余 強(qiáng) 曾國(guó)強(qiáng) 葛良全 魏世龍 劉璽堯 羅 群
(成都理工大學(xué) 核技術(shù)與自動(dòng)化工程學(xué)院 成都 610059)
微型X射線管燈絲電源的研制
余強(qiáng)曾國(guó)強(qiáng)葛良全魏世龍劉璽堯羅群
(成都理工大學(xué)核技術(shù)與自動(dòng)化工程學(xué)院成都610059)
根據(jù)微型X射線管的特點(diǎn),設(shè)計(jì)了基于推挽電路和閉環(huán)反饋電路的燈絲驅(qū)動(dòng)電源。采用了UCC3808 脈寬調(diào)制(Pulse Width Modulation, PWM)控制芯片產(chǎn)生兩路相位相反的PWM信號(hào),并經(jīng)由UCC37324 金屬氧化物半導(dǎo)體(Metal-Oxide-Semiconductor, MOS)管大電流驅(qū)動(dòng)器提高驅(qū)動(dòng)效率,實(shí)現(xiàn)推挽結(jié)構(gòu)的后端驅(qū)動(dòng)電路。閉環(huán)反饋電路的設(shè)計(jì)采用精密電阻將管電流的信號(hào)轉(zhuǎn)換為等比例的電壓信號(hào)引入到反饋環(huán)節(jié),通過系統(tǒng)自身的閉環(huán)控制實(shí)現(xiàn)管電流的恒定,并保持高效率和低功耗。實(shí)驗(yàn)首先通過外接1.5Ω的功率電阻測(cè)量系統(tǒng)功率,測(cè)得輸入功率為11W,效率超過80%,符合燈絲電源的功耗指標(biāo)要求;然后連接實(shí)際的X光管燈絲,經(jīng)測(cè)試可正常工作,管流穩(wěn)定度為0.227%。
微型X射線管,燈絲電源,推挽電路,管流閉環(huán)反饋電路
經(jīng)過一個(gè)多世紀(jì)的發(fā)展,X射線在各行業(yè)的應(yīng)用日益廣泛[1-3]。燈絲電源作為X射線管的重要核心組件,亦得到了廣泛的關(guān)注。燈絲電源使用的傳統(tǒng)技術(shù)是:工頻220V交流電輸入,功率因素校正后,經(jīng)降壓變壓器加熱燈絲。它有以下缺點(diǎn):體積大、輸出電壓紋波大、效率不高、燈絲電流穩(wěn)定性一般[4-6]。對(duì)中大型X射線管燈絲電源,能夠滿足要求。
本文針對(duì)于微型X射線管對(duì)管電流的控制要求,設(shè)計(jì)了推挽結(jié)構(gòu)、高頻電磁場(chǎng)耦合能量、閉環(huán)反饋電路的小體積、高效率的燈絲電源。
燈絲電源的整體框圖如圖1所示,分為兩個(gè)部分,電路上電工作時(shí),模擬開關(guān)自動(dòng)選定預(yù)加熱環(huán)路部分。初始加熱值設(shè)定為最大值,通過誤差放大器輸入到脈寬調(diào)制(Pulse Width Modulation, PWM)波發(fā)生電路,此時(shí)間段,輸出的兩路互補(bǔ)PWM波占空比均為50%,達(dá)到能量傳遞的最大值。驅(qū)動(dòng)電路用來增強(qiáng)電路的驅(qū)動(dòng)能力。當(dāng)管電流產(chǎn)生,通過取樣隔離反饋到前級(jí)電路與設(shè)定的控制電壓比較,若取樣電壓高于控制電壓,則觸發(fā)模擬開關(guān)轉(zhuǎn)換到第二回路。此回路在將電流信號(hào)轉(zhuǎn)換為電壓信號(hào)后,通過閉環(huán)反饋控制電路使得管電流趨于穩(wěn)定[7]。
圖1 燈絲電源整體框圖Fig.1 Integrated block diagram of filament power supply.
2.1核心激勵(lì)電路結(jié)構(gòu)與設(shè)計(jì)
燈絲電源的核心激勵(lì)選擇了推挽結(jié)構(gòu)。驅(qū)動(dòng)電路簡(jiǎn)單,電源電壓利用率很高,在輸入電壓很低的情況下仍能維持很大的輸出功率[8-10]。同時(shí),為了進(jìn)一步提升效率并減小推挽電路中的開關(guān)噪聲,引入了串聯(lián)諧振軟開關(guān)電路,串聯(lián)諧振軟開關(guān)推挽電路采用副邊串聯(lián)諧振電路,工作在零電流關(guān)斷、零電壓開通的條件下,相比與普通推挽電路,具有更高的效率和更低的噪聲[11-12]。其電路結(jié)構(gòu)如圖2所示。
由于推挽結(jié)構(gòu)需要一對(duì)相位相反的PWM波來驅(qū)動(dòng),所以需要設(shè)計(jì)一個(gè)能輸出兩路互補(bǔ)PWM波的電路。設(shè)計(jì)上采用了德州儀器(Texas Instruments)公司生產(chǎn)的高速、低功耗并能輸出兩路對(duì)稱PWM波的芯片UCC3808,電路如圖3所示。引入芯片UCC37324,以提高電路的驅(qū)動(dòng)能力。防止推挽電路的金屬氧化物半導(dǎo)體(Metal-Oxide-Semiconductor, MOS)管因關(guān)斷時(shí)的尖峰脈沖過高而損壞,在MOS管Q4、Q5旁均并聯(lián)了瞬態(tài)抑制二極管SMBJ,并且開關(guān)管選用耐壓值為100V的AM4492。
圖2 推挽變換電路Fig.2 Push-pull conversion circuit.
圖3 PWM波發(fā)生電路Fig.3 PWM wave generating circuit.
2.2電磁場(chǎng)耦合能量的設(shè)計(jì)
用錳鋅鐵氧體磁環(huán)作為磁芯,它的相對(duì)磁導(dǎo)率為20000,尺寸為18mm×10mm×10mm。在磁環(huán)上自行繞制優(yōu)良導(dǎo)線作為線圈,從線圈中間引出抽頭,使得輸入兩路互補(bǔ)的PWM波輸入到該激勵(lì)線圈時(shí), 環(huán)路能夠通過推挽的方式來通過電磁場(chǎng)傳遞能量,提高工作效率。在高頻錳鋅鐵氧體磁環(huán)初級(jí)所繞制的線圈的初次級(jí)匝數(shù)比為3:1,可適當(dāng)提高次級(jí)線圈的感生電流[13]。為了提高能量傳遞的效率,采用的設(shè)計(jì)有:在保證高效率的條件下,使推挽電路的驅(qū)動(dòng)頻率盡可能地高;所設(shè)計(jì)次級(jí)線圈的環(huán)路的直徑約為35mm;采用黃銅管來作為次級(jí)線圈的材料。
2.3閉環(huán)反饋電路的設(shè)計(jì)
閉環(huán)反饋環(huán)路1如圖4所示。En為電路使能信號(hào),Rdy為管電流超過預(yù)定值的觸發(fā)信號(hào)。Rdy默認(rèn)為低電平,電路使能后,僅當(dāng)管電流超過設(shè)定值時(shí)Rdy才跳轉(zhuǎn)為高電平。
圖4 閉環(huán)反饋環(huán)路1Fig.4 Closed-loop feedback loop 1.
系統(tǒng)上電后,En默認(rèn)為低電平,模擬開關(guān)1和2均導(dǎo)通,此時(shí)UCC3808無PWM波輸出。當(dāng)En變?yōu)楦唠娖胶?,模擬開關(guān)1斷開,環(huán)路1正常工作。電路對(duì)UCC3808輸出的PWM波取樣濾波后,將信號(hào)反饋入同向比例放大器,然后輸入到誤差放大器的同相端,與設(shè)定值(默認(rèn)最大值)比較后,誤差放大器輸出電壓至UCC3808的反饋輸入端FB,改變PWM波的占空比到電路趨于穩(wěn)定。由于誤差放大器反向端設(shè)定值默認(rèn)+5V,所以當(dāng)反饋環(huán)路穩(wěn)定時(shí),PWM波占空比達(dá)到最大值,燈絲上得到的能量也最大。
當(dāng)產(chǎn)生管電流后,首先對(duì)管電流進(jìn)行取樣,通過精密電阻將電流信號(hào)轉(zhuǎn)換為電壓信號(hào)。接著對(duì)轉(zhuǎn)換過后的電壓信號(hào)進(jìn)行了π型LC濾波。采用一般LC/LT型濾波器時(shí),通常會(huì)因?yàn)樵磁c濾波器端阻抗的不匹配而導(dǎo)致電路在某一頻率下和電路中其它元件產(chǎn)生諧振,從而影響到電路正常工作。在濾波器輸入端增加一個(gè)濾波電容,改變?yōu)V波器入端的阻抗,便構(gòu)成了π型濾波電路。來自“源”或“負(fù)載”的噪聲先經(jīng)過低阻抗的濾波電容回路,再進(jìn)入LC型濾波電路,這樣的濾波電路也可以同時(shí)抑制來自電源和電路側(cè)的噪聲和諧波信號(hào)。然后通過運(yùn)放的隔離與反向,將取樣濾波后的信號(hào)反饋到前級(jí)與設(shè)定的管電流初始值比較。若當(dāng)前的管電流大于設(shè)定值,比較器LM393將輸出高電平到D觸發(fā)器MC74HC74的時(shí)鐘端,MC74HC74的輸出端Rdy將輸出高電平,使得模擬開關(guān)2關(guān)閉、模擬開關(guān)3打開,電路進(jìn)入反饋環(huán)路2,如圖5。反饋環(huán)路2即一個(gè)閉環(huán)反饋控制調(diào)節(jié)。將反饋回來的管電流信號(hào)輸入到UCC3808,從而控制輸入PWM波的占空比和燈絲電流的大小,使管電流趨于穩(wěn)定[13?14]。
圖5 閉環(huán)反饋環(huán)路2Fig.5 Closed-loop feedback loop 2.
3.1電磁輻射干擾的抑制
燈絲電源中產(chǎn)生電磁干擾最大的部分就是MOS管用推挽的方式驅(qū)動(dòng)磁環(huán)的部分。當(dāng)MOS管工作頻率提高時(shí),開關(guān)的切換速度更快,電磁干擾也將增大。所以,開關(guān)器件的工作頻率與電磁干擾的大小成正比關(guān)系。降低電磁干擾采取的措施是設(shè)計(jì)了LC無源吸收電路,并在電路周圍披覆一層接地的金屬屏蔽膜。
3.2PWM發(fā)生電路實(shí)測(cè)波形
燈絲電源的效率受到頻率、PWM波的占空比、輸入電流、負(fù)載電阻、輸入功率的影響,最后綜合考慮,選擇了頻率為100kHz。將PWM波的占空比設(shè)置為50%,當(dāng)電路反饋環(huán)路建立穩(wěn)定后,用示波器測(cè)得PWM波發(fā)生電路輸出的兩路互補(bǔ)PWM波的波形如圖6所示。
圖6 兩路互補(bǔ)PWM的波形Fig.6 Two complementary PWM waveform.
3.3燈絲電源效率測(cè)試
微型X射線管的陰極燈絲的等效電阻為12Ω,所以將燈絲電流帶上1.5Ω的功率電阻進(jìn)行效率測(cè)試。在不同推挽頻率下,對(duì)燈絲電源的效率進(jìn)行了測(cè)試,結(jié)果如表1所示。
表1 不同推挽頻率燈絲電源的效率Table 1 Efficiency of different frequency of push-pull filament power supply.
3.4管流穩(wěn)定性測(cè)試
采用本文設(shè)計(jì)的燈絲電源的微型X光管,在管壓15kV、管流10μA條件下,激發(fā)銅箔,采用X射線SiPin探測(cè)器測(cè)量能譜曲線,獲得其銅峰的凈峰面積,每5s測(cè)量一次凈峰面積計(jì)數(shù),測(cè)試數(shù)據(jù)如下,計(jì)算可得平均計(jì)數(shù)為3 052.4,標(biāo)準(zhǔn)差為6.95,相對(duì)標(biāo)準(zhǔn)偏差為0.227%。
圖7 5s凈峰面積計(jì)數(shù)的連續(xù)測(cè)量數(shù)據(jù)散點(diǎn)圖Fig.7 5-s count net peak area of continuous measurement data scatter plot.
本文設(shè)計(jì)的X光管燈絲電源驅(qū)動(dòng)電路不同于常規(guī)設(shè)計(jì)的燈絲電源,增加了上電加熱環(huán)路,保證上電時(shí)最快速度加熱燈絲,使燈絲處于最佳工作狀態(tài),之后才開啟管流閉環(huán)控制使管流與預(yù)設(shè)值相等,繼而開始工作。相較于傳統(tǒng)比例積分微分(Proportion Integration Differentiation, PID)調(diào)節(jié)方式,當(dāng)預(yù)設(shè)管流較小時(shí),在PID調(diào)節(jié)初期實(shí)際驅(qū)動(dòng)燈絲的功率很小,需要較長(zhǎng)的時(shí)間才能達(dá)到調(diào)節(jié)穩(wěn)定狀態(tài),因此需要較長(zhǎng)時(shí)間的用戶等待時(shí)間。本文的設(shè)計(jì)方法可縮短等待時(shí)間。本文設(shè)計(jì)的X光管其管流的穩(wěn)定性目前實(shí)測(cè)只能達(dá)到0.227%,對(duì)于高精度X熒光元素分析還略有不足,其原因主要是高壓電源疊加在燈絲上的紋波,以及管流閉環(huán)反饋回路的參數(shù)未達(dá)到優(yōu)化,擬在今后工作中改進(jìn)提高。
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Development of the filament power supply of micro X-ray tube
YU QiangZENG GuoqiangGE LiangquanWEI ShilongLIU XiyaoLUO Qun
(College of Nuclear Technology and Automation Engineering, Chengdu University of Technology, Chengdu 610059, China)
Background: X-ray is widely used in various industries whilst the filament power supply is an important core component of X-ray tube. Traditional technology that applies 220-V AC input power frequency, power factor correction, the step-down transformer heating filament works fine for medium and large X-ray tube filament power supply, but is unsuitable for the micro X-ray tube. Purpose: This study aims to develop a novel filament power supply for practical micro X-ray tube. Methods: According to the characteristics of the micro X-ray tube, the push-pull circuit and a closed-loop feedback circuit were employed for the filament drive power supply. UCC3808 pulse width modulation (PWM) control chip was used to generate two opposite phase PWM signals, and the driving efficiency was improved through the UCC37324 metal-oxide-semiconductor (MOS) tube high current drive, so as to achieve the back-end of push-pull driving circuit. The tube current signal was converted into isometric voltage signal through the precision resistor, and then be introduced into the feedback loop through closed-loop control of the system to achieve a constant tube current, and maintain high efficiency and low power consumption. Results: The input power is measured to be 11W, and efficiency to be more than 80% when firstly connected to a 1.5-Ω power resistance, which meets the power consumption requirements of the filament power supply. Then an actual X light tube filament was connected for practical test, results show it is in working order with tube flow stability of 0.227%.Conclusions: The filament power supply proposed in this paper meets the requirements of practical micro X-ray tube, its stability is mainly induced by the ripple of high voltage power on the filament, and the unoptimized parameters of tube flow closed-loop feedback loop. These will be improved in the future work.
Micro X-ray tube, Filament power supply, Push-pull circuit, Tube current closed-loop feedback circuit
ZENG Guoqiang, E-mail: zgq@cdut.edu.cn
TL99
10.11889/j.0253-3219.2016.hjs.39.100402
國(guó)家高技術(shù)研究發(fā)展計(jì)劃項(xiàng)目(No.2012AA061803)、國(guó)家自然科學(xué)基金(No.41474159)、成都理工大學(xué)中青年骨干教師培養(yǎng)計(jì)劃項(xiàng)目(No.JXGG201408)、地學(xué)核技術(shù)四川省重點(diǎn)實(shí)驗(yàn)室開放基金項(xiàng)目(No.gnzds2014006)資助
余強(qiáng),男,1989年出生,2014年畢業(yè)于東華理工大學(xué),現(xiàn)為碩士研究生,研究方向?yàn)楹思夹g(shù)及應(yīng)用
曾國(guó)強(qiáng),E-mail: zgq@cdut.edu.cn
Supported by the National High Technology Research and Development Program (No.2012AA061803), National Natural Science Foundation of China(No.41474159), the Program of Cultivating Young Backbone Teachers in Chengdu University of Technology (No.JXGG201408), the Open Fund Project of Nuclear Technology Key Laboratory of Sichuan Province (No.gnzds2014006)First author: YU Qiang, male, born in 1989, graduated from the Donghua Polytechnic University in 2014, master student, major in nuclear technology and application
2016-05-09,
2016-08-10