Hi all, This may be slightly out of the SI realm, but should be interesting to most people out here. I am trying to kill some ringing in my synchronous buck converter (3.3V to 1.1V, load current 15A). The ringing is, as usual, at the junction of the main MOSFET, the synchronous MOSFET and the output inductor. The basic aim in this case is to reduce the peak overshoot as much as possible and damp out the oscillations to a minimum. I have to resort to a snubber (I did the best possible layout I could, and can't change it), and fortunately I do have provision for a RC snubber on the board. On a previous revision of the board, I could kill the ringing with RC snubbers across either switches (the snubber across the main switch took care of the turn-off ringing, whereas that across the synchronous switch reduced the turn-on ringing). The method chosen to arrive at the snubber values was one that is common in the literature - adding capacitance across the suspect node till the ringing frequency is halved, so that the parasitic capacitance of the tank circuit is known; and then adding appropriate resistance (ideally equal to the characteristic impedance of the tank circuit) to damp the oscillations. When the board underwent a revision, I made provision for a RC snubber right across each MOSFET. In addition, I added a little extra copper in the power path, a few more vias, and that's about it. On the current revision, though, the same technique doesn't work: - The ringing frequency (no snubber added) is the same as that on the previous board (62.5MHz) - Adding capacitance across either FET does alter the ringing frequency; the FET o/p capacitance is roughly 2.5nF, and adding a 4.7nF capacitor across it does cause the frequency to reduce to around 30MHz, but in addition there is a new frequency component of around 72MHz riding on it, though it's amplitude is quite low. - Upon adding a resistor (I've tried 1 ohm to 10 ohms) in series with the capacitor, the 30MHz is no longer seen, and the 72MHz ringing becomes prominent... almost as bad as the 62.5MHz ringing that I tried to kill. - For other reasons, I also happened to replace the MOSFET with a different device (a lower gate charge device, but capacitance characteristics are not way different). With this, the resonance occurs at around 83MHz, and is twice as nastier in terms of amplitude; nothing that I try, on the same lines as above, seems to alter the frequency or the amplitude of the ringing at all. The MOSFET datasheets do not mention any inductance characteristics of the device; assuming they are not very different, I guess the difference in behaviour is because of different turn-on/turn-off characteristics. Looking forward to any inputs that might be useful. Further, if somebody has worked on modelling such circuits (including parasitics) to a reasonable degree of accuracy, good enough for practical purposes but not too involved, I'd like some clues on how I could proceed with modelling this. Regards, Chandrashekhar K ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu