05-04-2020, 08:18 AM
My understanding is that the pulse rating is quite a lot down to the metallising.
Pulse currents flow from the leadout wire into the metallising, thence exist as displacement currents (I hate to say 'through the dielectric' but hopefully the meaning is clear!). So the bits of metallising remote from the leadout carry small currents. As you approach the leadout the current grows, and by the time you get to the junction between metallising and leadout, the current is at its maximum.
If the metallising can't carry this current, it degrades, stops conducting. In a stacked-foil type of capacitor (Siemens) the capacitance would reduce, because each layer is isolated from the rest. In a rolled-foil type, it wouldn't, because the layers are all linked, until the last death throes, but ESR would increase.
So high pulse rated capacitors need either foil electrodes rather than metallising, or lots more metallising.
Mike W's comments 'from the inside' are very informative - thanks!
Pulse currents flow from the leadout wire into the metallising, thence exist as displacement currents (I hate to say 'through the dielectric' but hopefully the meaning is clear!). So the bits of metallising remote from the leadout carry small currents. As you approach the leadout the current grows, and by the time you get to the junction between metallising and leadout, the current is at its maximum.
If the metallising can't carry this current, it degrades, stops conducting. In a stacked-foil type of capacitor (Siemens) the capacitance would reduce, because each layer is isolated from the rest. In a rolled-foil type, it wouldn't, because the layers are all linked, until the last death throes, but ESR would increase.
So high pulse rated capacitors need either foil electrodes rather than metallising, or lots more metallising.
Mike W's comments 'from the inside' are very informative - thanks!
