【选型】RS485通信口实现共模4KV，差模2KV防护设计如何选型？

时间： 2020-12-24 来源：世强

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本文将分享笔者对RS485通信口实现共模4KV，差模2KV（IEC61000-4-5 Level 4）的防护设计，已经于近期完成并通过测试。在该设计中，笔者选用了LITTELFUSE的系列防护器件，包含GDT（气体放电管）、PTC（自恢复保险丝）、TVS（瞬态电压抑制二极管）。

由于本案例应用产品部署在户外，其外部连接的RS485端口会暴露在雷击浪涌的环境下，客户要求必须能满足IEC61000-4-5 Level 4的标准，故笔者根据此要求进行设计。在雷击浪涌防护电路设计中，我们需要对防护对象分成两类进行设计，一类是电源端口，一类是信号端口，两者在防护设计构成上会略有不同，本文是对信号端口类进行防护设计。

对于信号线防护，需要考虑防护器件对信号的影响。其中，最为重要的参数为器件的结电容，会影响到信号波形，造成信号畸变。所以，在该类防护电路设计中，拥有很高结电容参数的器件——压敏电阻，就不适合应用。本文中拟用的防护电路构造为前级防护+中间退耦+后级防护的形式。

Littelfuse作为防护器件行业领先品牌，产品品质有保障，性价比高，笔者公司也有很多项目采用的力特产品，故本项目继续沿用。

在进行GDT的选型时，需要考虑直流击穿电压和通流量。因直流击穿电压必须要高于系统最高工作电压，避免常态会误触发，但是较低的击穿电压可以保证更快响应且后端电路残压更低。SL1003A系列GDT器件的结电容仅为1.5pF，且可提供最大10KA的通流量，针对本文应用可以可靠地进行泄放。通流量可达10KA，在10/1000us@200A的波形下拥有300次的寿命，足够保证在需要等级下测试的可靠。该对485线同时为RS232复用线对，RS232的最高电平可达15V，SL1003A系列中最接近的型号为SL1003A090，故选用SL1003A090较为合适。

而TVS的选型需要注意反向隔离电压，在本应用中，该对485线同时为RS232复用线对，RS232的最高电平可达15V，故为了不让器件在常态被误触发，需留有一定余量。SMBJ系列的TVS器件峰值功率可达600W，响应时间小至1ps，且反向泄露电流不大于1uA@VBR>12V。保证常态低电流损耗外，还可迅速响应和保护后端电路。故根据具体需求选用反向隔离电压为16V的SMBJ16CA。

因为本应用电路在常态电流较小（不大于10mA），为了加快整体电路响应速度，采用触发电流为0.3A的PTC较为合适，故PTC选用1812L010。

综合考虑需要同时防护共模和差模干扰，所以后端防护形式采用三点式TVS管接法，得出如下电路：

本文提及的电路在实际应用中，除了要注意器件选型外，还要注意电路通路设计，需要确保雷击浪涌信号先经过前级防护（大通流量器件）再到后级防护（快速响应器件），最后才到被保护电路。同时，还需要保证大地平面的连续和尽可能大面积，尽量避免换层，若面积有限可考虑增大铜厚参数来弥补面积不足的问题。

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型号- 30R135UU,72R300XF,1210L075/24PR,30R090UU,600R160-R1UR,1206L025YR,1812L,1812L350SL,1812L010/60DR,0603L050SLYR,2016L075/60,600R160-RAF,72R160XMR,0603L010YR,60R185XMR,1210L150TH,0805L110SLYR,16R1100G,1210L450SLWR,0402L050SLKR,0603L150SL,30R160UPR,1812L020PR,1206L110/12SL,2920L300DR,1206L175SLYR,72R090X,16R400GPR,16VT210SU,0805L260SLTH,1210L175WR,1812L200/12,LORHO,250S130¨CRCDR,30R300U,1812L200/16,16R1000GF,0805L100,16R250GU,20LR450,250R145-RA,2016L075/60MR,30R090,250R145-RAU,0402L020SL,2920L500/16,1206L260SLTHYR,1812L110/33MR,250S130¨CRC,250S130¨CRB,250S130¨CRA,16R135B,60R110XU,250S130V,2920L330/24MR,0603L075SL,0805L110,1206L175PR,250R145-RB,0603L,1206L012WR,16R500GU,24LT100,1812L150/12DR,1812L200/16DR,16R185BU,1210L380SL,600R160-RAUR,250S130¨CRBDR,30R800UH,15ST175U,250R145ZR,600R160F,16R1000G,16R110BPR,1206L380SLTHYR,16R250GPR,250R120-R3U,600R150-RA,600R150-RB,60R250,60R010,1812L075/33,250R120-R3,250R120-R2,250R120-R1,1210L200,1812L260/16MR,30R250UPR,72R135XMR,16R600GDR,1210L400SL,1206L300SLTHYR,30R700UMR,1206L450SL,250R120-RF,1206L200SLYR,250R120-RC,250R120-RA,0603L035YR,250R120-R2U,0603L175SL,1210L300SLYR,16R800GMR,60R375XH,16R110B,1812L190SLPR,16R1100GF,250R120TU,0805L075SLYR,16R1200G,250R120-RFZR,0402L,2920L200DR,2920L300,600R150F,2920L700SL,250R120-R1U,30R700UF,1210L075YR,2920L700SLPR,1210L110/16WR,60R110,1210L175SLYR,0805L010/24,16R600GU,2920L075/60MR,30R400U,60R050X,72R,60R,0402L010SLKR,72R090XPR,1206L050/15,1206L150SLYR,72R185XMR,2920L260PR,72R250XMR,1206L450SLWR,60R010XU,16VT210S,30R185,1812L075/24,1206L110SL,16R700GKLMR,1206L110TH,60R075X,LT SERIES,0603L100SLYR,1812L370SLPR,16R160BU,0805L110SL,60R025XPR,60R160X,0805L200SLTHYR,30R090UPR,2920L260/24,15LR380U,2920L,2920L185DR,16R500GPR,250R120T,60R090XPR,250R120U,30R300UU,1812L270SLPR,60R300X,1812L300SLPR,30R600U,72R075XU,0603L075SLYR,15LR260U,2016L,0402L020SLKR,06R075BU,06SL370GM,1206L075/16WR,1812L260SL,60R020XPR,20LR730,250R180MR,2920L330/24,06R155BPR,06SL190G,72R040XU,16R400G,1210L110THYR,2920L030DR,1812L160PR,2920L260/24DR,1812L075/33DR,1812L260TH,1210L035YR,1812L270SL,16R135BPR,1206L020YR,1206L075/13.2,30R250,0805L200SLTH,1812L050PR,600R150-RBF,0402L035SLKR,30R600UF,1210L350SL,16R300G,1206L075/13.6,72R075XPR,15LR260,15LR380,16R300GPR,30R185UPR,16R700GF,1812L014DR,60R160XMR,250R145U,0805L,1206L075SL,30R090U,2920L150DR,0805L050WR,1206L150TH,1812L200/12DR,0402L035SL,600R150-RAF,250R145T,1206L150SL,72R300XMR,1206L400SLWR,72R160XF,1812L150/24MR,30R500U,72R030XPR,2920L075DR,1206L075TH,1210L010WR,1812L350SLPR,LR,LT,1812L160/12DR,1812L150/24,60R025XU,2920L400/15MR,16R700GMR,0805L150SLYR,72R050XU,30R,16R1100GMR,60R010XPR,60R090,2920L200/24,1206L035YR,250R145-RBU,1206L050/15YR,16R800GF,1812L150/12,2920L400/15,VT SERIES,30R700U,1210L175SL,250R145TZR,0603L020YR,250S130¨CRADR,30R400UF,0805L300SL,1812L020/60,2016L050,1210L260SL,2016L050MR,16R250BU,1210L110/12WR,1812L010/60,06R120BPR,1812L075PR,30R400UMR,72R065XPR,60R135XF,SL SERIES,0805L010/24YR,16R250BPR,1812L370SL,30R160UU,20LR450U,0603L050SL,60R090X,1206L035/16,1210L020WR,1210L110/16,600R160-R1F,0402L010SL,250S130DR,60R185,24LT100U,72R020XPR,1206L150THWR,1210L380SLYR,1206L175SL,16R700GKL,72R300X,2920L050DR,2016L030,60R090XU,1210L110/12,2016L150,250R120ZR,16R185BPR,60R017XPR,1812L75/33,60R110XMR,1206L035/16YR,60R075XPR,60R025X,15ST175,250R180TMR,1812L150ZR,2920L050,1210L005WR,72R375X,1812L260THDR,1812L200THPR,72R050XPR,1812L125/16,12VL175XLU,0805L010YR,30R500UF,1812L75/24,16R700G,16R700GKLF,60R135X,2920L185,60R110X,1210L075,1812L160/12,16R900GF,1812L110,600R150-RBZR,600R,1812L125/6,60R040XPR,250R120-R1ZR,250R120-RFU,16R600G,2920L030,2920L150,30R800U,2016L100,1812L035/30DR,1812L075/24DR,16R135BU,250R120,30R185UU,SL,1812L010DR,2920L125PR,1206L200,1206L050YR,30R185U,ST,0603L150SLYR,1206L380SLTH,60R160XF,72R020X,30R600UMR,1812L300SL,60R050XU,0805L075SL,0805L260SLTHYR,72R250X,06SL370G,30R300,2016L075/060,1210L175,72R020XU,1812L260SLPR,250R180TF,16R1400GH,16R090BU,250R120-RAZR,0805L150SL,1812L050/30PR,30R160U,1210L260SLYR,60R065XPR,0805L020YR,1210L,1206L350SLTHYR,60R017X,1206L075SLYR,1206L110THYR,72R040XPR,2920L250DR,30R500,1210L110TH,1812L110/24DR,250R120-RCU,2016L200,0603L050YR,0805L300SLWR,1812L200,06R155BU,72R250XF,250R145,60R017XU,1812L020/60DR,1812L035/30,0603L050,1812L125/6PR,30R400,1206L400SL,60R030XPR,1210L350SLWR,60R300XF,VL,1206L260SLTH,06R120BU,72R065XU,1812L300MR,1210L400SLWR,VT,1210L450SL,16R,2920L500/16MR,1206L300SLTH,2920L075,250R120-RAU,72R030XU,250S130-RC,250S130-RB,16R110BU,250S130-RA,1812L300,2920L100PR,250S130,30R110UPR,30R250U,1206L110/12SLYR,0805L175SLYR,72R135X,0805L075WR,1206L016,16R500G,1206L150/12SL,0603L175SLYR,30R250UU,30R900U,16R900GMR,72R185XF,16R090B,1206L012,1206L075/13.2WR,600R150-RAZR,72R110X,1210L075/24,72R025X,0805L050,60R065XU,72R075X,1210L300SL,72R160X,0603L020,250R080ZR,72R090XU,1210L200SLYR,250R120TZR,60R185X,0603L025,0805L100WR,06R120B,2920L200/24DR,250R180,30R135U,60R040X,1206L025,06SL190GM,600R160-RA,72R135XF,06R075BPR,1206L020,1210L005,600R160UR,250R145-RBZR,12VL175XL,6

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