Chris, using Miller capacitance in a HV circuit can be very dangerous at turn-off time. It is very easy to destroy the gate of the MOSFET if you are not careful. Something else you could try is a sizeable powdered iron inductor. The cores are cheap, and the material is lossy, something that you want. The ideal solution will still be to effect a slow-start. If the couple of bucks for a soft-start controller is too much you can get reasonably close with a couple $.02 transistors and a few discretes. Steve. At 10:38 AM 3/9/2004 -0800, Christopher Jakubiec wrote: >Steve, > >The design is already in production, so I don't have the flexibility of >making major changes to the board. I have been experiementing with adding >Miller capacitance between the gate and drain of the FET, and it appears >to be helpful with controlling turn on transients. > >-Chris > > >steve weir wrote: > > > > Chris, I don't know where you are with the design, but would urge you to > > put some sort of controlled impedance between any 50V supply and > > capacitors. A 1uF 100V 1812 MLCC cap has only 26milliohms ESR. If you > > connect that to 50V, but for other parasitics, theoretically, there are > > almost 2000 peak amps available. There are lot's of little discrete > > circuits you can make with a couple of transistors to control turning that > > FET on in a controlled manner. An alternative is the tried and true NTC > > thermistor. The problem with those is that they need to cool down to > > reset. So, if this is a fan tray that someone might remove and reinsert > > before it cools, you will have lost your protection. If you are blowing a > > 2A fuse in under 1ms, then you must have a current on the order of 100A. > > > > Steve > > At 10:16 AM 3/9/2004 -0800, Christopher Jakubiec wrote: > > >Steve, > > > > > >The design that I am working with does not really have a hot-swap > > >controller. A power MOSFET is used to switch in the 48V return path based > > >on a couple of digital logic signals, so power does not come up as softly > > >as one might prefer. The 48V is powering cooling fans that are rated at > > >48V, 800mA. It is a single 1uF, 100V capacitor on the 48V input to the > > >fans that appears to be shorting and in turn blowing a 2A series fuse as > > >well. I am using a current probe on the positive 48V side just > > >before this capacitor to get an inrush current reading of 16A. > > > > > >Thanks for your input, > > > > > >Chris > > > > > > > > >steve weir wrote: > > > > > > > > Chris, generally the current is limited by power dissipation capacity > > > > determined by ESR. 16A destroying a big ceramic capacitor doesn't seem > > > > right. Are you sure it isn't more like 100A? > > > > > > > > It may be too late for your particular design right now, but usually on > > > > power entries, I try to use absolutely minimal capacitance on the > line side > > > > of the hot-swap switch to prevent arcing and deplating problems. > > > > > > > > Steve. > > > > At 08:37 AM 3/9/2004 -0800, Christopher Jakubiec wrote: > > > > >Group, > > > > > > > > > >Does anyone have experience and/or knowledge of the susceptability of > > > > >ceramic surface mount capacitors to surge/inrush currents? I have > a 48V > > > > >circuit that uses a 1uF, 100V, X7R SMD ceramic capacitor and it > appears > > > > >that the capacitors are shorting out in some cases. I have > measured the > > > > >inrush current during turn on, and I am finding a current spike on the > > > > >order of 16A for approximately 40uS, and then the current decays for > > > > >another 160uS or so. The voltage at turn on is peaking at about 58V. > > > > >Most capacitor manufacturer data sheets that I have looked at do not > > > > >really specify this specifically. > > > > > > > > > >Thanks, > > > > > > > > > >Chris Jakubiec > > > > >Sun Microsystems > > > > >------------------------------------------------------------------ > > > > >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 technical documents are available at: > > > > > http://www.si-list.org > > > > > > > > > >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 > > > > > ------------------------------------------------------------------ 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 technical documents are available at: http://www.si-list.org 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