今天小编要和大家分享的是布线技巧与EMC相关信息,接下来我将从ESD中的电容问题,max512esd这几个方面来介绍。
布线技巧与EMC相关技术文章ESD中的电容问题
ESD中的电容问题
如果电子模块有不用的引脚,那么我们如何去处理呢?我们在处理时,可以考虑直接连接到地平面上去。(参考福特公司的处理方案ELCOMP17)(Modules that have spare pins (never used in any version using the same PCB); these pins shall be tied to the module ground plane.)如果有射频接受模块的话,这些引脚的接地可能会造成射频接受性能的降低,这时候就不能使用接地的方式了,必须使用ESD的电容和放电电阻来消除静电的影响了。还有一种例外也是很常见的,如果这些不用的引脚附近有电源或者HSD输出的时候,我们就必须考虑把这些引脚接电容或者地了。显而易见的是,如果这些引脚相邻小于5mm,在不断的插拔过程中,引脚可能弯曲,这样会引起严重的断路事故,这是不被允许的。当然ESD电容和放电电阻不一定必须接到地,也可以接到阻抗低的电源。以上所有的措施都是为了防止静电从意想不到的路径传递到板上。我们看看ISO10605的描述我们在设计ESD电容的时候,电容的容值是有限的,额定电压也是有限地,该如何去选取呢?总结如下:Applied Voltage Levels
- For a specific ESD test Level, the lower the value of capacitance under test, the greater the voltage applied.
- Higher values of capacitance can withstand high levels of ESD pulses。
- The actual applied voltage is also limited by air discharge, which is a function of the case size.
- Capacitor Capabilites
- C0G will wishstand higher levels of ESD for the same voltage rating and capacitance value
- Higher voltage ratings are important if higher ESD levels were going to be involved
- For the same chip size, as the voltage rating increases, the maximum capacitance available decreases
- Chip size has little effect on basic ESD capability,providing the same capacitance value is available at the same voltage rating
- For smaller chip sizes, the maximumu availabe capacitance at the same voltage rating decreases
- Reduction of chip size should be evaluated carefully for ESD critical appliactions.This is especially true if it is necessary to trade off voltage rating or capacitance value
- Use of 0603 chip sizes will most likely result in lower ESD levels.Air Breakdown is a factor to be considered
- Chip sizes samller than 0603 should not be used in ESD critical Applications
- C0G dielectric materials are close to ideal, and are not affected by voltage coefficients
- X7R dielectric materials are Ferro electric, this effect increase the voltage applied
- Higher voltage ratings are again desirable to reduce the impact of the voltage coefficient on the voltage applied
- Smaller size chips may also influence the impact of the voltage coefficient
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