[SI-LIST] Re: Capacitor surge current

  • From: Christopher Jakubiec <Christopher.Jakubiec@xxxxxxx>
  • To: weirsp@xxxxxxxxxx
  • Date: Thu, 11 Mar 2004 08:26:05 -0800

Steve,

Your safety point is well taken, and I am raising this issue within my company. 
 This is a design that has been around for some time, and it is only recently 
that we have observed the failures.  The problem with boards that have been in 
production for a considerable amount of time is that components tend to get 
replaced due to end of life issues, lead free changes, etc, and then the 
circuits sometimes start to behave in different manners.

Thank You for all the help.  I think that we can call this one closed for now.

-Chris



steve weir wrote:
> 
> Christopher, after thinking about your problem some more, from the
> description, I really think you have a serious safety hazard.  I know you
> don't want to make changes to the PCB, but the liability of a fire is a
> serious issue.  I am surprised that the assembly made it through safety
> agency testing.  The fact that you are blowing fuses and charring
> capacitors is not something to take lightly.  If you manage to just damage
> a capacitor without blowing the fuse, then you have a recipe for a fire.
> 
> As we discussed previously, you need to insert some sort of significant
> impedance to protect the components.  Using miller capacitance from the
> power-FET seems wonderful at turn-on, but at turn-off can easily destroy
> the FET.  There are lots of hot-swap controllers on the market now, some
> better than others.  If the hot-swap controllers are too rich for your
> blood, then you can build a discrete circuit that performs well for about
> $0.50 plus the cost of the power FET.
> 
> If you have logic control of the FET a less desirable but sometimes
> workable solution is to provide a precharge resistor to the bulk cap and
> hold-off the FET long enough to let the cap charge up.  If you really have
> only the one 1uF cap, that might be a viable way to go.  The resistor
> values and start-up times are usually prohibitive for millifarad range
> capacitor banks.  Other alternatives include NTCs, but with the caution
> about reset times.  Also if you have just the one cap, a choke on garbage
> iron could work with a little series resistor.   Another alternative is if
> you can afford the reduction in insertion loss, a small resistor in series
> with the capacitor alone.
> 
> Whatever you do, I implore you to get to the root cause of this thing
> before your equipment causes a fire that injures someone.
> 
> Regards,
> 
> 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
>
------------------------------------------------------------------
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
  

Other related posts: