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| Ceramic capacitor will get damaged if repeatedly overvoltaged by spike? |
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| T3sl4co1l:
--- Quote from: AndyC_772 on November 15, 2019, 07:06:40 am ---Applying a high voltage to a ceramic cap effectively saturates the outer layers of the conductive plates with electrons, which means any further electrons have to occupy space which is further inside the conductive material. This boundary layer effectively increases the separation between plates, which reduces the capacitance (recall C = εA/d). --- End quote --- This is an interesting observation, but I'm not sure you are aware what the electron density is in metals--? It is an important figure for semiconductors, though. I wonder if silicon capacitors are heavily enough doped that this isn't a factor? Nonlinear capacitance would be apparent (a MOS or SOS capacitor has less capacitance near zero bias, IIRC, because the depleted surfaces effectively thicken the capacitor; under bias, the boundary layer (holes or electrons) comes right up to the insulator). Tim |
| AndyC_772:
I've no doubt the reduction in capacitance is a complex function involving the combined electrostatic field of all those electrons, and the definition of the Volt. Once upon a time, I could probably have calculated it. Nowadays, I think I prefer a simplified explanation, possibly backed up with some real world measurements to get the point across. I do have an impedance analyser which could do it, but sadly real work always seems to get in the way. |
| T3sl4co1l:
Saturation of ferroics seems to be a statistical process, averaging to a simple power-law asymptote. I don't think I've seen what function a typical ferrite B-H curve approximates, but the typical curves for capacitors gives an approximately linear E(V) curve. Which implies C ~ 1/V or thereabouts. (Or probably more like sqrt(1 / (V^2 + Eo)) since it's symmetrical and has a linear range near zero. Also an offset in V for electrets.) Tim |
| SeanB:
--- Quote from: treez on November 15, 2019, 06:37:10 am ---Thanks, I must admit, I once worked in a huge multinational aerospace company. I was doing a board where I needed more capacitance but didn’t have room. We weren’t allowed electrolytics, had to be ceramics. I was publicly trashed in front of the whole office by the Chief Engineer who told me that I should have used ceramic capacitors which were only rated for about half the rail voltage (because they were smaller and I would fit more of them on) …because as he said, ceramic MLCC capacitors can easily withstand this, and he said it was a mark of my ignorance that I was not aware of this. --- End quote --- Now we know who the Boeing designer of MCAS was........ There is a reason why you can get MLCC units with a built in fuse, and just because they cost more is not one of the reasons. |
| Gyro:
Yes, MLCCs do blow. At low voltage, high current they will burn up and damage the board. At 630V+ with reasonable reservoir capacitance and/or a relatively direct link to the mains involved (it looks like the steady state voltage is about 340V), it is likely to be rather more catastrophic. |
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