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  1. #1
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    pcb breakdown voltage

    Anybody can help me to figure out some hints on the PCB characteristics under high AC voltage? It is ESD generator discharged current/voltage, the max voltage can be up to ~4KV, even higher, and the rising edge could be 0.7~1ns.

    I am considering if the FR4 eproxy material Dielectric constant and loss tangent would change at such high voltage and quick rising edge.
    Anybody can provide some papers or application notes on the max voltage the FR4 PCB can support?
    we can always see a lot of document and simulation results saying, what is the max frequency of the FR4 based PCB, but somehow few lines about the maximum AC voltage it can stand.

    Appreciate of your help :)

    Thanks
    - X

    •   Alt26th August 2008, 16:22

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    document pcb characteristic

    Normal FR4 manufacturing process varies, that is why you have to specify if certain traces have to be controlled impedance. Manufacturer will adjust traces to get desired impedance.
    Check Polar web site for application info and manufacturer web sites to see what would be suitable for your purpose.



    •   Alt26th August 2008, 16:47

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    pcb material voltage breakdown

    FR4 material standards show an electric strength (breakdown voltage) around 40 kV/mm, which is quite a lot, I think. Tracking resistance is another question, it may be particularly a problem with high frequency high voltage gradients at the surface, but this common to most organic isolators. Protective coating of high voltage circuits can reduce the danger of partial discharges slowly destroying the isolators surface.

    A lot of high voltage circuits have been manufactured on FR4, I think. With fast pulses, the geometry has to be designed in a way, that field strengths are kept in a safe range also with transients and at conductor edges. Verifying, that the PCB is free from partial discharges during operation may be regarded as an indicator to my opinion.


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    •   Alt26th August 2008, 18:28

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    pcb, breakdown voltage

    yes, I checked the standard of PCB material Dielectric BreakDown Voltage data, and it shows normally it could be up to 1KV/mil, which is 40KV/mm, it is the boundary.

    so how can I understand this value? can I view the material characteristic constant when voltage is below 40KV/mm, or there is some changes in the material characteristics, but there is a formula or model to describe the changes? or the changes are dependent on other variables? is there any theory/paper can explain this?

    Thanks
    -X


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    fr4 dielectric breakdown voltage

    I'm not sure that I understand the question. For ESD tests you should worry most about current (5-10 ampere) not voltage. If the 4kV you are talking about is related to the ESD gun test then this basically is a capacitor of 150pF charged up to e.g. 4kV. The capacitor is discharged through a 330 ohm resistance to the device under test (your system). The rise time mentioned is similar to the IEC 61000-4-2 standards

    Typically the board has some protection devices (EMI-ESD filter) that clamp the voltage to safe voltage of 10V-20V. Moreover, your board will have parasitic or specifically included discrete capacitance that might be bigger than the 150pF causing the voltage to divide.

    Finally, the on-chip ESD devices of the IC's on your board may trigger as well, again clamping the voltage... possibly till destruction of the IC as they are typically not created to sustain 4kV IEC stress.

    Hope this helps!



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