外文翻譯----高度穩(wěn)壓直流電源

1、<p><b>　　高精度穩(wěn)壓直流電源</b></p><p>　　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)電機組有關(guān)),重點是在電路的經(jīng)濟適用性上,而不是要達到最好的性能。當然，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型機組及和它的性能

2、特點有關(guān)的的一些科目。</p><p><b>　　介紹</b></p><p>　　從商業(yè)的角度來看供電領(lǐng)域可以得到這樣一個事實，在相對較低的成本下就可以可以獲得標準類型的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的費用而把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，比如說線路和負載調(diào)解率。本文將

3、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。 </p><p>　　目前中等功率直流電源通常采用預穩(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)缺點還有待觀察。</p><p><b>　　溫度系數(shù)</b></p><p>　　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

4、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，還要仔細的分析低TC繞線電阻器。</p><p>　　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。&l

5、t;/p><p><b>　　圖1 電源輸入級</b></p><p>　　圖2 等效的齊納參考電路</p><p>　　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

6、無論是主要的還是次要的影響。</p><p><b>　　R3的影響</b></p><p>　　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。</p><p>　　如果進一步假定IB <<

7、 Iz;從（1）可以得到</p><p><b>　　同時，</b></p><p>　　消除Iz，由（2b）可得</p><p><b>　　并且</b></p><p><b>　　現(xiàn)在，假設(shè)</b></p><p><b>　　那么，&l

8、t;/b></p><p>　　方程式（2b）也可以寫成</p><p><b>　　例1：</b></p><p><b>　　齊納二極管</b></p><p>　　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

9、部分由齊納貢獻。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是眾所周知的，各文獻對于它的描述非常好，這里將不予討論[2]。</p><p>　　基級與放射級電壓的變化</p><p>　　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真實的參考電壓不

10、是E1而是E2+(Vbe1-Vbe2)。</p><p>　　因為，對于大多數(shù)的實際應(yīng)用</p><p>　　TC的參考價值將比齊納的TC優(yōu)先</p><p>　　考慮到很難獲得高達50 V/°C的差，這就會變得相當明顯，在大多數(shù)情況下,TC可能會超出額定值。</p><p>　　例2:一個30AV/°C下安全的，低成本的

11、設(shè)計。與一個1N752并聯(lián)，整體的TC將會是</p><p>　　實驗，筆者計算出在室溫情況下13個標準的鍺晶體管的信號，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

12、更好的TC。</p><p><b>　　基極電流的變化</b></p><p>　　該基級晶體管的電流由下式給出</p><p>　　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，因為對于所使用的晶體管的I∞和β來說它減少了系統(tǒng)的增益和需求。亨特[4]指出α的值域范圍是在+0.2％/℃至

13、-0.2％之間，還有I∞可能近似于</p><p>　　其中A0的值由T0決定。</p><p>　　β還取決于溫度，施泰格[3]還通過實驗證明了它的變化范圍是在0.5％/ ° C到0.9％/° C之間。</p><p><b>　　并且</b></p><p>　　圖3 Q2的輸入電路</p&

14、gt;<p>　　當前情況下ΔIB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 (and ΔEB)所束縛著。因此，如果要看溫度從T1變到T2時ΔEB的變化</p><p><b>　　輸出電壓的變化</b></p><p><b>　　并且，</b></p>

15、<p>　　例3:假設(shè)有Q3(在25攝氏度)</p><p><b>　?。ㄍ?）∴</b></p><p><b>　　R1的變化</b></p><p>　　R1A和R1B的TC的變化的影響是很明顯的，這里不做討論。</p><p><b>　　短期漂移</b&g

16、t;</p><p>　　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

17、現(xiàn)補償，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的機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。</p><p>　　圖4 12V的電源晶

18、體管具有百分之0.01的調(diào)節(jié)精度。注意，保護盒是用來給第一放大器和參考組件進行隔熱保護。</p><p>　　圖5 和圖4類似，電源提供了短期漂移，并且沒有保護措施。該元件是沒有覆蓋的，直到t1。盒子里面的溫度上升，電壓隨著時間 t1而變化。</p><p>　　如果電位器用于輸出地調(diào)節(jié)（例如R1），應(yīng)該謹慎的選擇價位和設(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

19、必要的。用低電阻的合金和低分辨率的元件可以輪流休息，來縮小范圍可以達到同樣令人滿意的效果。當然，還要考慮到線路的抗腐蝕性等問題。有機硅潤滑脂可以得到很好的效果。接觸臂的周期的運用對元件的腐蝕有很好的“療效”。</p><p><b>　　熱漂移</b></p><p>　　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

20、內(nèi)輸出的變化。溫漂通常與線路電壓和負載有關(guān)”[5]。</p><p>　　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）的耗材有滿負載的百分之0.05到0.15之間的漂移，這非常的罕見。事實上，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

21、好位于通風良好處。應(yīng)該注意到，只能在百分之0.01和0.05之間索取。</p><p><b>　　瞬態(tài)響應(yīng)</b></p><p>　　大多數(shù)該類型的電源有一個還很受爭議的負載端電容器。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，晶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

22、感時，反應(yīng)會變得很小。其等效電源如圖6所示。引線從電源到負載電阻R處引入，設(shè)備的感應(yīng)電流Is是相對穩(wěn)定的。</p><p><b>　　在平衡條件下，</b></p><p>　　圖7表明，一個突然的負荷變化會導致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，</p><p><b>

23、　　其中</b></p><p><b>　　并且，</b></p><p>　　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荷。</p><p>　　圖6 遠程輸出傳感的等效電路</p><p>　　圖7 瞬態(tài)響應(yīng)，遙感。</p>&l

24、t;p><b>　　圖8 框圖。</b></p><p>　　使用預穩(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致負載的飽和，那是前置穩(wěn)壓器在響應(yīng)時間內(nèi)產(chǎn)生的。</p><p>　　開關(guān)前置穩(wěn)壓器型機組</p><p>　　傳統(tǒng)類- A

25、型晶體管電源供應(yīng)變得相當笨重，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。</p><p>　　最近對高壓、大功率開關(guān)晶體管的開關(guān)晶

26、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。通常是獨立的電源頻率導致了高開關(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〉?因為它減少了在預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。</p><p>　　一個晶體管作為電源開關(guān)操作適合具有

27、大電流，高電壓等級低漏電流耦合。不幸的是，這些特點是實現(xiàn)了熱容量犧牲，同時使電壓和電流的條件導致很高的峰值功率可能是災(zāi)難性的。因此，它成為強制性的設(shè)計負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系統(tǒng)。</p><p>　　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)閥電流負載條件下,除了在穩(wěn)態(tài)(包括短路)。考慮一下,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

28、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,當前將會上升直至穩(wěn)態(tài)已恢復、電路電阻引起限制,或不足開車使開關(guān)出來的飽和度。上述第二種情況導致開關(guān)失敗。</p><p>　　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)

29、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</p><p><b>　　其中</b></p><p>　　C0 = output capacity.</p><p>　　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</p><p>

30、　　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,</p><p><b>　　其中</b></p><p>　　ER = passing stage voltage</p><p>　　Tl = time constant of reference supply.</p><p>　　策略合作關(guān)系S

31、CR的使用來代替電晶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,嚴峻的限制,在當前的收視率現(xiàn)限制使用它們。</p><p><b>　　參考文獻</b></p><p>　　[1] j.G.Truxal，控制工程師手冊。紐約：McGraw Hill出版社，1958年，

32、11到19頁。</p><p>　　[2]摩托羅拉齊納二極管/整流器手冊，第二版。 1961。</p><p>　　[3]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。</p><p>　　[4]唱片獵人，半導體電子手冊。紐約：McGraw Hill出版社，1956年，第13-3。<

33、;/p><p>　　[5]“標準出版物的監(jiān)管電子直流電源，“（未發(fā)表稿）電子電源集團，半導體電源轉(zhuǎn)換器部分，NEMA等。</p><p>　　[6]Muchnick，“遠程傳感晶體管電源”電子產(chǎn)品，1962年9月。</p><p>　　[7]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。</p><p>　　[8]四漢考克和B

34、Kurger，“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。</p><p>　　[9]河四米德爾，差分放大器。紐約：Wiley，1963。</p><p>　　[10]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。</p><p>　　本文摘自：IEEE TRANSACTIONS

35、 ON INDUSTRY AND GENERAL APPLICATIONS VOL. IGA-2, NO.5 SEPT/OCT 1966</p><p>　　Highly Regulated DC Power Supplies</p><p>　　Abstract-The design and application of highly regulated dc power supplie

36、s present many subtle, diverse, and interesting problems. This paper discusses some of these problems (especially inconnection with medium power units) but emphasis has been placed more on circuit economics rather than o

37、n ultimate performance.Sophisticated methods and problems encountered in connection with precision reference supplies are therefore excluded. The problems discussed include the subjects of temperature coeffic</p>

38、<p>　　INTRODUCTION</p><p>　　ANY SURVEY of the commercial de power supply field will uncover the fact that 0.01 percent regulated power supplies are standard types and can be obtained at relatively low c

39、osts. While most users of these power supplies do not require such high regulation, they never-theless get this at little extra cost for the simple reason that it costs the manufacturer very little to give him 0.01 perce

40、nt instead of 0.1 percent. The performance of a power supply, however, includes other factors besides line</p><p>　　TEMPERATURE COEFFICIENT</p><p>　　A decade ago, most commercial power supplies

41、were made to regulation specifications of 0.25 to 1 percent. The reference elements were gas diodes having temperature coefficients of the order of 0.01 percent [1]. Consequently, the TC (temperature coefficient) of the

42、supply was small compared to the regulation specifications and often ignored. Today, the reference element often carries aTC specification greater than the regulation specification.While the latter may be improved consid

43、erably at littl</p><p>　　Fig. 1. Input stage of power supply.</p><p>　　Fig. 2. Equivalent circuit of zener reference.</p><p>　　Let it be assumed that e3, the output of the stage, fe

44、edsadditional differential amplifiers, and under steady-state conditions e3 = 0. A variation of any of the parameters could cause the output to drift; while this is also true of the other stages, the effects are reduced

45、by the gain of all previous stages. Consequently, the effects of other stages will be neglected. The following disculssion covers the effects of all elements having primary and secondary influences on the overall TC.<

46、/p><p>　　Effect of R3 </p><p>　　The equivalent circuit of CRI -R3 branch is shown in Fig. 2. The zener ha's been replaced with its equivalent voltage source E/' and internal impedance R,. F

47、or high gain regulators, the input of the differential amplifier will have negligible change with variations of R3 so that</p><p>　　before and after a variation of R3 is made.</p><p>　　If it is

48、further assumed that IB << Iz; then from (1)</p><p><b>　　Also,</b></p><p>　　Eliminating I, from (2b),</p><p><b>　　and</b></p><p>　　Now, as

49、suming that</p><p><b>　　then,</b></p><p>　　Equation (2b) can also be written</p><p>　　The Zener Diode</p><p>　　The zener diode itself has a temperature coef

50、ficient andusually is the component that dominates the overall TCof the unit. For the circuit of Fig. 1, the TC of the circuit describes, in essence, the portion of the regulator TC contributed by the zener. If the bridg

51、e circuit shown in Fig. 1 were used in conjunction with a dropping resistor so that only a portion of the output voltage appeared across the bridge circuit shown, the TC of the unit and the zener would be different. Sinc

52、e the character</p><p>　　Variation of Base-Emitter Voltages</p><p>　　Not only do the values of V,, of the differential am-plifier fail to match, but their differentials with tem perature also fa

53、il to match. This should not, however,suggest that matched pairs are required. The true reference voltage of Fig. 1 is not the value E,, but E, + (Vie, -Vbe2)-Since, for most practical applicatioins</p><p>　

54、　the TC of the reference will be the TC of the zener plus</p><p>　　Considering that it is difficult to obtain matched pairs that have differentials as poor as 50 V/°C, it becomes rather apparent that, i

55、n most cases, a matched pair bought specifically for TC may be overdesigning.</p><p>　　Example 2: A standard available low-cost matched pair laims 30AV/°C. In conjunction with a 1N752, the ontribution t

56、o the overall TC would be</p><p>　　Tests, performed by the author on thirteen standard germanium signal transistors in the vicinity of room temperature and at a collector current level of 3 mA,indicated that

57、 it is reasonable to expect that 90 to 95 percent of the units would have a base-emitter voltage variation of -2.1 to -2.4 mV/°C. Spreads of this magnitude have also been verified by others (e.g., Steiger[3]). The w

58、orst matching of transistors led to less than 400 ,V/°C differential. In conjunction with a 1N752,even this would</p><p>　　Variation of Base Currents</p><p>　　The base current of the transi

59、stors is given by</p><p>　　A variation of this current causes a variation in signal voltage at the input to the differential amplifier due to finite source impedances. Matching source impedances is not parti

60、cularly desirable, since it reduces the gain of the system and requires that transistors matched for I,o and A be used. Hunter [4 ] states that the TC of a is in the range of +0.2%/0C to -0.2%7/'C and that 1,, may be

61、 approximated by</p><p>　　where Ao is the value at To.</p><p>　　β is also temperature dependent and Steiger [3] experimentally determined the variation to be from about 0.5%/°C to 0.9%/0C.&

62、lt;/p><p><b>　　And，</b></p><p>　　Fig. 3. Input circuit of Q2.</p><p>　　The current AIB flows through the source impedance per Fig. 3. The drops in the resistance string, ho

63、wever, are subject to the constraint that EB (and AEB) are determined by the zener voltage and the base-emitter drops of Q1 and Q2. Consequently, if in going from temperature T1to T2 a change AEB occurs,</p><p

64、>　　The change in output voltage is</p><p><b>　　And</b></p><p>　　Example 3: For Q2 (at 25°C)</p><p>　　(see Example 1)∴</p><p>　　Variation of R,</p

65、><p>　　The effects of a variation of the TC between RIA and RIB is sufficiently self-evident so that a discussion of the contribution is not included.</p><p>　　SHORT-TERM DRIFT</p><p>

66、　　The short-term drift of a supply is defined by the National Electrical Manufacturers Association (NEMA) as "a change in output over a period of time, which change is unrelated to input, environment, or load [5].&q

67、uot;Much of the material described in the section on temperature coefficient is applicable here as well. It has been determined experimentally, however, that thermal air drafts in and near the vicinity of the power suppl

68、y contributes enormously to the short-term characteristics. The cooling </p><p>　　Oftentimes, a matched transistor differential amplifier in a common envelope is used for the first amplifier just to equalize

69、 and eliminate the difference in cooling effects between the junctions. Approximations to this method include cementing or holding the transistors together, imbedding the transistors in a common metal block, etc. Excelle

70、nt results were achieved by the author by placing the input stage and zener reference in a separate enclosure. This construction is shown in Fig. 4. The im</p><p>　　Fig. 5. Short-term drift of a power supply

71、 similar to the one shown in Fig. 4 with and without protective covers. The unit was operated without the cover until time tl, when the cover was attached. The initial voltage change following t, is due to a temperature

72、rise inside the box.</p><p>　　Fig. 5. Short-term drift of a power supply similar to the one shown n Fig. 4 with and without protective covers. The unit was operated without the cover until time tl, when the

73、cover was attached. The initial voltage change following t, is due to a temperature rise inside the box.</p><p>　　If potentiometers are used in the supply for output adjustment (e.g., RI), care should be use

74、d in choosing the value and design. Variations of the contact resistance can cause drift. It is not always necessary, however, to resort to the expense of high-resolution multiturn precision units to obtain low drift. A

75、reduction in range of adjustment, use of low-resistance alloys and low-resolution units which permit the contact arm to rest firmly between turns, may be just as satisfactory. Of course, </p><p>　　THERMAL DR

76、IFT</p><p>　　NEMA defines thermal drift as "a change in output over a period of time, due to changes in internal ambient temperatures not normally related to environmental changes. Thermal drift is usua

77、lly associated with changes in line voltage and/or load changes [5]."</p><p>　　Thermal drift, therefore, is strongly related to the TC of the supply as well as its overall thermal design. By proper plac

78、ement of critical components it is possible to greatly reduce or even eliminate the effect entirely. It is not uncommon for supplies of the 0.01 percent(regulation) variety to have drifts of between 0.05 to 0.15 percent

79、for full line or full load variations. In fact, one manufacturer has suggested that anything better than 0.15 percent is good. Solutions to reducing thermal </p><p>　　TRANSIENT RESPONSE</p><p>　

80、　Most power supplies of the type being discussed have a capacitor across the load terminals. This is used for stabilization purposes and usually determines the dominant time constant of the supply. The presence of this c

81、apacitor unfortunately leads to undesirable transient phenomena when the supply is used in the remote sensing mode①. Normally, transistorized power supplies respond in microseconds, but as the author has pointed out [6],

82、 the response can degenerate severely in remote sensing .The </p><p>　　Under equilibrium conditions,</p><p>　　A sudden load change will produce the transient of Fig. 7. The initial "spike&q

83、uot; is caused by an inductive surge Ldi/dt; the longer linear discharge following is the result of the capacitor trying to discharge (or charge). The discharge time is</p><p><b>　　where</b></

84、p><p><b>　　and</b></p><p>　　The limitations of I,, are usually not due to available drive of the final amplifier stages but to other limitations, current limiting being the most common.

85、 Units using pre regulators of the switching type (transistor or SCR types) should be looked at carefully if the characteristics mentioned represent a problem.</p><p>　?、賀emote sensing is the process by whic

86、h the power supply senses voltage directly at the load.</p><p>　　Fig. 6. Output equivalent circuit at remote sensing.</p><p>　　Fig. 7. Transient response, remote sensing.</p><p>　　F

87、ig. 8. Block diagram.</p><p>　　Preregulated supplies are used to reduce size and losses by monitoring and controlling the voltage across the class-A-type series passing stage (Fig. 8). Since the main regulat

88、or invariably responds much quicker than the preregulator, sufficient reserve should always be built into the drop across the passing stage. Failure to provide this may result in saturation of the passing stage when load

89、 is applied, resulting in a response time which is that of the preregulator itself.</p><p>　　SWITCHING PREREGULATOR-TYPE UNITS</p><p>　　The conventional class-A-type transistorized power supply

90、becomes rather bulky, expensive, and crowded with passing stages, as the current and power level of the supply increases. The requirement of wide output adjustment range, coupled with the ability of the supply to be remo

91、tely programmable, aggravates the condition enormously. For these reasons the high-efficiency switching regulator has been employed as a preregulator in commercial as well as military supplies for many years. The overwhe

92、lm</p><p>　　Recent improvements in high-voltage, high-power switching transistors has made the switching transistor pproach more attractive. This system offers a somewhat lower-cost, lower-volume approach co

93、upled with a submillisecond response time. This is brought about by a high switching rate that is normally independent of line frequency. The switching frequency may be fixed, a controlled variable or an independent self

94、-generated (by the LC filter circuit) parameter [7], [8]. Faster response time is hig</p><p>　　A transistor suitable for operating as a power switch has a high-current, high-voltage rating coupled with low l

95、eakage current. Unfortunately, these characteristics are achieved by a sacrifice in thermal capacity, so that simultaneous conditions of voltage and current leading to high peak power could be disastrous. It therefore be

96、comes mandatory to design for sufficient switch drive during peak load conditions and also incorporate current-limiting or rapid overload protection systems.</p><p>　　Commercial wide-range power supplies inv

97、ariably have output current limiting, but this does not limit the preregulator currents except during steady-state load conditions (including short circuits). Consider, for example, a power supply operating at short circ

98、uit and the short being removed suddenly. Referring to Fig. 8, the output would rise rapidly, reduce the passing stage voltage, and close the switching transistor. The resulting transient extends over many cycles (switch

99、ing rate) so that th</p><p>　　Other operating conditions that would produce similar transients include output voltage programming and initial turn-on of the supply. Momentary interruption of input power shou

100、ld also be a prime consideration.</p><p>　　One solution to the problem is to limit the rate of change of voltage that can appear across the passing stage to a value that the preregulator can follow. This can

101、 be done conveniently by the addition of sufficient output capacitance. This capacitance in conjunction with the current limiting characteristic would produce a maximum rate of change of</p><p><b>　　wh

102、ere</b></p><p>　　C0 = output capacity.</p><p>　　Assuming that the preregulator follows this change and has a filter capacitor Cl, then the switch current is</p><p>　　During po

103、wer on, the preregulator reference voltage rise must also be limited. Taking this into account,</p><p><b>　　where</b></p><p>　　ER = passing stage voltage</p><p>　　Tl = t

104、ime constant of reference supply.</p><p>　　The use of SCR's to replace the transistors would be a marked improvement due to higher surge current ratings, but turning them off requires large energy source