【正文】 r conduction time lengthened, so that switching transistor during this period will tend to continuous conduction, and easily damaged. To this end, the requirements of this paragraph in the boot time, the switch to switch the output modulation circuit transistor base drive signal of the pulse width modulation, can guarantee the switching transistor by the cutoff switches are being 3 more and more normal state, therefore the protection of the setting up of a boot to tie in with the soft start. overcurrent protection When the load shortcircuit, overload control circuit failure or unforeseen circumstances, such as would cause the flow of switching voltage regulator transistor current is too large, so that increased power tubes, fever, if there is no overcurrent protection device, high power switching transistor may be damaged. Therefore, the switching regulator in the overcurrent protection is monly used. The most economical way is to use simple fuse. As a result of the heat capacity of small transistors, general fuse protection in general can not play a role in the rapid fuse mon fuse. This method has the advantage of the protection of vulnerable, but it needs to switch transistor in accordance with specific security requirements of the work area to select the fuse specifications. This disadvantage is overcurrent protection measures brought about by the inconvenience of frequent replacement of fuses. Linear voltage regulator monly used in the protection and current limiting to protect the cutoff in the switching regulator can be applied. However, according to the characteristics of switching regulators, the protection circuit can not directly control the output transistor switches, and overcurrent protection must be converted to pulse output mands to control the modulator to protect the transistor switch. In order to achieve overcurrent protection are generally required sampling resistor in series in the circuit, this will affect the efficiency of power supply, so more for lowpower switching regulator of occasions. In the highpower switching power supply, by taking into account the power consumption should be avoided as far as possible access to the sampling resistor. Therefore, there will usually be converted to overcurrent protection, and undervoltage protection. overvoltage protection Switching regulator39。晶體管和集成器件耐受電、熱沖擊的能力較差。最簡單的極性保護(hù)電路如圖 1 所示。在開機(jī)瞬間 ,濾波電容器會(huì)流過很大的浪涌電流 ,這個(gè)浪涌電流可以為正常輸入電流的數(shù)倍。這可用開機(jī)程序控制電路來保證。由于晶體管的熱容量小 ,普通保險(xiǎn)絲一般不能起到保護(hù)作用 ,常用的是快速熔斷保險(xiǎn)絲。 9 過電壓保護(hù) 開關(guān)穩(wěn)壓器的過電壓保護(hù)包括輸入過電壓保護(hù)和輸出過電壓保護(hù)。常用的方法是晶閘管短路保護(hù)。簡單的欠電壓保護(hù)如圖 6所示。 過熱保護(hù) 開關(guān)穩(wěn)壓器的高集成化和輕量小體積，使其單位體積內(nèi)的功率密度大大提高，電源裝置內(nèi)部的元器件對(duì)其工作環(huán)境溫度的要求也相應(yīng)提高。根據(jù) p型控制柵熱晶閘管 (TT102)的特性，由 RT 值確定該器件的導(dǎo)通溫度， RT越大，導(dǎo)通溫度越低。根據(jù)集電極電流的最大值來確定輸入過電流的保護(hù)值。一般告警措施有聲警和光警兩種。這就要求保護(hù)的邏輯嚴(yán)密 ,電路簡單、元器件最少，除此而外還要考慮到保護(hù)電路本身出故障時(shí)維修難度和其所保護(hù)的電源損壞程 度。在大功率 ,多路電源的場(chǎng)合 ,總是用交、直流斷路器 ,高靈敏繼電器等構(gòu)成自動(dòng)保護(hù)措施 ,切斷電源的輸入使系統(tǒng)停止工作 ,免受損害。在所要求的工作溫度范圍內(nèi) ,開關(guān)穩(wěn)壓器的輸出電壓的上、下限就是輸出過、欠電壓保護(hù)的電壓值。它亦可以用作功率晶體管的過熱保護(hù)，晶體開關(guān)管的基極電流被 n 控制柵型熱晶閘管TT201 旁路，開關(guān)管截止，切斷集電極電流 ,防止過熱。亦可將溫度繼電器置于開關(guān)三極管的附近，一般大功率管允許的最高管殼溫度是 75℃ ，調(diào)節(jié)溫度整定值為 60℃ 。特別在升壓型或反相升壓型的直流開關(guān)穩(wěn)壓器中欠電壓的保護(hù)是跟過電流保護(hù)緊密相關(guān)的，因而更加重要。另外，穩(wěn)壓管存在著參數(shù)的離散性，型號(hào)相同但過電壓起動(dòng)值卻各不相同，給調(diào)試帶來了困難。輸入 電源的極性保護(hù)電路可以跟輸入過電壓保護(hù)結(jié)合在一起 ,構(gòu)成極性 保護(hù)鑒別與過電壓保護(hù)電路。但是 ,根據(jù)開關(guān)穩(wěn)壓器的特點(diǎn) ,這種保護(hù)電路的輸出不能直接控制開關(guān)三極管 ,而必須使過電流保護(hù)的輸出轉(zhuǎn)換為脈沖指令 ,去控制調(diào)制器以保護(hù)開關(guān)三極管。為此 ,要求在開機(jī)這一段時(shí)間內(nèi) ,開關(guān)調(diào)制電路輸出給開關(guān)三極管基極的脈寬調(diào)制驅(qū)動(dòng)信號(hào)，能保證開關(guān)三極管由截止逐漸趨于正常的開關(guān)狀態(tài)，故而要加設(shè)開機(jī)保護(hù)以配合軟啟動(dòng)。為了不使該限流電阻消耗過多的功率，以致影響開關(guān)穩(wěn)壓器的正常工作 ,而在開機(jī)暫態(tài)過程結(jié)束后，用一個(gè)繼電器自動(dòng)短接它 ,使直流電源直接 對(duì)開關(guān)穩(wěn)壓器供電。 程序保護(hù) 開關(guān)穩(wěn)壓電源的電路比較復(fù)雜 ,基本上可以分為小功率的控制部分和大功率的開關(guān)部分。 極性保護(hù) 直流開關(guān)穩(wěn)壓器的輸入一般都是未穩(wěn)壓直流電源。 also fed back to the switching regulator control circuit, so that switching transistor cutoff or cut off the input power. overtemperature protection Switching regulator and the high level of integration of lightweight small volume, with its unit volume greatly increased the power density, power supply ponents to its work within the requirements of the ambient temperature is also a corresponding increase. Otherwise, the circuit performance will deteriorate, premature ponent failure. Therefor