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DC Switching Power Supply Protection Technology Abstract: The DC switching power supply protection system, protection system design principles and machine protection measures, an analysis of switching power supply in the range of protected characteristics and its design methodology, introduced a number of practical protection circuit. Keywords: switching power supply protection circuit system design A、Introduction DC switching regulator used in the price of more expensive high-power switching devices, the control circuit is also more complex, In addition, the load switching regulators are generally used a large number of highly integrated electronic systems installed devices. Transistors and integrated device tolerance electricity, less heat shocks. Switching Regulators therefore should take into account the protection of voltage regulators and load their own safety. Many different types of circuit protection, polarity protection, introduced here, the program protection, over-current protection, over-voltage protection, under-voltage protection and over-temperature protection circuit. Usually chosen to be some combination of protection, constitutes a complete protection system. B、Polarity protection DC switching regulator input is generally not regulated DC power supply. Operating errors or accidents as a result of the situation will take its wrong polarity; switching power supply will be damaged. Polarity protection purposes, is to make the switching regulator only when the correct polarity is not connected to DC power supply regulator to work at. Connecting a single device can achieve power polarity protection. Since the diode D to flow through switching regulator input total current, this circuit applied in a low-power switching regulator more suitable. Power in the larger occasion, while the polarity protection circuit as a procedure to protect a link, save the power required for polarity protection diodes, power consumption will be reduced. In order to easy to operate, make it easier to identify the correct polarity or not, collect the next light. C、Procedures to protect Switching power supply circuit is rather complicated, basically can be divided into low-power and high-power part of the control part of the switch. Switch is a high-power transistors, for the protection of the transistor switch is turned on or off power safety, we must first modulator, amplifier and other low-power control circuit. To this end, the boot to ensure the correct procedures. Switching Regulators generally take the input of a small inductor, the input filter capacitor. Moment in the boot, filter capacitor will flow a lot of surge current, the surge current can be several times more than the normal input current. Such a large surge current may contact the general power switch or relay contact melting, and the input fuse. In addition, the capacitor surge current will damage to shorten the life span of premature damage. To this end, the boot should be access to a current limiting resistor, through the current limiting resistor to capacitor charging. In order not to make the current limiting resistor excessive power consumption, thus affecting the normal switching regulator, and the transient process in the boot after a short period then automatically relays it to DC power supply directly to the switching regulator power supply. This circuit switching regulator called a "soft start" circuit. Switching regulator control circuit of the logic components required or op-amp auxiliary power supply. To this end, the auxiliary power supply must be in the switch circuit. This control circuit can be used to ensure the boot. Normal boot process is: to identify the polarity of input power, voltage protection procedures → boot → auxiliary power supply circuit and through current limiting resistor R of the switching regulator input capacitor C → charge modulation switching regulator circuit, → short-circuit current limiting resistor stability switching regulator. In the switching regulator, the machines just because the output capacitance, and charge to the rated output voltage value of the need for a certain period of time. During this time, sampling the output amplifier with low input voltage sampling, closed-loop regulation characteristics of the system will force the switching of the transistor 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 cut-off switches are becoming more and more normal state, therefore the protection of the setting up of a boot to tie in with the soft start. D、Over-current protection When the load short-circuit, 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 over-current protection device, high power switching transistor may be damaged. Therefore, the switching regulator in the over-current protection is commonly 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 common 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 over-current protection measures brought about by the inconvenience of frequent replacement of fuses. Linear voltage regulator commonly used in the protection and current limiting to protect the cut-off 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 over current protection must be converted to pulse output commands to control the modulator to protect the transistor switch. In order to achieve over-current protection are generally required sampling resistor in series in the circuit, this will affect the efficiency of power supply, so more for low-power switching regulator of occasions. In the high-power 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 over-current protection, and under-voltage protection. E、Over-voltage protection Switching regulators input over-voltage protection, including over-voltage protection and output over-voltage protection. Switching regulator is not used in DC power supply voltage regulator and rectifier, such as battery voltage, if too high, so switching regulator is not working properly, or even damage to internal devices, therefore, it is necessary to use the input over-voltage protection circuit. Using transistors and relays protection circuit. In the circuit, when the input DC power supply voltage higher than the voltage regulator diode breakdown voltage value, the breakdown voltage regulator tube, a current flowing through resistor R, so that V turn-on transistor, relay, normally closed contact off open, cut off the input. Voltage regulator voltage regulator which controls the value of Vs. = Earwax-UBE. The polarity of input power with the input protection circuit can be combined with over-voltage protection, polarity protection constitute a differential circuit and over voltage protection. Output over-voltage protection switching power supply is essential. In particular, for the 5V output of the switching regulator, it is a lot of load on a high level of integration of the logic device. If at work, switching regulator sudden damage to the switch transistor, the output potential may be increased immediately to the importation of non-regulated DC power supply voltage value, causing great loss instantaneous. Commonly used method is short-circuit protection thirsted. The simplest over-voltage protection circuit. When the output voltage is too high, the regulator tube breakdown triggered thirstier turn-on, the output short-circuit, resulting in over-current through the fuse or circuit protective device to cut off the input to protect the load. This circuit is equivalent to the response time of the opening time of thirstier is about 5 ~ 10μs. The disadvantage is that its action is fixed voltage, temperature coefficient, and action points of instability. In addition, there is a voltage regulator control parameters of the discrete, model over-voltage start-up the same but has different values, difficult to debug. Esc a sudden increase in output voltage, transistors V1, V2 conduction, the thruster conduction. Reference voltage Vs. by type. F、Under-voltage protection Output voltage below the value to reflect the input DC power supply, switching regulator output load internal or unusual occurrence. Input DC power supply voltage drops below the specified value would result in switching regulator output voltage drops, the input current increases, not only endanger the switching transistor, but also endanger the input power. Therefore, in order to set up due to voltage protection. Due to simple voltage protection. When no voltage regulator input normal, ZD breakdown voltage regulator tube, transistors V conduction, the relay action, contact pull-in, power-switching regulator. When the input below the minimum allowable voltage value, the regulator tube ZD barrier, V cut-off, contact Kai-hop, switching regulator can not work. Internal switching regulator, as the control switch transistor circuit disorders or failure will decrease the output voltage; load short-circuit output voltage will also decline. Especially in the reversed-phase step-up or step-up switching regulator DC voltage of the protection due to over-current protection with closely related and therefore more important. Implementation of Switching Regulators in the termination of the output voltage comparators. Normally, there is no comparator output, once the voltage drops below the allowable value in the comparator on the flip, drive alarm circuit; also fed back to the switching regulator control circuit, so that switching transistor cut-off or cut off the input power. G、Over-temperature protection Switching regulator and the high level of integration of light-weight small volume, with its unit volume greatly increased the power density, power supply components to its work within the requirements of the ambient temperature is also a corresponding increase. Otherwise, the circuit performance will deteriorate premature component failure. Therefore, in high-power switching regulator should be set up over-temperature protection. Relays used to detect the temperature inside the power supply temperature, when the internally generated power supply overheating, the temperature of the relay on the action, so that whole circuit in a warning alarm, and the realization of the power supply over-temperature protection. Temperature relay can be placed in the vicinity of the switching transistor, the general high-power tube shell to allow the maximum temperature is 75 ℃, adjust the temperature setting to 60 ℃. When the shell after the temperature exceeds the allowable value to cut off electrical relay on the switch protection. Semiconductor switching device thermal "hot thirstier," in the over-temperature protection, played an important role. It can be used as directed circuit temperature. Under the control of p-hot-gate thirstier (TT102) characteristics, by RT value to determine the temperature of the device turn-on, RT greater the temperature the lower the turn-on. When placed near the power switching transistor or power device, it will be able to play the role of temperature instructions. When the power control the temperature of the shell or the internal device temperature exceeds the allowed value, the heat conduction thirstier on, so that LED warning light. If the opt coupler with, would enable the whole circuit alarm action to protect the switching regulator. It can also be used as a power transistor as the over-temperature protection, crystal switch the base current by n-type gate control thirstier TT201 thermal bypass, cut-off switch to cut off the collector current to prevent overheating. I、Conclusion Discussed above in the switching regulator of a variety of conservation, and introduces a number of specific ways to achieve. Of a given switching power supply is concerned, but also protection from the whole to consider the following points: 1) The switching regulator used in the switching transistor in the DC security restrictions on the work of regional work. The transistor switches selected by the manual available transistors get DC safe working area. According to the maximum collector current to determine the input value of over-current protection. However, the instantaneous maximum value should be converted to the average current. At rated output current and output voltage conditions, the switch of the dynamic load line does not exceed a safe working area DC maximum input voltage, input over-voltage protection is the voltage value. 2) The switching regulator output limit given by the technical indicators within. Work within the required temperature range, the switching regulators output voltage, the lower limit of the output is off, due to the voltage value of voltage protection. Over-current protection can be based on the maximum output current to determine. False alarm in order not to protect the value of a certain margin to remain appropriate. 3) From the above two methods to determine the protection after the power supply device in accordance with the needs of measures to determine the alarm. Measures the general alarm sound and light alarm two police. Voice of the police applied to more complex machines, power supply parts and do not stand out in a place, it can give staff an effective warning of failure; optical Police instructions can be eye-catching and fault alarm and pointed out that the fault location and type. Protection measures should be protected as to determine the location. In the high-power, multi-channel power supply, always paying, DC circuit breakers, relays, etc. high-sensitivity auto-protection measures, to cut off the input power supply to stop working the system from damage. Through the logic control circuit to make the appropriate program cut-off switch transistor is sensitive it is convenient and economic. This eliminated large, long response time, the price of your high-power relay or circuit breaker. 4) The power of putting in the protection circuit will be affected after the reliability of the system, for which want to protect the reliability of the circuit itself is higher in order to improve the reliability of the entire power system, thereby increasing its own power supply MTBF. This requires the protection of strict logic, the circuit is simple, at least components, In addition to the protection circuit should also be considered a failure of maintenance of their difficulty and their power to protect the damage. Therefore, we must be comprehensive and systematic consideration of a variety of switching power supply protection measures to ensure the normal operation of switching power supplies and high-efficiency and high reliability. 直流開關(guān)穩(wěn)壓電源的保護技術(shù) 摘 要：討論了直流開關(guān)穩(wěn)壓電源的保護系統(tǒng),提出保護系統(tǒng)設(shè)計的原則和整機保護的措施，分析了開關(guān)穩(wěn)壓電源中的各種保護的特點及其設(shè)計方法,介紹了幾種實用保護電路。 關(guān)鍵字：開關(guān)電源 保護電路 系統(tǒng)設(shè)計 一、 引 言 　　直流開關(guān)穩(wěn)壓器中所使用的大功率開關(guān)器件價格較貴,其控制電路亦比較復(fù)雜,另外,開關(guān)穩(wěn)壓器的負載一般都是用大量的集成化程度很高的器件安裝的電子系統(tǒng)。晶體管和集成器件耐受電、熱沖擊的能力較差。因而開關(guān)穩(wěn)壓器的保護應(yīng)該兼顧穩(wěn)壓器本身和負載的安全。保護電路的種類很多,這里介紹極性保護、程序保護、過電流保護、過電壓保護、欠電壓保護以及過熱保護等電路。通常選用幾種保護方式加以組合,構(gòu)成完善的保護系統(tǒng)。 二、極 性 保 護 直流開關(guān)穩(wěn)壓器的輸入一般都是未穩(wěn)壓直流電源。由于操作失誤或者意外情況會將其極性接錯,將損壞開關(guān)穩(wěn)壓電源。極性保護的目的,就是使開關(guān)穩(wěn)壓器僅當以正確的極性接上未穩(wěn)壓直流電源時才能工作。利用單向?qū)ǖ钠骷梢詫崿F(xiàn)電源的極性保護。最簡單的極性保護電路如圖1所示。由于二極管D要流過開關(guān)穩(wěn)壓器的輸入總電流,因此這種電路應(yīng)用在小功率的開關(guān)穩(wěn)壓器上比較合適。在較大功率的場合,則把極性保護電路作為程序保護中的一個環(huán)節(jié),可以省去極性保護所需的大功率二極管,功耗也將減小。為了操作方便,便于識別極性正確與否,在二極管之后接指示燈。 三、 程序保護 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。開關(guān)晶體管則屬大功率，為保護開關(guān)晶體管在開啟或關(guān)斷電源時的安全,必須先讓調(diào)制器、放大器等小功率的控制電路工作。為此,要保證正確的開機程序。開關(guān)穩(wěn)壓器的輸入端一般接有小電感、大電容的輸入濾波器。在開機瞬間,濾波電容器會流過很大的浪涌電流,這個浪涌電流可以為正常輸入電流的數(shù)倍。這樣大的浪涌電流會使普通電源開關(guān)的觸點或繼電器的觸點熔化,并使輸入保險絲熔斷。另外,浪涌電流也會損害電容器,使之壽命縮短,過早損壞。為此,開機時應(yīng)該接入一個限流電阻,通過這個限流電阻來對電容器充電。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,而在開機暫態(tài)過程結(jié)束后，用一個繼電器自動短接它,使直流電源直接對開關(guān)穩(wěn)壓器供電。這種電路稱之謂開關(guān)穩(wěn)壓器的“軟啟動”電路。 開關(guān)穩(wěn)壓器的控制電路中的邏輯組件或者運算放大器需用輔助電源供電。為此,輔助電源必須先于 開關(guān)電路工作。這可用開機程序控制電路來保證。一般的開機程序是:輸 入電源的極性鑒別,電壓保護→開機程 序電路工作→輔助電源工作并通過限流電阻 R對開關(guān)穩(wěn)壓器的輸入電容器C充電→ 開關(guān)穩(wěn)壓器的調(diào)制電路工作，短路限流電阻→開關(guān)穩(wěn)壓器 穩(wěn)定工作。 在開關(guān)穩(wěn)壓器中,剛開機時,因為其輸出電容容量大,充到額定輸出電壓值需要一定時間。在這段時間內(nèi),取樣放大器輸入低的輸出電壓采樣,根據(jù)系統(tǒng)閉環(huán)調(diào)節(jié)特性將迫使開關(guān)三極管的導(dǎo)通時間加長,這樣一來,開關(guān)三極管就會在這段期間內(nèi)趨于連續(xù)導(dǎo)通,而容易損壞。為此,要求在開機這一段時間內(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)動信號，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)開機保護以配合軟啟動。 四、 過電流保護 　　當出現(xiàn)負載短路、過載或者控制電路失效等意外情況時,會引起流過穩(wěn)壓器中開關(guān)三極管的電流過大，使管子功耗增大,發(fā)熱,若沒有過流保護裝置,大功率開關(guān)三極管就有可能損壞。故而在開關(guān)穩(wěn)壓器中過電流保護是常用的。最經(jīng)濟簡便的方法是用保險絲。由于晶體管的熱容量小,普通保險絲一般不能起到保護作用,常用的是快速熔斷保險絲。這種方法具有保護容易的優(yōu)點，但是,需要根據(jù)具體開關(guān)三極管的安全工作區(qū)要求來選擇保險絲的規(guī)格。這種過流保護措施的缺點是帶來經(jīng)常更換保險絲的不便。 在線性穩(wěn)壓器中常用的限流保護和電流截止保護在開關(guān)穩(wěn)壓器中均能應(yīng)用。但是,根據(jù)開關(guān)穩(wěn)壓器的特點,這種保護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的輸出轉(zhuǎn)換為脈沖指令,去控制調(diào)制器以保護開關(guān)三極管。為了實現(xiàn)過電流保護一般均需要用取樣電阻串聯(lián)在電路中,這會影響電源的效率,因此多用于小功率開關(guān)穩(wěn)壓器的場合。而在大功率的開關(guān)穩(wěn)壓電源中,考慮到功耗，應(yīng)盡量避免取樣電阻的接入。因此,通常將過電流保護轉(zhuǎn)換為過、欠電壓保護。 五、 過電壓保護 開關(guān)穩(wěn)壓器的過電壓保護包括輸入過電壓保護和輸出過電壓保護。開關(guān)穩(wěn)壓器所使用的未穩(wěn)壓直流電源諸如蓄電池和整流器的電壓如果過高,使開關(guān)穩(wěn)壓器不能正常工作,甚至損壞內(nèi)部器件,因此,有必要使用輸入過電壓保護電路。 在該電路中，當輸入直流電源的電壓高于穩(wěn)壓二極管的擊穿電壓值時，穩(wěn)壓管擊穿，有電流流過電阻R， 使晶體管V導(dǎo)通，繼電器動作，常閉接點斷開，切斷輸入。其中穩(wěn) 壓管的穩(wěn)壓值Vz=ESrmax－UBE。輸入 電源的極性保護電路可以跟輸入過電壓保護結(jié)合在一起,構(gòu)成極性 保護鑒別與過電壓保護電路。 輸出過電壓保護在開關(guān)穩(wěn)壓電源中是至關(guān)重要的。特別對輸出為5V的開關(guān)穩(wěn)壓器來說,它的負載是大量的高集成度的邏輯器件。如果在工作時,開關(guān)穩(wěn)壓器的開關(guān)三極管突然損壞,輸出電位就可能立即升高到輸入未穩(wěn)壓直流電源的電壓值,瞬時造成很大的損失。常用的方法是晶閘管短路保護。當輸出電壓過高時,穩(wěn)壓管被擊穿,觸發(fā)晶閘管導(dǎo)通,把輸出端短路,造成過電流,通過保險絲或電路保護器將輸入切斷,保護了負載。這種電路的響應(yīng)時間相當于晶閘管的開通時間，約為5～10μs。它的缺點是動作電壓是固定的，溫度系數(shù)大，動作點不穩(wěn)定。另外，穩(wěn)壓管存在著參數(shù)的離散性，型號相同但過電壓起動值卻各不相同，給調(diào)試帶來了困難。 六、 欠電壓保護 　　輸出電壓低于規(guī)定值時，反映了輸入直流電源、開關(guān)穩(wěn)壓器內(nèi)部或者輸出負載發(fā)生了異常。輸入直流電源電壓下降到規(guī)定值之下時，會導(dǎo)致開關(guān)穩(wěn)壓器的輸出電壓跌落，輸入電流增大，既危及開關(guān)三極管，也危及輸入電源。因此，要設(shè)欠電壓保護。簡單的欠電壓保護如圖6所示。 當未穩(wěn)壓輸入的電壓值正常時，穩(wěn)壓管ZD擊穿，晶體管V導(dǎo)通，繼電器動作，觸點吸合，開關(guān)穩(wěn)壓器加電。當輸入低于所允許的最低電壓值時，穩(wěn)壓管ZD不通，V截止，觸點跳開，開關(guān)穩(wěn)壓器不能工作。開關(guān)穩(wěn)壓器內(nèi)部，由于控制電路失?；蛘唛_關(guān)三極管失效會使輸出電壓下降；負載發(fā)生短路也會使輸出電壓下降。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是跟過電流保護緊密相關(guān)的，因而更加重要。實現(xiàn)方法是在開關(guān)穩(wěn)壓器的輸出端接電壓比較器。 正常時，比較器沒有輸出，一旦電壓跌落在允許值之下比較器就翻轉(zhuǎn)，驅(qū)動告警電路;同時反饋到開關(guān)穩(wěn)壓器的控制電路，使開關(guān)三極管截止或切斷輸入電源。 七、 過熱保護 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)部的元器件對其工作環(huán)境溫度的要求也相應(yīng)提高。否則，會使電路性能變壞，元器件過早失效。因此在大功率開關(guān)穩(wěn)壓器中應(yīng)該設(shè)過熱保護。 采用溫度繼電器來檢測電源裝置內(nèi)部的溫度，當電源裝置內(nèi)部產(chǎn)生過熱時，溫度繼電器就動作，使整機告警電路處于告警狀態(tài)，實現(xiàn)對電源的過熱保護。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℃。當管殼溫度超過允許值后繼電器就切斷電器，對開關(guān)管進行保護。半導(dǎo)體熱敏開關(guān)器件“熱晶閘管”在超溫保護方面有重要作用。它可以用作溫度指示電路。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。 當將其放置功率開關(guān)三極管附近，或在電源裝置內(nèi)時，它就能起到溫度指示作用。當功率管的管殼溫度或者裝置內(nèi)部的溫度超過允許值時，熱晶閘管就導(dǎo)通，使發(fā)光二極管發(fā)亮告警。倘若配合光電耦合器，就可使整機告警電路動作，保護開關(guān)穩(wěn)壓器。它亦可以用作功率晶體管的過熱保護，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。 九、 結(jié)語 以上分別討論了在開關(guān)穩(wěn)壓器中的各種保護方式,并介紹了一些具體實現(xiàn)的方法。對一個給定的開關(guān)穩(wěn)壓電源來說，還應(yīng)從整機保護方面考慮以下幾點： （1）把開關(guān)穩(wěn)壓器中所應(yīng)用的開關(guān)三極管限制在直流安全工作區(qū)域之內(nèi)工作。對于選定的開關(guān)三極管,由晶體管手冊可查得其直流安全工作區(qū)。根據(jù)集電極電流的最大值來確定輸入過電流的保護值。但是,這個瞬時最大值應(yīng)轉(zhuǎn)換為電流的平均值。在額定輸出電流與輸出電壓的條件下,開關(guān)管的動態(tài)負載線不超過直流安全工作區(qū)的最大輸入電壓,就是輸入過電壓保護的電壓值。 （2）把開關(guān)穩(wěn)壓器的輸出限制在所給定的技術(shù)指標之內(nèi)。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、欠電壓保護的電壓值。過電流保護則可根據(jù)最大輸出電流來確定。為了不誤告警,保護值應(yīng)適當留一定的余量。 （3）由以上兩點確定保護方式之后,再根據(jù)電源裝置的需要來確定告警措施。一般告警措施有聲警和光警兩種。聲警適用于整機比較復(fù)雜、電源部分又裝在不顯眼的地方,它可以給工作人員以有效的故障告警;光警可以醒目地指示故障告警并指出故障發(fā)生的部位和類型。保護措施要視所保護的部位來確定。在大功率,多路電源的場合,總是用交、直流斷路器,高靈敏繼電器等構(gòu)成自動保護措施,切斷電源的輸入使系統(tǒng)停止工作,免受損害。通過邏輯控制 電路使相應(yīng)的開關(guān)三極管截止的方案顯得既靈敏方便又經(jīng)濟。這樣可以省去體積大,響應(yīng)時間長,價格貴的大功率繼電器或斷路器。 　?。?）電源中加設(shè)了保護電路之后會影響系統(tǒng)的可靠性，為此要求保護電路本身的可靠性要高，以提高整個電源系統(tǒng)的可靠性，進而提高電源本身的MTBF。這就要求保護的邏輯嚴密,電路簡單、元器件最少，除此而外還要考慮到保護電路本身出故障時維修難度和其所保護的電源損壞程度。 所以必須全面系統(tǒng)地考慮開關(guān)電源各種保護措施，確保開關(guān)電源的正常工作和高效率與高可靠性。 附錄A外文文獻及翻譯 31 本科畢業(yè)設(shè)計(論文) 外文翻譯 院 （系）： 電子信息工程學(xué)院 專 業(yè)： 電氣工程及自動化 班 級： 090416 學(xué) 生： 丁瑞朋 學(xué) 號： 090416102 指導(dǎo)教師： 張宏鵬 2013年 5月 畢業(yè)設(shè)計(論文)開題報告 題目：高壓輸電線路