App note: Ceramic input capacitors can cause overvoltage transients
This app note from Linear Technology describes why ceramic capacitors aren’t an ideal replacement for tantalum based caps. The higher internal inductance of ceramic capacitors creates high voltage spikes:
This entry was posted in app notes and tagged app note, Linear Technology.Applying a voltage step to a ceramic capacitor causes a large current surge that stores energy in the inductances of the power leads. A large voltage spike is created when the stored energy is transferred from these inductances into the ceramic capacitor. These voltage spikes can easily be twice the amplitude of the input voltage step.
Comments
It’s not that the ceramics have high internal inductance (they don’t) it’s that they have such low internal resistance that they allow the parasitic inductance of the power leads an wall-wart to charge up. A TVS is an easy way to deal with it and will help other overvoltage problems as well.
Yep well this is wrong:
“The input ripple current is usually in the range of 1A to 2A.”, the cap is a short until it is charged.
I worked on some PSU designs back in the late 80′s and in some cases we were seeing inrush >120A
Basically only limited by the resistance /inductance of the supply cables.
We corrected it by adding an inductor.
This happens because ceramic caps are too good. They have very little resistance and their Q is very high. To fix this problem, just add a little series resistance. There is no need for a protection circuit. A little series resistance also helps with stability when used with LDO regulators. Do not add too much resistance.