?電感是中常用的(de)(de)元件，由于(yu)它的(de)(de)電(dian)流(liu)、電(dian)壓相(xiang)位不(bu)同，所(suo)以理論上(shang)(shang)損耗為(wei)零。電(dian)感常為(wei)儲能元件，也(ye)常與電(dian)容一起(qi)用在輸入(ru)濾波和(he)輸出濾波電(dian)路上(shang)(shang)， 用來(lai)平(ping)滑電(dian)流(liu)。電(dian)感也(ye)被稱為(wei)扼流(liu)圈，特點(dian)是流(liu)過其上(shang)(shang)的(de)(de)電(dian)流(liu)有“很(hen)(hen)大(da)的(de)(de)慣性(xing)”。換句(ju)話說，由于(yu)磁(ci)通連(lian)續(xu)特性(xing)，電(dian)感上(shang)(shang)的(de)(de)電(dian)流(liu)必須是連(lian)續(xu)的(de)(de)，否則(ze)將會產(chan)生很(hen)(hen)大(da)的(de)(de) 電(dian)壓尖峰。

　　電感(gan)為磁性元(yuan)件，自然(ran)有(you)磁飽和的問題。有(you)的應用允許電感(gan)飽和存在(zai)(zai)一(yi)個(ge)不可忽視的(de)問題，即電(dian)(dian)(dian)(dian)(dian)(dian)感的(de)繞(rao)(rao)線將導致兩個(ge)分布參數(shu)(或寄生參數(shu))，一(yi)個(ge)是不可避免的(de)繞(rao)(rao)線電(dian)(dian)(dian)(dian)(dian)(dian)阻，另一(yi)個(ge)是與繞(rao)(rao)制(zhi)工藝(yi)、材料有(you)關的(de)分布式 雜散電(dian)(dian)(dian)(dian)(dian)(dian)容(rong)(rong)。雜散電(dian)(dian)(dian)(dian)(dian)(dian)容(rong)(rong)在(zai)(zai)低頻(pin)時(shi)影(ying)響不大，但隨(sui)頻(pin)率的(de)提(ti)高而(er)漸顯出來，當(dang)頻(pin)率高到某(mou)個(ge)值以上時(shi)，電(dian)(dian)(dian)(dian)(dian)(dian)感也(ye)許變成電(dian)(dian)(dian)(dian)(dian)(dian)容(rong)(rong)特性了。如果將雜散電(dian)(dian)(dian)(dian)(dian)(dian)容(rong)(rong)“集(ji)中”為一(yi)個(ge)電(dian)(dian)(dian)(dian)(dian)(dian)容(rong)(rong)， 則從電(dian)(dian)(dian)(dian)(dian)(dian)感的(de)等效電(dian)(dian)(dian)(dian)(dian)(dian)路可以看出在(zai)(zai)某(mou)一(yi)頻(pin)率后(hou)所呈(cheng)現的(de)電(dian)(dian)(dian)(dian)(dian)(dian)容(rong)(rong)特性。

　　當(dang)分析電(dian)(dian)(dian)感在線路中(zhong)的工作(zuo)狀況或者(zhe)繪制電(dian)(dian)(dian)壓(ya)電(dian)(dian)(dian)流波形圖時，不妨考慮(lv)下面幾個特點：

　　1. 當電(dian)感L中有電(dian)流(liu)I流(liu)過時(shi)，電(dian)感儲存的能量為：

　　E=0.5×L×I2 (1)

　　2. 在一個開關周(zhou)期中，電感電流的變化(紋波(bo)電流峰(feng)峰(feng)值)與(yu)電感兩端電壓的關系為：

　　V=(L×di)/dt (2)

　　由此可看出，紋波電流的(de)大小跟電感值(zhi)有關(guan)。

　　3. 就像電(dian)容有(you)充(chong)、放電(dian)電(dian)流(liu)一樣，電(dian)感(gan)器(qi)也有(you)充(chong)、放電(dian)電(dian)壓(ya)(ya)過(guo)程。電(dian)容上(shang)的電(dian)壓(ya)(ya)與(yu)電(dian)流(liu)的積(ji)分(安·秒)成正比，電(dian)感(gan)上(shang)的電(dian)流(liu)與(yu)電(dian)壓(ya)(ya)的積(ji)分(伏·秒)成正比。只要電(dian)感(gan)電(dian)壓(ya)(ya)變(bian)(bian)化，電(dian)流(liu)變(bian)(bian)化率di/dt也將變(bian)(bian)化;正向電(dian)壓(ya)(ya)使電(dian)流(liu)線(xian)性(xing)上(shang)升(sheng)，反向電(dian)壓(ya)(ya)使電(dian)流(liu)線(xian)性(xing)下降(jiang)。

　　計算出正確的(de)(de)電(dian)(dian)(dian)感(gan)值對選用合(he)適的(de)(de)電(dian)(dian)(dian)感(gan)和(he)輸出電(dian)(dian)(dian)容(rong)以(yi)獲得(de)最小的(de)(de)輸出電(dian)(dian)(dian)壓紋波(bo)(bo)(bo)而言(yan)非常重要。流過開(kai)關電(dian)(dian)(dian)源電(dian)(dian)(dian)感(gan)器(qi)的(de)(de)電(dian)(dian)(dian)流由(you)交(jiao)流和(he)直(zhi)流兩種分(fen)量組成，因為交(jiao)流分(fen)量具有較(jiao)高的(de)(de)頻率，所以(yi)它會通過輸出電(dian)(dian)(dian)容(rong)流入地，產生(sheng)相(xiang)應的(de)(de)輸出紋波(bo)(bo)(bo)電(dian)(dian)(dian)壓dv=di×RESR。這(zhe)個紋波(bo)(bo)(bo)電(dian)(dian)(dian)壓應盡可能(neng)低，以(yi)免影響電(dian)(dian)(dian)源系統的(de)(de)正常操(cao)作(zuo)，一般(ban)要求峰(feng)峰(feng)值為10mV~500mV。

　　紋波電(dian)(dian)流(liu)(liu)的(de)(de)大(da)小(xiao)同樣(yang)會影(ying)響電(dian)(dian)感器和輸出電(dian)(dian)容的(de)(de)尺寸(cun)，紋波電(dian)(dian)流(liu)(liu)一般設定為最大(da)輸出電(dian)(dian)流(liu)(liu)的(de)(de)10%~30%，因此對(dui)降壓(ya)型電(dian)(dian)源來說，流(liu)(liu)過(guo)電(dian)(dian)感的(de)(de)電(dian)(dian)流(liu)(liu)峰值比電(dian)(dian)源輸出電(dian)(dian)流(liu)(liu)大(da)5%~15%。

　　降壓型開關電源的電感選擇

　　為降壓(ya)型開(kai)關電(dian)(dian)源選擇電(dian)(dian)感(gan)器時，需要確定(ding)最大輸(shu)入電(dian)(dian)壓(ya)、輸(shu)出電(dian)(dian)壓(ya)、電(dian)(dian)源開(kai)關頻(pin)率、最大紋波電(dian)(dian)流、占空比。下面以(yi)圖2為例說(shuo)明降壓(ya)型開(kai)關電(dian)(dian)源電(dian)(dian)感(gan)值的(de)計(ji)算，首先假設開(kai)關頻(pin)率為300kHz、輸(shu)入電(dian)(dian)壓(ya)范(fan)圍12V±10%、輸(shu)出電(dian)(dian)流為1A、最大紋波電(dian)(dian)流300mA。

最大輸入電壓值為13.2V，對應的占空比為：

　　D=Vo/Vi=5/13.2=0.379 (3)

　　其中，Vo為(wei)輸出(chu)電(dian)壓、Vi為(wei)輸出(chu)電(dian)壓。當開關(guan)管(guan)導通時(shi)，電(dian)感器上的電(dian)壓為(wei)：

　　V=Vi-Vo=8.2V (4)

　　當(dang)開關(guan)管關(guan)斷時，電(dian)感器上(shang)的電(dian)壓為：

　　V=-Vo-Vd=-5.3V (5)

　　dt=D/F (6)

　　把公(gong)式(shi)2/3/6代入公(gong)式(shi)2得出：

　　升壓型的電感選(xuan)擇

　　對于升壓(ya)型開關的(de)電(dian)感(gan)(gan)值計算，除(chu)了占(zhan)空比(bi)與電(dian)感(gan)(gan)電(dian)壓(ya)(ya)的(de)關系式(shi)有(you)所改變外，其(qi)它過(guo)程跟降壓(ya)(ya)型(xing)開(kai)(kai)關電(dian)源的(de)計算方式(shi)一樣。以圖3為(wei)例(li)進行計算，假設開(kai)(kai)關頻率(lv)為(wei) 300kHz、輸入電(dian)壓(ya)(ya)范圍5V±10%、輸出電(dian)流(liu)為(wei)500mA、效(xiao)率(lv)為(wei)80%，則最大紋波電(dian)流(liu)為(wei)450mA，對應的(de)占(zhan)空比(bi)為(wei)：

　　D=1-Vi/Vo=1-5.5/12=0.542 (7)

　　當開關管(guan)導通時(shi)，電感器上的(de)電壓為：

　　V=Vi=5.5V (8)

　　當開關管關斷(duan)時，電感器(qi)上的電壓(ya)為：

　　V=Vo+Vd-Vi=6.8V (9)

　　把公式6/7/8代(dai)入公式2得(de)出：

　　請注意(yi)，升壓電(dian)(dian)(dian)源(yuan)與(yu)降(jiang)壓電(dian)(dian)(dian)源(yuan)不同，前(qian)者的(de)負載電(dian)(dian)(dian)流(liu)并不是一(yi)直由(you)電(dian)(dian)(dian)感電(dian)(dian)(dian)流(liu)提供(gong)。當開關管(guan)導通時，電(dian)(dian)(dian)感電(dian)(dian)(dian)流(liu)經過(guo)開關管(guan)流(liu)入(ru)地，而(er)負載電(dian)(dian)(dian)流(liu)由(you)輸(shu)出(chu)電(dian)(dian)(dian)容(rong)提供(gong)，因(yin)此輸(shu) 出(chu)電(dian)(dian)(dian)容(rong)必須有足夠大的(de)儲能容(rong)量來提供(gong)這一(yi)期間負載所需的(de)電(dian)(dian)(dian)流(liu)。但在(zai)開關管(guan)關斷期間，流(liu)經電(dian)(dian)(dian)感的(de)電(dian)(dian)(dian)流(liu)除(chu)了提供(gong)給負載，還(huan)給輸(shu)出(chu)電(dian)(dian)(dian)容(rong)充電(dian)(dian)(dian)。

　　一般(ban)而言，電(dian)(dian)感(gan)值變(bian)大，輸出紋波會變(bian)小(xiao)(xiao)(xiao)，但電(dian)(dian)源的(de)動(dong)態響應也會相應變(bian)差，所以(yi)電(dian)(dian)感(gan)值的(de)選取可以(yi)根據(ju)電(dian)(dian)路的(de)具體(ti)應用要求來(lai)調整以(yi)達到最理想(xiang)效果(guo)。開關頻率的(de)提 高可以(yi)讓電(dian)(dian)感(gan)值變(bian)小(xiao)(xiao)(xiao)，從而讓電(dian)(dian)感(gan)的(de)物理尺寸(cun)變(bian)小(xiao)(xiao)(xiao)，節省電(dian)(dian)路板空間(jian)，因此目(mu)前的(de)開關電(dian)(dian)源有往高頻發展(zhan)的(de)趨勢，以(yi)適應電(dian)(dian)子(zi)產品的(de)體(ti)積越來(lai)越小(xiao)(xiao)(xiao)的(de)要求。?