電子 電流 外文翻譯 外文文獻(xiàn) 英文文獻(xiàn) 高度穩(wěn)壓直流電源

1、高精度穩(wěn)壓直流電源文摘:目前對(duì)于可調(diào)式直流電源的設(shè)計(jì)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,而不是要到達(dá)最好的性能。當(dāng)然，對(duì)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(gè)事實(shí)，在相對(duì)較低的本錢下就可以可以獲得標(biāo)準(zhǔn)類型的0.01%供電調(diào)節(jié)。大局部的供電用戶并不需要這么高的規(guī)格，但是供給商不會(huì)為了減少客戶這么一點(diǎn)的費(fèi)用而把0.1%改成0.01%。并且電力供給的性能還包括

2、其他一些因素，比方說線路和負(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/體積比和本錢，但是只有某些電力供給采用這樣的做法。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)電力供給為規(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0.011。因此，人們往往會(huì)無視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(huì)有很大的提高，但是這并不是真正的TC。因此，如果本錢要保持在一個(gè)低的水平，可以采用TC非常低的齊納二極管，安裝上差動(dòng)放大電路，還要仔細(xì)的分析低TC繞線電阻器

3、。如圖1所示，一個(gè)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1 電源輸入級(jí) 圖2 等效的齊納參考電路假設(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(huì)引起輸出的漂移；對(duì)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(duì)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(duì)于高增益調(diào)節(jié)器，其中R3的變化對(duì)差分放大器的輸入來說可以忽略不計(jì)，所以前后的變化由R3決定。如果進(jìn)一步假定IB

4、Iz;從1可以得到同時(shí)，消除Iz，由2b可得并且現(xiàn)在，假設(shè)那么，方程式2b也可以寫成例1：齊納二極管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(duì)于電路圖1，TC電路的介紹，從本質(zhì)上講，穩(wěn)壓器的TC局部由齊納奉獻(xiàn)。如果橋接如電路圖1顯示.被用于并聯(lián)一個(gè)下降電阻,只有局部輸出電壓出現(xiàn)過了橋顯示電流,TC的單位和齊納會(huì)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(duì)于它的描述非常好，這里將不予討論2。基級(jí)與放射級(jí)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-V

5、be2)。 因?yàn)椋瑢?duì)于大多數(shù)的實(shí)際應(yīng)用TC的參考價(jià)值將比齊納的TC優(yōu)先考慮到很難獲得高達(dá)50 V/C的差，這就會(huì)變得相當(dāng)明顯，在大多數(shù)情況下,TC可能會(huì)超出額定值。例2:一個(gè)30AV/C下平安的，低本錢的設(shè)計(jì)。與一個(gè)1N752并聯(lián)，整體的TC將會(huì)是 實(shí)驗(yàn)，筆者計(jì)算出在室溫情況下13個(gè)標(biāo)準(zhǔn)的鍺晶體管的信號(hào)，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(gè)-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(gè)1N752并聯(lián)甚至可能會(huì)給出一個(gè)0.007%/C更好的TC。基極電流的變化 該基

6、級(jí)晶體管的電流由下式給出由于有限源阻抗變化，一個(gè)電流變化造成了差分放大器的信號(hào)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?duì)于所使用的晶體管的I和來說它減少了系統(tǒng)的增益和需求。亨特4指出的值域范圍是在+0.2/至-0.2之間，還有I可能近似于其中A0的值由T0決定。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。并且圖3 Q2的輸入電路當(dāng)前情況下IB流經(jīng)圖3上的每一個(gè)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 EB)所束縛著。因此，如果要看溫度從T1變到T2時(shí)EB的變化輸出電壓的變化并且，例

7、3:假設(shè)有Q3(在25攝氏度)同例1R1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(huì)NEMA提供，可以這樣說“這段時(shí)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(duì)溫度系數(shù)的描述在這里也適用。據(jù)試驗(yàn)測(cè)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(dòng)空氣的冷卻效果是眾所周知的，然而人們通常不會(huì)意識(shí)到就算空氣在齊納二極管和晶體管中移動(dòng)的很緩慢，它對(duì)溫度的影響也是很顯著地。如果提供比擬大的TC，那么輸出會(huì)有很大的變化。會(huì)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(shí)間常數(shù)仍會(huì)受到干擾。一個(gè)常用的方法

8、是使用第一個(gè)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(gè)方案，將晶體管嵌入在一個(gè)共同的金屬塊中，等等。筆者通過把輸入級(jí)和參考齊納放置到一個(gè)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4 12V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t1。盒子里面的溫度上升，電壓隨著時(shí)間 t1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(jià)位和設(shè)計(jì)。接觸電阻的變化可引起漂移。用有高精

9、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，來縮小范圍可以到達(dá)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(duì)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(shí)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的供給以及整體散熱的設(shè)計(jì)有關(guān)。通過對(duì)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(gè)制造商曾經(jīng)說過百分之0.

10、15會(huì)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。比方說在關(guān)鍵的放大器和散熱元件之間放置熱障。外外表最好位于通風(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(gè)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(huì)決定主要的電源時(shí)間常數(shù)。這個(gè)電容器會(huì)導(dǎo)致在遙感模式下短暫的電力供給不良的現(xiàn)象。通常情況下，晶體管電源會(huì)在很短的時(shí)間內(nèi)作出反響，但是筆者曾經(jīng)指出6，在遙感時(shí)，反響會(huì)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(duì)穩(wěn)定的。在平衡條件下，圖7說明，一個(gè)突然的負(fù)荷變化會(huì)導(dǎo)致Ldi/dt的瞬間激

11、增，我們稱之為“尖峰；以及線性放電時(shí)間越長時(shí)電容充放電的情況。放電時(shí)間是，其中并且，對(duì)于Is來說，通常它不會(huì)在放大器的最后階段提供驅(qū)動(dòng)，但是會(huì)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8 框圖。使用預(yù)穩(wěn)壓電源可以減少監(jiān)測(cè)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(chǔ)藏來使這個(gè)階段下降。如果不這樣可能會(huì)導(dǎo)致負(fù)載的飽和，那是前置穩(wěn)壓器在響應(yīng)時(shí)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型晶體管電源供給變得相當(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

12、的供給是遠(yuǎn)程可編程的,會(huì)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)的供給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(duì)高壓、大功率開關(guān)晶體管的開關(guān)晶體管的方法更具有吸引力。該系統(tǒng)提供了一種低本錢,發(fā)行量較小的方法,再加上一個(gè)submillisecond響應(yīng)時(shí)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(gè)被控制的變量或一個(gè)獨(dú)立的自生自儲(chǔ)LC濾波器電路參數(shù)7,8。更快的反響時(shí)間是非?？扇〉?因?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

13、電力需求在壓力調(diào)節(jié)閥過濾器。一個(gè)晶體管作為電源開關(guān)操作適合具有大電流，高電壓等級(jí)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(shí)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(jì)負(fù)荷頂峰期間有足夠的條件，也包含開關(guān)驅(qū)動(dòng)電流限制或快速過載保護(hù)系統(tǒng)。商業(yè)開展電力供給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(gè)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(huì)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(huì)

14、上升直至穩(wěn)態(tài)已恢復(fù)、電路電阻引起限制,或缺乏開車使開關(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(huì)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(gè)解決這個(gè)問題的方法就是限制的電壓變化的速率在可出現(xiàn)一值,通過舞臺(tái)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(huì)同限流特性會(huì)產(chǎn)生一種最大的改變的比率其中C0 = output capacity.假設(shè)這個(gè)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,其中ER = passing stage voltageTl

15、= time constant of reference supply.策略合作關(guān)系SCR的使用來代替電晶體的將是一個(gè)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)，控制工程師手冊(cè)。紐約：McGraw Hill出版社，1958年，11到19頁。2摩托羅拉齊納二極管/整流器手冊(cè)，第二版。 1961。3瓦特施泰格，“一個(gè)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。4唱片獵人，半導(dǎo)體電子手冊(cè)。紐約：M

16、cGraw Hill出版社，1956年，第13-3。5“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)換器局部，NEMA等。6Muchnick，“遠(yuǎn)程傳感晶體管電源電子產(chǎn)品，1962年9月。7路勞克斯，“在開關(guān)型穩(wěn)壓器的設(shè)計(jì)考慮固態(tài)設(shè)計(jì)，1963年4月。8四漢考克和B Kurger，“高效率穩(wěn)壓電源采用高速交換在AIEE冬季干事提交會(huì)議，紐約，紐約，1月27號(hào)至2月1日，1963年。9河四米德爾，差分放大器。紐約：Wiley，1963。10索倫森控制功率目錄和手冊(cè)。索倫森，雷神公司，南諾沃克，美國康涅狄格州單位。本文摘自：IEEE TRANSACTIONS ON INDUS

17、TRY AND GENERAL APPLICATIONS VOL. IGA-2, NO.5 SEPT/OCT 1966Highly Regulated DC Power SuppliesAbstract-The design and application of highly regulated dc power supplies present many subtle, diverse, and interesting problems. This paper discusses some of these problems (especially inconnection with med

18、ium power units) but emphasis has been placed more on circuit economics rather than on ultimate performance.Sophisticated methods and problems encountered in connection with precision reference supplies are therefore excluded. The problems discussed include the subjects of temperature coefficient,sh

19、ort-term drift, thermal drift, transient response degeneration caused by remote sensing, and switching preregualtor-type units and some of their performance characteristics.INTRODUCTIONANY SURVEY of the commercial de power supply field will uncover the fact that 0.01 percent regulated power supplies

20、 are standard types and can be obtained at relatively low costs. 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 percent instead of 0.1 per

21、cent. The performance of a power supply, however, includes other factors besides line and load regulation. This paper will discuss a few of these-namely, temperature coefficient, short-term drift, thermal drift, and transient response. Present medium power dc supplies commonly employ preregulation a

22、s a means of improving power/volume ratios and costs, but some characteristics of the power supply suffer by this approach. Some of the short-comings as well as advantages of this technology will be examined.TEMPERATURE COEFFICIENT A decade ago, most commercial power supplies were made to regulation

23、 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 supply was small compared to the regulation specifications and often ignored. Today, the reference elemen

24、t often carries aTC specification greater than the regulation specification.While the latter may be improved considerably at little cost increase, this is not necessarily true of TC. Therefore,the use of very low TC zener diodes, matched differential amplifier stages, and low TC wire wound resistors

25、 must be analyzed carefully, if costs are to be kept low.A typical first amplifier stage is shown in Fig. 1. CRI is the reference zener diode and R, is the output adjustment potentiometer. Fig. 1. Input stage of power supply.Fig. 2. Equivalent circuit of zener reference.Let it be assumed that e3, th

26、e output of the stage, feedsadditional 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 by the gain of all previous stages. Consequently, the e

27、ffects of other stages will be neglected. The following disculssion covers the effects of all elements having primary and secondary influences on the overall TC.Effect of R3 The equivalent circuit of CRI -R3 branch is shown in Fig. 2. The zener has been replaced with its equivalent voltage source E/

28、 and internal impedance R,. For high gain regulators, the input of the differential amplifier will have negligible change with variations of R3 so thatbefore and after a variation of R3 is made.If it is further assumed that IB Iz; then from (1)Also,Eliminating I, from (2b),andNow, assuming thatthen,

29、Equation (2b) can also be writtenThe Zener Diode The zener diode itself has a temperature coefficient 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

30、. If the bridge 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. Since the characteristic of zeners is so well known and so well descri

31、bed in the literature, a discussion will not be given here 2.Variation of Base-Emitter VoltagesNot only do the values of V, of the differential am-plifier fail to match, but their differentials with tem perature also fail to match. This should not, however,suggest that matched pairs are required. Th

32、e true reference voltage of Fig. 1 is not the value E, but E, + (Vie, -Vbe2)-Since, for most practical applicatioinsthe TC of the reference will be the TC of the zener plusConsidering that it is difficult to obtain matched pairs that have differentials as poor as 50 V/C, it becomes rather apparent t

33、hat, in most cases, a matched pair bought specifically for TC may be overdesigning.Example 2: A standard available low-cost matched pair laims 30AV/C. In conjunction with a 1N752, the ontribution to the overall TC would beTests, performed by the author on thirteen standard germanium signal transisto

34、rs in the vicinity of room temperature and at a collector current level of 3 mA,indicated that 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., Steiger3).

35、 The worst matching of transistors led to less than 400 ,V/C differential. In conjunction with a 1N752,even this would give a TC of better than 0.007%/0C.Variation of Base CurrentsThe base current of the transistors is given by A variation of this current causes a variation in signal voltage at the

36、input to the differential amplifier due to finite source impedances. Matching source impedances is not particularly 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

37、 and that 1, may be approximated by where Ao is the value at To. is also temperature dependent and Steiger 3 experimentally determined the variation to be from about 0.5%/C to 0.9%/0C.And，F(xiàn)ig. 3. Input circuit of Q2.The current AIB flows through the source impedance per Fig. 3. The drops in the resi

38、stance string, however, 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,The change in output voltage isAndExample 3: For Q2 (at 25C)(see Example 1)Varia

39、tion of R,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.SHORT-TERM DRIFTThe short-term drift of a supply is defined by the National Electrical Manufacturers Association (NEMA) as a change in output over

40、a period of time, which change is unrelated to input, environment, or load 5.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 supply contr

41、ibutes enormously to the short-term characteristics. The cooling effects of moving air are quite well known, but it is not often recognized that even extremely slow air movements over such devices as zeners and transistors cause the junction temperature of these devices to change rapidly. If the TC

42、of the supply is large compared to the regulation, then large variations in the output will be observed. Units having low TCs achieved by compensation-that is, by canceling out the effects of some omponents by equal and opposite effects of others may still be plagued by these drafts due to the diffe

43、rence in thermal time constants of the elements.Oftentimes, a matched transistor differential amplifier in a common envelope is used for the first amplifier just to equalize and eliminate the difference in cooling effects between the junctions. Approximations to this method include cementing or hold

44、ing the transistors together, imbedding the transistors in a common metal block, etc. Excellent 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 improvement in drift obtained by means of the addition

45、 of the metal cover is demonstrated dramatically in Fig. 5.Fig. 5. Short-term drift of a power supply 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

46、due to a temperature rise inside the box.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 cover was attached. The initial voltage change following t, is due to a temperatur

47、e rise inside the box.If potentiometers are used in the supply for output adjustment (e.g., RI), care should be used 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 prec

48、ision units to obtain low drift. A 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, other considerations should include the ability of both the arms and the wire

49、to resist corrosion. Silicone greases are helpful here. Periodic movement of contact arms has been found helpful in healing corroded elements.THERMAL DRIFTNEMA defines thermal drift as a change in output over a period of time, due to changes in internal ambient temperatures not normally related to e

50、nvironmental changes. Thermal drift is usually associated with changes in line voltage and/or load changes 5.Thermal drift, therefore, is strongly related to the TC of the supply as well as its overall thermal design. By proper placement of critical components it is possible to greatly reduce or eve

51、n 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 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

52、 thermal drift other than the obvious approach of improving the TC and reducing internal losses include a mechanical design that sets up a physical and thermal barrier between the critical amplifier components and heat dissipating elements. Exposure to outside surfaces with good ventilation is recom

53、mended. With care, 0.01 to 0.05 percent is obtainable.TRANSIENT RESPONSEMost 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 capacitor

54、unfortunately leads to undesirable transient phenomena when the supply is used in the remote sensing mode. Normally, transistorized power supplies respond in microseconds, but as the author has pointed out 6, the response can degenerate severely in remote sensing .The equivalent circuit is shown in

55、Fig. 6. The leads from the power supply to the load introduce resistance r. Is is the sensing current of the supply and is relatively constant.Under equilibrium conditions,A sudden load change will produce the transient of Fig. 7. The initial spike is caused by an inductive surge Ldi/dt; the longer

56、linear discharge following is the result of the capacitor trying to discharge (or charge). The discharge time iswhereandThe 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. Units using pre regulat

57、ors of the switching type (transistor or SCR types) should be looked at carefully if the characteristics mentioned represent a problem. Remote sensing is the process by which the power supply senses voltage directly at the load.Fig. 6. Output equivalent circuit at remote sensing.Fig. 7. Transient re

58、sponse, remote sensing.Fig. 8. Block diagram.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 regulator invariably responds much quicker than the preregulator, sufficient reserve s

59、hould always be built into the drop across the passing stage. Failure to provide this may result in saturation of the passing stage when load is applied, resulting in a response time which is that of the preregulator itself.SWITCHING PREREGULATOR-TYPE UNITSThe conventional class-A-type transistorize

60、d power supply 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 remotely programmable, aggravates the condition enormously. For these