FPS400-24輸入模擬量模塊,P0922YU使用進(jìn)展
并入繼電器后發(fā)生觸點(diǎn)短路失效的原因，通過原理、實(shí)驗(yàn)測(cè)試、驗(yàn)證及繼電器材料方面，詳細(xì)解析繼電器在電路設(shè)計(jì)應(yīng)用過程中的問題點(diǎn)，為繼電器在開關(guān)電源產(chǎn)品上的設(shè)計(jì)提供參考。
01 失效現(xiàn)象及來源實(shí)際工程設(shè)計(jì)中，把圖1的PFC電路設(shè)計(jì)到產(chǎn)品上，測(cè)試發(fā)現(xiàn)沖擊電流超標(biāo)（設(shè)計(jì)目標(biāo)≤25A），到達(dá)70A。本方案中，熱敏電阻RT1的阻值為10Ω，理論上計(jì)算，按照輸入電壓為90VAC，即在相位90°或270°時(shí)，
FPS400-24輸入模擬量模塊,P0922YU使用進(jìn)展有最大的輸入峰值電壓為90*√2≈127V,輸入最大峰值電流（輸入沖擊電流）為Imax=127/10=12.7A，測(cè)試結(jié)果和理論計(jì)算完全偏離。結(jié)合圖1分析，影響輸入沖擊電流的器件主要是熱敏電阻RT1和繼電器K1，有以下4種組合情況：
1）熱敏電阻開路和繼電器未吸合，此時(shí)輸入屬于開路狀態(tài)，產(chǎn)品應(yīng)該無輸入；
2）熱敏電阻開路和繼電器吸合，輸入電流經(jīng)過繼電器直接給到后端電路，熱敏電阻在電路中不起作用，輸入沖擊電流大；
3）熱敏電阻正常和繼電器未吸合，輸入電流經(jīng)過熱敏電阻給到后端電路，輸入沖擊電流受抑制而減少；
4）熱敏電阻正常和繼電器吸合，輸入電流主要經(jīng)繼電器給到后端電路，熱敏電阻在電路中不起作用，輸入沖擊電流大；
對(duì)熱敏電阻和繼電器進(jìn)行檢測(cè)，結(jié)果為熱敏電阻阻值正常，繼電器在沒有供電的情況下常開點(diǎn)處于吸合狀態(tài)，也就是繼電器為異常器件。更換新的繼電器后，測(cè)得的沖擊電流僅為7.4A。之前產(chǎn)品測(cè)試沖擊電流超標(biāo)屬于第④種情況，輸入電流主要經(jīng)繼電器給到后端電路，熱敏電阻在電路中不起作用，導(dǎo)致輸入沖擊電流大。
電路圖1的工作原理為：繼電器K1并聯(lián)在輸入熱敏電阻RT1的兩端，由PFC電感L2的輔助繞組經(jīng)過線性穩(wěn)壓后供電。當(dāng)開關(guān)電源上電啟動(dòng)后，因?yàn)槔^電器K1此時(shí)沒有供電電壓，繼電器K1處于開路狀態(tài)，輸入電流通過熱敏電阻RT1給大電解電容C8充電，從而限制了開機(jī)的輸入沖擊電流。當(dāng)功率管Q1接收的驅(qū)動(dòng)信號(hào)后，PFC電感L2輔助繞組電壓建立，即繼電器K1供電電壓建立。當(dāng)供電電壓達(dá)到9V左右時(shí)，繼電器開始工作，觸點(diǎn)閉合把熱敏電阻RT1短路，降低產(chǎn)品工作時(shí)的輸入線路阻抗，減少損耗，提高產(chǎn)品的效率。
02 繼電器觸點(diǎn)短路失效的原因
EMC design of printed circuit board (PCB)PCB is the support of circuit components and devices in single chip microcomputer system. It provides the electrical connection between circuit components and devices. With the rapid development of electronic technology, the density of PCB is getting higher and higher. The quality of PCB design has a great impact on the electromagnetic compatibility of single chip microcomputer system. Practice has proved that even if the circuit schematic design is correct and the printed circuit board design is improper, it will also have an adverse impact on the reliability of single chip microcomputer system. For example, if the two thin parallel lines of the printed board are close together, the delay of the signal waveform will be formed, and the reflected noise will be formed at the end of the transmission line. Therefore, when designing PCB, we should pay attention to the correct method, abide by the general
