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| Ferrite bead/ Capacitor filter causes ringing in output? |
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| Sjoertdb:
I am testing the output voltage on a 3V3-buck regulator (TPS62175). On the output i see around 18 mV peak to peak ripple, which is completely fine. However, once the signal passes a ferrite bead/ capacitor filter, the signal is distorted with alot of ringing. Could this be a resonant filter? I've read a small article on Q-factors that might be too high. If someone has experience with this, could you share this? Thanks Sjoert Design Engineer |
| Siwastaja:
Easily. Very typical. Either an AC lossy inductor, or an AC lossy capacitor, or both, tend to help. For the inductor, being lossy means equivalent parallel resistance; for the capacitor, it's equivalent series resistance. This means turning some part of high frequency components to heat, instead of storing and releasing as reactive energy. Ferrite beads are supposed to be lossy, but they always aren't, and even those that are, stop being lossy once saturated, and the saturation current tends to be unspecified (a lot less than the thermal max current). So make sure the bead doesn't saturate; and if the ringing is still an issue, consider using a resistive capacitor. If it's an MLCC, add a series resistor, or use a tantalum (with proper derating & peak current analysis) or aluminium electrolytic (with proper lifetime derating/analysis). |
| Sjoertdb:
Thank you very much for your reply Siwastaja :) :) I will replace the ferrite for a type that does not saturate. I've spent days looking for this problem :P Thumbs up for you :-+ :-+ |
| Sjoertdb:
I've measured the Q-factor using an LCR-meter. In the oscilloscope plot there clearly is a periodic pattern of 13 kHz visible. When the LCR meter is set to 13 kHz, the Q factor is 1.74 and the inductance is 10.9 uH. The filter consists of a 100 nF -> bead (1000R at 100 MHz) -> 1 nF capacitor MLCC. Should this configuration be sensitive for ringing? |
| ConKbot:
A bead that is 1k ohm impedance at 100 MHz (if it's normal 0805-0402 size) is more meant for signal lines, though still perfectly usable for a few mA of power. Look at the datasheet for it. It should have curves for resistance, inductance and combined impedance vs frequency. Down at 13khz, almost no ferrite beads are going to be very resistive, so you're working with an inductor that isn't very lossy at that frequency. Parallel a resistor, in the 50-500 ohm range, across the ferrite bead. It will damp the LC ringing, at the expense of less attenuation from the bead. |
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