問題現象描述

某視頻播(bo)放(fang)(fang)顯示設(she)備(bei)，在(zai)USB模式(shi)下播(bo)放(fang)(fang)視頻文件時，在(zai)對其USB端(duan)子進(jin)行±4KV接觸放(fang)(fang)電(dian)時，出(chu)現USB設(she)備(bei)掉線問(wen)題。需(xu)要拔插USB設(she)備(bei)，操作進(jin)入USB模式(shi)重(zhong)新播(bo)放(fang)(fang)視頻，不符合靜(jing)電(dian)放(fang)(fang)電(dian)測(ce)試B等級判定(ding)結果。

對設(she)備(bei)(bei)的其它端口(kou)進(jin)(jin)行±4KV接觸放電時，USB設(she)備(bei)(bei)掉線概(gai)率較低，靠(kao)近USB端子越近的位置(zhi)進(jin)(jin)行接觸放電出(chu)現USB設(she)備(bei)(bei)掉線的概(gai)率越高。

問題現象排查過程

設計工(gong)程師嘗試(shi)在(zai)USB差分(fen)(fen)信號(hao)線上增(zeng)加不同(tong)規格的(de)TVS管器(qi)件，在(zai)USB差分(fen)(fen)信號(hao)線上串聯(lian)電(dian)阻，靜(jing)電(dian)放(fang)電(dian)USB設備(bei)掉線問題依(yi)然(ran)存(cun)(cun)在(zai)。更換不同(tong)的(de)USB設備(bei)測(ce)試(shi)，問題現象依(yi)然(ran)存(cun)(cun)在(zai)，排除USB設備(bei)抗靜(jing)電(dian)能(neng)力弱的(de)可(ke)能(neng)性。

更換(huan)不同(tong)的(de)測(ce)試(shi)(shi)場地、更換(huan)不同(tong)的(de)靜電放電槍，問(wen)題現(xian)象依然存在，排(pai)除測(ce)試(shi)(shi)場地、測(ce)試(shi)(shi)環境(jing)、測(ce)試(shi)(shi)設備的(de)影響，確認是(shi)視頻播放設備本(ben)身(shen)抗(kang)靜電能力較(jiao)弱。

設(she)備連接(jie)有線(xian)網絡，播放視頻進行靜電(dian)(dian)放電(dian)(dian)測試(shi)，測試(shi)結果(guo)是通(tong)(tong)過，設(she)備視頻來使(shi)用HDMI設(she)備播放視頻進行靜電(dian)(dian)放電(dian)(dian)測試(shi)，結果(guo)也(ye)是通(tong)(tong)過，確認USB設(she)備播放視頻靜電(dian)(dian)放電(dian)(dian)才會出現掉線(xian)問題(ti)。   

USB設備(bei)(bei)的差(cha)分信(xin)號采用直連設計，中間無USB Hub、USB開(kai)關、USB Redriver等電(dian)(dian)(dian)路(lu)，而USB設備(bei)(bei)電(dian)(dian)(dian)路(lu)設計經過多次級聯，如果在靜電(dian)(dian)(dian)放電(dian)(dian)(dian)測試過程中USB設備(bei)(bei)的供(gong)電(dian)(dian)(dian)電(dian)(dian)(dian)源設備(bei)(bei)出現電(dian)(dian)(dian)壓跌落(luo)，當電(dian)(dian)(dian)壓跌落(luo)到USB設備(bei)(bei)供(gong)電(dian)(dian)(dian)電(dian)(dian)(dian)壓的下(xia)限(xian)，則會導(dao)致USB設備(bei)(bei)因供(gong)電(dian)(dian)(dian)電(dian)(dian)(dian)壓不符合規格(ge)要求，出現USB設備(bei)(bei)掉線。

問題分析驗證過程

根(gen)據USB設(she)備供(gong)(gong)電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)源(yuan)(yuan)電(dian)(dian)(dian)(dian)路(lu)結構設(she)計，可知(zhi)USB設(she)備的(de)供(gong)(gong)電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)源(yuan)(yuan)分別經過DC-DC轉換電(dian)(dian)(dian)(dian)路(lu)、時序切(qie)換電(dian)(dian)(dian)(dian)路(lu)、USB電(dian)(dian)(dian)(dian)源(yuan)(yuan)限流開關。為驗證(zheng)問題產(chan)生的(de)原因可能與USB供(gong)(gong)電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)源(yuan)(yuan)穩定性相關，在(zai)(zai)靠近USB端子處供(gong)(gong)電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)源(yuan)(yuan)增加100uF電(dian)(dian)(dian)(dian)解電(dian)(dian)(dian)(dian)容，在(zai)(zai)前(qian)級(ji)電(dian)(dian)(dian)(dian)路(lu)出(chu)現電(dian)(dian)(dian)(dian)壓跌(die)(die)落(luo)瞬(shun)間(jian)進行補(bu)償，防止USB設(she)備供(gong)(gong)電(dian)(dian)(dian)(dian)出(chu)現因供(gong)(gong)電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)壓跌(die)(die)落(luo)出(chu)現設(she)備掉線。

靠(kao)近(jin)USB設備(bei)供電電源引腳增加100uF電容，對USB端(duan)子(zi)進行靜電放電測(ce)試(shi)過(guo)程(cheng)中未出現USB設備(bei)掉線的問題，反復驗證多次，測(ce)試(shi)結果均(jun)是(shi)滿足靜電放電測(ce)試(shi)判定結果的。

為(wei)驗證USB設備(bei)電源(yuan)出現跌落的(de)具體原因，嘗試將USB限流(liu)開(kai)(kai)關(guan)進(jin)行短路后，對USB端子(zi)進(jin)行靜(jing)電放電測試，USB設備(bei)掉線的(de)問題(ti)消失，反復驗證確認與USB限流(liu)開(kai)(kai)關(guan)相關(guan)。   

問題根因分析

通過(guo)(guo)分(fen)析排查試(shi)驗(yan)驗(yan)證，確認是USB限(xian)流(liu)開(kai)關本身受(shou)到干擾(rao)，導致內部判斷(duan)出(chu)現過(guo)(guo)流(liu)，自動關閉(bi)輸(shu)出(chu)，靜電放(fang)電干擾(rao)消除后，限(xian)流(liu)開(kai)關恢復(fu)正常，在此過(guo)(guo)程中USB供電電源出(chu)現跌落，后端又沒(mei)有電解電容進(jin)行(xing)補償。

問(wen)題產生的(de)根因是USB限流(liu)開關(guan)靠近外部端(duan)(duan)子(zi)較(jiao)近，外部端(duan)(duan)子(zi)附近的(de)參考(kao)地平面破壞嚴重，靜電(dian)(dian)(dian)電(dian)(dian)(dian)流(liu)流(liu)過時地平面騷(sao)動(dong)大(da)，USB限流(liu)開關(guan)因參考(kao)地騷(sao)擾出現過流(liu)保護。驗證(zheng)過程(cheng)中發現在(zai)限流(liu)開關(guan)的(de)輸(shu)出引(yin)腳(jiao)，增加電(dian)(dian)(dian)解電(dian)(dian)(dian)容緩解電(dian)(dian)(dian)壓跌落可以(yi)解決USB設備掉線的(de)問(wen)題，在(zai)輸(shu)出端(duan)(duan)增加1000pF的(de)高頻電(dian)(dian)(dian)容，吸收靜電(dian)(dian)(dian)放電(dian)(dian)(dian)過程(cheng)中通過輸(shu)出引(yin)腳(jiao)耦合(he)的(de)高頻噪聲也可以(yi)解決USB設備掉線的(de)問(wen)題。

問題解決方案

根據問題原(yuan)因分析，使(shi)用Pin to Pin 抗(kang)(kang)靜(jing)電能力較好的SY6280型號USB限(xian)流(liu)開(kai)關(guan)(guan)，替代(dai)原(yuan)始設計使(shi)用的抗(kang)(kang)靜(jing)電能力較弱(ruo)的TMI6263BH信號的限(xian)流(liu)開(kai)關(guan)(guan)，進行靜(jing)電放(fang)電測試，USB設備掉線問題消失，靜(jing)電放(fang)電測試結果(guo)PASS。

問題解(jie)決方案（二）：使用原(yuan)始設計的TMI6263BH型號(hao)的USB限(xian)流(liu)開(kai)(kai)關芯(xin)片，在輸出引腳增加濾波電(dian)(dian)解(jie)電(dian)(dian)容來補償其電(dian)(dian)壓(ya)跌落，或者(zhe)增加1000pF電(dian)(dian)容來吸(xi)收(shou)靜(jing)電(dian)(dian)放(fang)電(dian)(dian)過程高頻噪(zao)聲通過輸出引腳耦合進入USB限(xian)流(liu)開(kai)(kai)關，誤觸發其過流(liu)保護機制。對策后進行靜(jing)電(dian)(dian)放(fang)電(dian)(dian)測試，USB設備(bei)掉線問題消失，靜(jing)電(dian)(dian)放(fang)電(dian)(dian)測試結果PASS。