6ES7321-1BH02-0AA0 在之前的中，我向大家介紹了如何借助低側(cè)電流感應(yīng)控制電機(jī)，并分享了為成本型應(yīng)用設(shè)計(jì)低側(cè)電流感應(yīng)電路的三個(gè)步驟。在本篇文章中，我將介紹如何使用應(yīng)用印刷電路板（PCB）技術(shù)，采用一款 （Op amp）來設(shè)計(jì)精確的、低成本的低側(cè)電流感應(yīng)電路。
圖1是之前的文章引用的低側(cè)電流感應(yīng)電路原理圖，圖一中使用的是超小型運(yùn)算放大器。
圖1：低側(cè)電流感應(yīng)原理圖
公式1是計(jì)算圖1所示電路的傳遞函數(shù)：
其中
精確的低側(cè)電流感應(yīng)設(shè)計(jì)對印刷電路板的設(shè)計(jì)有兩大要求。首先要確保分流電阻（Rshunt）直接連接到放大器的同相輸入端和RG的接地端，這通常被稱為“開爾文接法”（Kelvin connection）。如果不使用開爾文接法，會(huì)產(chǎn)生與分流電阻（Rshunt）串聯(lián)的寄生電阻，導(dǎo)致系統(tǒng)產(chǎn)生增益誤差。圖2顯示了系統(tǒng)中寄生電阻的位置。
圖2：與分流電阻（Rshunt）串聯(lián)的寄生電阻
公式2是計(jì)算圖2中電路的傳遞函數(shù)：
第二個(gè)設(shè)計(jì)要求是要將電阻RG的接地端盡可能地靠近分流電阻（Rshunt）的接地端。當(dāng)電流流過印刷電路板的接地層時(shí)，接地層上會(huì)產(chǎn)生壓降，致使印刷電路板上不同位置的接地層電壓出現(xiàn)差異。這會(huì)使系統(tǒng)出現(xiàn)偏移電壓。在圖3中，連接到RG的地面電壓源符號代表了地電位的不同。
圖3：接地層電壓差異
公式3是計(jì)算圖3所示電路的傳遞函數(shù)：
圖4顯示了正確的印刷電路板布局示意圖。
圖4：正確的布局示意圖
圖5展示了我之前建議的適合低側(cè)電流感應(yīng)設(shè)計(jì)的印刷電路板布局。頂層是紅色，底層是藍(lán)色的。印刷電路板布局中的R5和C1指示負(fù)載電阻和去耦電容應(yīng)該放置的的位置。
圖 5：正確的低側(cè)電流感應(yīng)印刷電路板布局
需要注意的是從分流電阻（Rshunt）發(fā)出的軌跡線使用開爾文接法且RG盡可能靠近分流電阻 （Rshunt）。您能夠使用小型（0.8mm×0.8mm）五引腳X2SON封裝的TLV9061運(yùn)算放大器將所有無源器件放置在頂層分流電阻的兩個(gè)焊盤之間。您可以從這里方便地將底層的分流電阻（Rshunt）線路穿過通孔與頂層的同相引腳和RG連接起來。
在您今后為低側(cè)電流感應(yīng)設(shè)計(jì)印刷電路板布局時(shí)，請務(wù)必遵循以下準(zhǔn)則，以減少設(shè)計(jì)中潛在的錯(cuò)誤：
在分流電阻（Rshunt）上使用開爾文接法。
RG盡可能放置在靠近分流電阻（Rshunt）接地端的地方。
去耦電容盡可能靠近電源引腳。
至少要有一個(gè)可靠的接地層。
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