！?constant例如果進(jìn)一步假定IB<<Iz;從（1）可以得到同時，消除Iz，由（2b）可得E。=房堂&(&+砌+矽JillJ一IL3并且現(xiàn)在，假設(shè)=Ez;KEZrIZR.那么，（2b）也可以寫成方程式J￥o=30volts;CH,=1N752;Z.=20mA二

Rz=llii;Ez—（2b）也可以寫成Rs=12OOS2carbon(TC=(0.1%)=—0.004%/。。11(0.02)(0.1%)=—0.004%/。。So齊納二極管擁有自己的溫度系數(shù)，通常，它在TC的整體中占有很重要的位置。對于電路圖1，TC電路的介紹，從本質(zhì)上講，穩(wěn)壓器的TC部分由齊納貢獻(xiàn)。如果橋接如電路圖1顯示.被用于并聯(lián)一個下降電阻，只有部分輸出電壓出現(xiàn)過了橋顯示電流,TC的單位和齊納會有所不同。由于齊納二極管的特點(diǎn)是眾所周知的，各文獻(xiàn)對于它的描述非常好，這里將不予討論［2］?；壟c放射級電壓的變化不只是差分放大器Vbe的值不匹配，溫度的差距也不匹配。不應(yīng)該這樣，無論怎樣，互相協(xié)調(diào)是有必要的。圖1真實(shí)的參考電壓不是E1而是E2+(Vbe1-Vbe2)。因?yàn)?，對于大多?shù)的實(shí)際應(yīng)用TC的參考價值將比齊納的TC優(yōu)先考慮到很難獲得高達(dá)50V/°C的差，這就會變得相當(dāng)明顯，在大多數(shù)情況下,TC可能會超出額定值。例2:一個30AV/°C下安全的，低成本的設(shè)計。與一個1N752并聯(lián)，整體的TC將會是3。-0.0005%/°C.實(shí)驗(yàn)，筆者計算出在室溫情況下13個標(biāo)準(zhǔn)的鍺品體管的信號，集電極的電流水平為3mA，說明了它的合理值是90%到95%，基極和集電極之間將有一個-2.1至-2.4毫伏的變化。人們已經(jīng)驗(yàn)證出了如此龐大的利差(例如，施泰格［3］)。最糟糕的情況是電晶體導(dǎo)致不到400V/°C微分。與一個1N752并聯(lián)甚至可能會給出一個0.007%/°C更好的TC。基極電流的變化該基級晶體管的電流由下式給出由于有限源阻抗變化，一個電流變化造成了差分放大器的信號電壓輸入的變化。所用的電源的阻抗不是特別的理想，因?yàn)閷τ谒褂玫钠敷w管的I8和P來說它減少了系統(tǒng)的增益和需求。亨特［4］指出a的值域范圍是在+0.2%/°C至-0.2%之間，還有I8可能近似于其中A0的值由T0決定。p還取決于溫度，施泰格［3］還通過實(shí)驗(yàn)證明了它的變化范圍是在0.5%/°C到0.9%/°C之間。并且dIB…dTuo機(jī)尸—孔).(8)O圖并且dIB…dTuo機(jī)尸—孔).(8)O當(dāng)前情況下AIB流經(jīng)圖3上的每一個電源阻抗，在電阻串中變小，是由齊納電壓值和基極與發(fā)射極之間Q1和Q2之間的落差所造成的EB(andAEB)所束縛著。因此，如果要看溫度從T1變到T2時AEB的變化AZjRw=A/7b(9)△Eg=—M)?輸出電壓的變化并且，例3：假設(shè)有Q3（在25攝氏度）6=踞；=。-7%尸C；—1）=3/iA;Ic=3niA;用=30voks;出』=5600

頊乙-U,…羅法（同例1）二器=-0.68MA/°C皿=2440（-0.68X10p=-L6fimV*=—0.0055%尸C.R1的變化R1A和R1B的TC的變化的影響是很明顯的，這里不做討論。短期漂移短期漂移是由國家電氣制造商協(xié)會（NEMA）提供，可以這樣說“這段時間的輸出與輸入，環(huán)境和負(fù)載無關(guān)”[5]。在上一節(jié)中對溫度系數(shù)的描述在這里也適用。據(jù)試驗(yàn)測定，在電源里面和它附近的熱空氣極大地提高了短期特性。流動空氣的冷卻效果是眾所周知的，然而人們通常不會意識到就算空氣在齊納二極管和晶體管中移動的很緩慢，它對溫度的影響也是很顯著地。如果提供比較大的TC，那么輸出會有很大的變化。會有低TC實(shí)現(xiàn)補(bǔ)償，也就是說，如果消除了了一些元器件相同或相反的影響，這些元器件的熱時間常數(shù)仍會受到干擾。一個常用的方法是使用第一個放大器來消除和平衡掉交界處冷卻效果上的差異。可以通過晶體管的固定或保持來近似模擬這個方案，將晶體管嵌入在一個共同的金屬塊中，等等。筆者通過把輸入級和參考齊納放置到一個單獨(dú)的機(jī)箱中取得了很好的效果。如圖4所示。在圖5中通過金屬的覆蓋，漂移得到了很好的改善。圖412V的電源晶體管具有百分之0.01的調(diào)節(jié)精度。注意，保護(hù)盒是用來給第一放大器和參考組件進(jìn)行隔熱保護(hù)。圖5和圖4類似，電源提供了短期漂移，并且沒有保護(hù)措施。該元件是沒有覆蓋的，直到ti。盒子里面的溫度上升，電壓隨著時間ti而變化。如果電位器用于輸出地調(diào)節(jié)（例如R1），應(yīng)該謹(jǐn)慎的選擇價位和設(shè)計。接觸電阻的變化可引起漂移。用有高精密線圈的元件來獲得低漂移是沒有必要的。用低電阻的合金和低分辨率的元件可以輪流休息，來縮小范圍可以達(dá)到同樣令人滿意的效果。當(dāng)然，還要考慮到線路的抗腐蝕性等問題。有機(jī)硅潤滑脂可以得到很好的效果。接觸臂的周期的運(yùn)用對元件的腐蝕有很好的“療效”。熱漂移符合NEMA定義的熱漂移就是“由于不正常的環(huán)境的變化引起有關(guān)的內(nèi)部環(huán)境溫度的變化而照成在一定時間內(nèi)輸出的變化。溫漂通常與線路電壓和負(fù)載有關(guān)”［5］。溫漂與TC的供應(yīng)以及整體散熱的設(shè)計有關(guān)。通過對關(guān)鍵部件妥善安置是有可能大大減少甚至完全消除影響。百分之0.01(規(guī)定)的耗材有滿負(fù)載的百分之0.05到0.15之間的漂移，這非常的罕見。事實(shí)上，一個制造商曾經(jīng)說過百分之0.15會更好。減少熱漂移除了提高TC以外還可以通過減少內(nèi)部的消耗來解決。比如說在關(guān)鍵的放大器和散熱元件之間放置熱障。外表面最好位于通風(fēng)良好處。應(yīng)該注意到，只能在百分之0.01和0.05之間索取。瞬態(tài)響應(yīng)大多數(shù)該類型的電源有一個還很受爭議的負(fù)載端電容器。這是出于穩(wěn)定的目的，通常會決定主要的電源時間常數(shù)。這個電容器會導(dǎo)致在遙感模式下短暫的電力供應(yīng)不良的現(xiàn)象①。通常情況下，品體管電源會在很短的時間內(nèi)作出反應(yīng)，但是筆者曾經(jīng)指出［6］，在遙感時，反應(yīng)會變得很小。其等效電源如圖6所示。引線從電源到負(fù)載電阻R處引入，設(shè)備的感應(yīng)電流Is是相對穩(wěn)定的。在平衡條件下，瓦=(Je+IJr+瓦’(11)圖7表明，一個突然的負(fù)荷變化會導(dǎo)致Ldi/dt的瞬間激增，我們稱之為“尖峰”；以及線性放電時間越長時電容充放電的情況。放電時間是，如=G孕(12)1SS其中Iss=finalloadcurrentfordecreaseofload=I職一It.forincreaseofload并且，ISi=maximumshortcircuitdeliverablecurrent.①對于Is來說，通常它不會在放大器的最后階段提供驅(qū)動，但是會出現(xiàn)限流現(xiàn)象。遙感是指電源電壓電感的直接負(fù)荷。

圖6遠(yuǎn)程輸出傳感的等效電路圖7瞬態(tài)響應(yīng)，遙感。圖6遠(yuǎn)程輸出傳感的等效電路圖7瞬態(tài)響應(yīng)，遙感。圖8框圖。使用預(yù)穩(wěn)壓電源可以減少監(jiān)測和控制的A型階段電壓的使用和損耗。由于主要的調(diào)節(jié)器往往比預(yù)調(diào)節(jié)器響應(yīng)更快，應(yīng)該建立足夠的儲備來使這個階段下降。如果不這樣可能會導(dǎo)致負(fù)載的飽和，那是前置穩(wěn)壓器在響應(yīng)時間內(nèi)產(chǎn)生的。開關(guān)前置穩(wěn)壓器型機(jī)組傳統(tǒng)類-A型品體管電源供應(yīng)變得相當(dāng)笨重，昂貴，與傳遞階段擁擠，作為供給增加電流和功率的水平。要求輸出調(diào)節(jié)范圍更大,再加上電力的供應(yīng)是遠(yuǎn)程可編程的，會極大地提高條件的要求。正是由于這些原因，高效利用的開關(guān)調(diào)節(jié)器作為一種壓力調(diào)節(jié)閥在商業(yè)和軍事用品應(yīng)用了許多年。絕大多數(shù)的供應(yīng)整流器可控硅使用與控制元件。從60-cycle操作的系統(tǒng)壓力調(diào)節(jié)閥響應(yīng)來源，在20至50ms之間。最近對高壓、大功率開關(guān)晶體管的開關(guān)品體管的方法更具有吸引力。該系統(tǒng)提供了一種低成本，發(fā)行量較小的方法，再加上一個submillisecond響應(yīng)時間。通常是獨(dú)立的電源頻率導(dǎo)致了高開關(guān)率。開關(guān)頻率就可能被固定的，一個被控制的變量或一個獨(dú)立的自生自儲（LC濾波器電路）參數(shù)[7],[8]。更快的反應(yīng)時間是非?？扇〉模?yàn)樗鼫p少了在預(yù)備役電壓值必須通過階段或倉庫（的數(shù)量）的電力需求在壓力調(diào)節(jié)閥過濾器。一個晶體管作為電源開關(guān)操作適合具有大電流，高電壓等級低漏電流耦合。不幸的是，這些特點(diǎn)是實(shí)現(xiàn)了熱容量犧牲，同時使電壓和電流的條件導(dǎo)致很高的峰值功率可能是災(zāi)難性的。因此，它成為強(qiáng)制性的設(shè)計負(fù)荷高峰期間有足夠的條件，也包含開關(guān)驅(qū)動電流限制或快速過載保護(hù)系統(tǒng)。商業(yè)發(fā)展電力供應(yīng)總是有輸出電流限制，但這并不限制壓力調(diào)節(jié)閥電流負(fù)載條件下，除了在穩(wěn)態(tài)（包括短路）?？紤]一下，例如，一個電源工作在短路、短被刪除突然叫起來。指圖8、9月的產(chǎn)量將會快速上漲，減少通過階段電壓，關(guān)閉開關(guān)晶體管。由此產(chǎn)生的瞬態(tài)傳達(dá)很多周期的交換率），這樣電感的壓力調(diào)節(jié)閥過濾變得完全不夠的限制流量。因此，當(dāng)前將會上升直至穩(wěn)態(tài)已恢復(fù)、電路電阻引起限制，或不足開車使開關(guān)出來的飽和度。上述第二種情況導(dǎo)致開關(guān)失敗。其他營業(yè)狀況會產(chǎn)生相似的瞬變包括輸出電壓編程和初始刺激的用度。輸入功率瞬間的中斷也應(yīng)首先要考慮的事。一個解決這個問題的辦法就是限制的電壓變化的速率在可出現(xiàn)一值，通過舞臺了壓力調(diào)節(jié)閥可以遵循。這能被做方便加上足夠的輸出電容。這電容會同限流特性會產(chǎn)生一種最大的改變的比率其中C0=outputcapacity.假設(shè)這個壓力調(diào)節(jié)閥遵循這種變化和有濾波電容器冠軍杯,然后開關(guān)電流=孩（I+旨）.（14）在電源在上，壓力調(diào)節(jié)閥的參考電壓上升也必須是有限的。采取這一考慮,Amax=+5）+C,—（15）其中ER=passingstagevoltage

Tl=timeconstantofreferencesupply.策略合作關(guān)系SCR的使用來代替電品體的將是一個明顯改善由于較高的增兵電流的收視率，卻轉(zhuǎn)身他們?nèi)チ?，需要大量的能源。而門策略合作關(guān)系SCR討厭似乎將為員工提供良好的折衷，全部的問題，嚴(yán)峻的限制，在當(dāng)前的收視率現(xiàn)限制使用它們。參考文獻(xiàn)j.G.Truxal,控制工程師手冊。紐約：McGrawHill出版社，1958年，11到19頁。摩托羅拉齊納二極管/整流器手冊，第二版。1961。瓦特施泰格，“一個晶體管的溫度分析及其應(yīng)用差分放大器，“愛爾蘭跨。在儀器儀表，第一卷。1-8，頁82-9，1959年12月。唱片獵人，半導(dǎo)體電子手冊。紐約：McGrawHill出版社，1956年，第13-3。“標(biāo)準(zhǔn)出版物的監(jiān)管電子直流電源，“(未發(fā)表稿)電子電源集團(tuán)，半導(dǎo)體電源轉(zhuǎn)換器部分，NEMA等。Muchnick，"遠(yuǎn)程傳感品體管電源”電子產(chǎn)品，1962年9月。路勞克斯，“在開關(guān)型穩(wěn)壓器的設(shè)計考慮”固態(tài)設(shè)計，1963年4月。四漢考克和BKurger，“高效率穩(wěn)壓電源采用高速交換”在AIEE冬季干事提交會議，紐約，紐約，1月27號至2月1日，1963年。河四米德爾，差分放大器。紐約：Wiley，1963。索倫森控制功率目錄和手冊。索倫森，雷神公司，南諾沃克，美國康涅狄格州單位。本文摘自：IEEETRANSACTIONSONINDUSTRYANDGENERAL

APPLICATIONSVOL.IGA-2,NO.5SEPT/OCT1966HighlyRegulatedDCPowerSuppliesAbstract-Thedesignandapplicationofhighlyregulateddcpowersuppliespresentmanysubtle,diverse,andinterestingproblems.Thispaperdiscussessomeoftheseproblems(especiallyinconnectionwithmediumpowerunits)butemphasishasbeenplacedmoreoncircuiteconomicsratherthanonultimateperformance.Sophisticatedmethodsandproblemsencounteredinconnectionwithprecisionreferencesuppliesarethereforeexcluded.Theproblemsdiscussedincludethesubjectsoftemperaturecoefficient,short-termdrift,thermaldrift,transientresponsedegenerationcausedbyremotesensing,andswitchingpreregualtor-typeunitsandsomeoftheirperformancecharacteristics.INTRODUCTIONANYSURVEYofthecommercialdepowersupplyfieldwilluncoverthefactthat0.01percentregulatedpowersuppliesarestandardtypesandcanbeobtainedatrelativelylowcosts.Whilemostusersofthesepowersuppliesdonotrequiresuchhighregulation,theynever-thelessgetthisatlittleextracostforthesimplereasonthatitcoststhemanufacturerverylittletogivehim0.01percentinsteadof0.1percent.Theperformanceofapowersupply,however,includesotherfactorsbesideslineandloadregulation.Thispaperwilldiscussafewofthese-namely,temperaturecoefficient,short-termdrift,thermaldrift,andtransientresponse.Presentmediumpowerdcsuppliescommonlyemploypreregulationasameansofimprovingpower/volumeratiosandcosts,butsomecharacteristicsofthepowersupplysufferbythisapproach.Someoftheshort-comingsaswellasadvantagesofthistechnologywillbeexamined.TEMPERATURECOEFFICIENTAdecadeago,mostcommercialpowersuppliesweremadetoregulationspecificationsof0.25to1percent.Thereferenceelementsweregasdiodeshavingtemperaturecoefficientsoftheorderof0.01percent[1].Consequently,theTC(temperaturecoefficient)ofthesupplywassmallcomparedtotheregulationspecificationsandoftenignored.Today,thereferenceelementoftencarriesaTCspecificationgreaterthantheregulationspecification.Whilethelattermaybeimprovedconsiderablyatlittlecostincrease,thisisnotnecessarilytrueofTC.Therefore,theuseofverylowTCzenerdiodes,matcheddifferentialamplifierstages,andlowTCwirewoundresistorsmustbeanalyzedcarefully,ifcostsaretobekeptlow.AtypicalfirstamplifierstageisshowninFig.1.CRIisthereferencezenerdiodeandR,istheoutputadjustmentpotentiometer.Fig.1.Inputstageofpowersupply.

Fig.2.Fig.2.Equivalentcircuitofzenerreference.Letitbeassumedthate3,theoutputofthestage,feedsadditionaldifferentialamplifiers,andundersteady-stateconditionse3=0.Avariationofanyoftheparameterscouldcausetheoutputtodrift;whilethisisalsotrueoftheotherstages,theeffectsarereducedbythegainofallpreviousstages.Consequently,theeffectsofotherstageswillbeneglected.ThefollowingdisculssioncoverstheeffectsofallelementshavingprimaryandsecondaryinfluencesontheoverallTC.EffectofR3TheequivalentcircuitofCRI-R3branchisshowninFig.2.Thezenerha'sbeenreplacedwithitsequivalentvoltagesourceE/'andinternalimpedanceR,.Forhighgainregulators,theinputofthedifferentialamplifierwillhavenegligiblechangewithvariationsofR3sothat—=constant例beforeandafteravariationofR3ismade.(2時IfitisfurtherassumedthatIB<<Iz;thenfrom(1)頑+E/(2b)Also,E&=+Rz)+EliminatingI,from(2b),

E。=房受&+砌+磯'A21J—IL3andNow,assumingthatE；=Ez(2時(2b)andNow,assumingthatE；=Ez;KEZRIZR.then,Equation(2b)canalsobewrittenEo=3(}volts;CR,=1N752;Z￡=20mAa

Rz=llfi;Ez—5.6volts;Rz=12OOS2carbon(TC=0.1%尸C)Eq11(0.02)(0.1%)=—0.004%/。。?TheEq11(0.02)(0.1%)=—0.004%/。。?ThezenerdiodeitselfhasatemperaturecoefficientandusuallyisthecomponentthatdominatestheoverallTCoftheunit.ForthecircuitofFig.1,theTCofthecircuitdescribes,inessence,theportionoftheregulatorTCcontributedbythezener.IfthebridgecircuitshowninFig.1wereusedinconjunctionwithadroppingresistorsothatonlyaportionoftheoutputvoltageappearedacrossthebridgecircuitshown,theTCoftheunitandthezenerwouldbedifferent.Sincethecharacteristicofzenersissowellknownandsowelldescribedintheliterature,adiscussionwillnotbegivenhere[2].VariationofBase-EmitterVoltagesNotonlydothevaluesofV,,ofthedifferentialam-plifierfailtomatch,buttheir

differentialswithtemperaturealsofailtomatch.Thisshouldnot,however,suggestthatmatchedpairsarerequired.ThetruereferencevoltageofFig.1isnotthevalueE,,butE,+(Vie,-Vbe2)-Since,formostpracticalapplicatioinsEe>theTCofthereferencewillbetheTCofthezenerplusConsideringthatitisdifficulttoobtainmatchedpairsthathavedifferentialsaspooras50V/°C,itbecomesratherapparentthat,inmostcases,amatchedpairboughtspecificallyforTCmaybeoverdesigning.Example2:Astandardavailablelow-costmatchedpairlaims30AV/°C.Inconjunctionwitha1N752,theontributiontotheoverallTCwouldbe30X30X10~6

5^6a0_0005%尸CTests,performedbytheauthoronthirteenstandardgermaniumsignaltransistorsinthevicinityofroomtemperatureandatacollectorcurrentlevelof3mA,indicatedthatitisreasonabletoexpectthat90to95percentoftheunitswouldhaveabase-emittervoltagevariationof-2.1to-2.4mV/°C.Spreadsofthismagnitudehavealsobeenverifiedbyothers(e.g.,Steiger[3]).Theworstmatchingoftransistorsledtolessthan400,V/°Cdifferential.Inconjunctionwitha1N752,eventhiswouldgiveaTCofbetterthan0.007%/oC.VariationofBaseCurrentsThebasecurrentofthetransistorsisgivenbyAvariationofthiscurrentcausesavariationinsignalvoltageattheinputtothedifferentialamplifierduetofinitesourceimpedances.Matchingsourceimpedancesisnotparticularlydesirable,sinceitreducesthegainofthesystemandrequiresthattransistorsmatchedforI,oandAbeused.Hunter[4]statesthattheTCofaisintherangeof+0.2%/0Cto-0.2%7/'Candthat1,,maybeapproximatedby玲=糜麒FwhereAoisthevalueatTo.PisalsotemperaturedependentandSteiger[3]experimentallydeterminedthevariationtobefromabout0.5%/°Cto0.9%/oC.頊=?_A。(1+；)⑺And,??碧"Y矗-。?眼4+9*5?⑻Sr□Fig.3.InputcircuitofQ2.ThecurrentAIBflowsthroughthesourceimpedanceperFig.3.Thedropsintheresistancestring,however,aresubjecttotheconstraintthatEB(andAEB)aredeterminedbythezenervoltageandthebase-emitterdropsofQ1andQ2.Consequently,ifingoingfromtemperatureT1toT2achangeAEBoccurs,AZiJ?ia=A/*7a(9)Rw=*1占(A/g—△/】),ThechangeinoutputvoltageisAndExample3:ForQ2(at25°C)。=50;=。二％尸C;An—3*A;Ic=3niA；=30vdts;RiA=560QAA',.nA'.,.-:有！W(seeExamPleD.??器=^Q.6SMA/°C△1%=2440(-0.68X1〔)t)=mV費(fèi)」=-0.0055%/°C.VariationofR,TheeffectsofavariationoftheTCbetweenRIAandRIBissufficientlyself-evidentsothatadiscussionofthecontributionisnotincluded.SHORT-TERMDRIFTTheshort-termdriftofasupplyisdefinedbytheNationalElectricalManufacturersAssociation(NEMA)as"achangeinoutputoveraperiodoftime,whichchangeisunrelatedtoinput,environment,orload[5].”Muchofthematerialdescribedinthesectionontemperaturecoefficientisapplicablehereaswell.Ithasbeendeterminedexperimentally,however,thatthermalairdraftsinandnearthe

vicinityofthepowersupplycontributesenormouslytotheshort-termcharacteristics.Thecoolingeffectsofmovingairarequitewellknown,butitisnotoftenrecognizedthatevenextremelyslowairmovementsoversuchdevicesaszenersandtransistorscausethejunctiontemperatureofthesedevicestochangerapidly.IftheTCofthesupplyislargecomparedtotheregulation,thenlargevariationsintheoutputwillbeobserved.UnitshavinglowTC'sachievedbycompensation-thatis,bycancelingouttheeffectsofsomeomponentsbyequalandoppositeeffectsofothersmaystillbeplaguedbythesedraftsduetothedifferenceinthermaltimeconstantsoftheelements.Oftentimes,amatchedtransistordifferentialamplifierinacommonenvelopeisusedforthefirstamplifierjusttoequalizeandeliminatethedifferenceincoolingeffectsbetweenthejunctions.Approximationstothismethodincludecementingorholdingthetransistorstogether,imbeddingthetransistorsinacommonmetalblock,etc.Excellentresultswereachievedbytheauthorbyplacingtheinputstageandzenerreferenceinaseparateenclosure.ThisconstructionisshowninFig.4.TheimprovementindriftobtainedbymeansoftheadditionofthemetalcoverisdemonstrateddramaticallyinFig.5.Fig.5.Short-termdriftofapowersupplysimilartotheoneshowninFig.4withand

withoutprotectivecovers.Theunitwasoperatedwithoutthecoveruntiltimetl,when

thecoverwasattached.Theinitialvoltagechangefollowingt,isduetoatemperature

riseinsidethebox.Fig.5.Short-termdriftofapowersupplysimilartotheoneshownnFig.4withandwithoutprotectivecovers.Theunitwasoperatedwithoutthecoveruntiltimetl,whenthecoverwasattached.Theinitialvoltagechangefollowingt,isduetoatemperatureriseinsidethebox.Ifpotentiometersareusedinthesupplyforoutputadjustment(e.g.,RI),careshouldbeusedinchoosingthevalueanddesign.Variationsofthecontactresistancecancausedrift.Itisnotalwaysnecessary,however,toresorttotheexpenseofhigh-resolutionmultiturnprecisionunitstoobtainlowdrift.Areductioninrangeofadjustment,useoflow-resistancealloysandlow-resolutionunitswhichpermitthecontactarmtorestfirmlybetweenturns,maybejustassatisfactory.Ofcourse,otherconsiderationsshouldincludetheabilityofboththearmsandthewiretoresistcorrosion.Siliconegreasesarehelpfulhere.Periodicmovementofcontactarmshasbeenfoundhelpfulin"healing"corrodedelements.THERMALDRIFTNEMAdefinesthermaldriftas"achangeinoutputoveraperiodoftime,duetochangesininternalambienttemperaturesnotnormallyrelatedtoenvironmentalchanges.Thermaldriftisusuallyassociatedwithchangesinlinevoltageand/orloadchanges[5]."Thermaldrift,therefore,isstronglyrelatedtotheTCofthesupplyaswellasitsoverallthermaldesign.Byproperplacementofcriticalcomponentsitispossibletogreatlyreduceoreveneliminatetheeffectentirely.Itisnotuncommonforsuppliesofthe0.01percent(regulation)varietytohavedriftsofbetween0.05to0.15percentforfulllineorfullloadvariations.Infact,onemanufacturerhassuggestedthatanythingbetterthan0.15percentisgood.SolutionstoreducingthermaldriftotherthantheobviousapproachofimprovingtheTCandreducinginternallossesincludeamechanicaldesignthatsetsupaphysicalandthermalbarrierbetweenthecriticalamplifiercomponentsandheatdissipatingelements.Exposuretooutsidesurfaces

withgoodventilationisrecommended.Withcare,0.01to0.05percentisobtainable.TRANSIENTRESPONSEMostpowersuppliesofthetypebeingdiscussedhaveacapacitoracrosstheloadterminals.Thisisusedforstabilizationpurposesandusuallydeterminesthedominanttimeconstantofthesupply.Thepresenceofthiscapacitorunfortunatelyleadstoundesirabletransientphenomenawhenthesupplyisusedintheremotesensingmode?.Normally,transistorizedpowersuppliesrespondinmicroseconds,butastheauthorhaspointedout[6],theresponsecandegenerateseverelyinremotesensing.TheequivalentcircuitisshowninFig.6.Theleadsfromthepowersupplytotheloadintroduceresistancer.Isisthesensingcurrentofthesupplyandisrelativelyconstant.Underequilibriumconditions,EcEc=(Zr+If)r+既,(11)AsuddenloadchangewillproducethetransientofFig.7.Theinitial"spike"iscausedbyaninductivesurgeLdi/dt;thelongerlineardischargefollowingistheresultofthecapacitortryingtodischarge(orcharge).ThedischargetimeiswhereIss=finalloadcurrentfordecreaseofload=I職—It.forincreaseofloadandISi=maximumshortcircuitdeliverablecurrent.ThelimitationsofI,,areusuallynotduetoavailabledriveofthefinalamplifierstagesbuttootherlimitations,currentlimitingbeingthemostcommon.Unitsusingpreregulatorsoftheswitchingtype(transistororSCRtypes)shouldbelookedatcarefullyifthecharacteristicsmentionedrepresentaproblem.?Remotesensingistheprocessbywhichthepowersupplysensesvoltagedirectlyattheload.Fig.6.Outputequivalentcircuitatremotesensing.Fig.7.Transientresponse,remotesensing.Fig.8.Blockdiagram.Preregulatedsuppliesareusedtoreducesizeandlossesbymonitoringandcontrollingthevoltageacrosstheclass-A-typeseriespassingstage(Fig.8).Sincethemainregulatorinvariablyrespondsmuchquickerthanthepreregulator,sufficientreserveshouldalwaysbebuiltintothedropacrossthepassingstage.Failuretoprovidethismayresultinsaturationofthepassingstagewhenloadisapplied,resultinginaresponsetimewhichisthatofthepreregulatoritself.SWITCHINGPREREGULATOR-TYPEUNITSTheconventionalclass-A-typetransistorizedpowersupplybecomesratherbulky,expensive,andcrowdedwithpassingstages,asthecurrentandpowerlevelofthesupplyincreases.Therequirementofwideoutputadjustmentrange,coupledwiththeabilityofthesupplytoberemotelyprogrammable,aggravatestheconditionenormously.Forthesereasonsthehigh-efficiencyswitchingregulatorhasbeenemployedasapreregulatorincommercialaswellasmilitarysuppliesformanyyears.Theoverwhelmingmajorityofthesuppliesusedsiliconcontrolledrectifiersasthecontrolelement.Forsystemsoperatingfrom60-cyclesources,thispreregulatorrespondsin20to50ms.Recentimprovementsinhigh-voltage,high-powerswitchingtransistorshasmadetheswitchingtransistorpproachmoreattractive.Thissystemoffersasomewhatlower-cost,lower-volumeapproachcoupledwithasubmillisecondresponsetime.Thisisbroughtaboutbyahighswitchingratethatisnormallyindependentoflinefrequency.Theswitchingfrequencymaybefixed,acontrolledvariableoranindependentself-generated(bytheLCfiltercircuit)parameter[7],[8].Fasterresponsetimeishighlydesirablesinceitreducestheamountofreservevoltagerequiredacrossthepassingstageortheamountof(storage)capacityrequiredinthepreregulatorfilter.Atransistorsuitableforoperatingasapowerswitchhasahigh-current,high-voltageratingcoupledwithlowleakagecurrent.Unfortunately,thesecharacteristicsareachievedbyasacrificeinthermalcapacity,sothatsimultaneousconditionsofvoltageandcurrentleadingtohighpeakpowercouldbedisastrous.Itthereforebecomesmandatorytodesignforsufficientswitchdriveduringpeakloadconditionsandalsoincorporatecurrent-limitingorrapidoverloadprotectionsystems.Commercialwide-rangepowersuppliesinvariablyhaveoutputcurrentlimiting,butthisdoesnotlimitthepreregulatorcurrentsexceptduringsteady-stateloadconditions(includingshortcircuits).Consider,forexample,apowersupplyoperatingatshortcircuitandtheshortbeingremovedsuddenly.ReferringtoFig.8,theoutputwouldriserapidly,reducethepassingstagevoltage,andclosetheswitchingtransistor.Theresultingtransientextendsovermanycycles(switchingrate)sothattheinductanceofthepreregulatorfilterbecomestotallyinadequatetolimitcurrentflow.Therefore,thecurrentwillriseuntilsteadystateisresumed,circuitresistancecauseslimiting,o