Buck converter failures
in [Design]
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From: miso on 13 Apr 2010 01:02 On Apr 12, 3:39 pm, D Yuniskis <not.going.to...(a)seen.com> wrote: > Hi, > > I've got an external SATA-USB cradle that gave up > the ghost. > > 12VDC supply (used by the spindle motor) which is > then dropped down to 5V (for the logic) via a > buck converter (I am guessing this based on visually > examining the foil traces -- I'll have to drag out > the stereoscope to read the markings on the device). > > Note that the design is prone to the load being > removed or applied at will -- can't expect folks > to power the cradle down while inserting/removing > (I mention this only in that it might be a contributing > factor to the failure). > > Converter (SOIC8) gets nice and toasty! > > Voltmeter shows recirculating diode is shorted. > This would be consistent with the overheating as > the output of the converter would just be shorted > through the diode! > > Is this a common failure (or am I making some bad > assumptions)? > > I'll have to check the markings on the diode, as well. > But, I assume it should be sized to see ~12V and ~whatever > the current draw by the SATA drive is (note "~" as approx). > > So, roughly an amp @12V (without any derating). > > (of course, ripple current will have some impact on the > *actual* current through the device) > > Any other issues? I assume this should be a reasonably fast > diode, as well? Make and model so we don't buy one?
From: E on 13 Apr 2010 08:46 "D Yuniskis" <not.going.to.be(a)seen.com> kirjoitti viestiss�:hq075u$fr6$1(a)speranza.aioe.org... > Hi, > > I've got an external SATA-USB cradle that gave up > the ghost. > > 12VDC supply (used by the spindle motor) which is > then dropped down to 5V (for the logic) via a > buck converter (I am guessing this based on visually > examining the foil traces -- I'll have to drag out > the stereoscope to read the markings on the device). > > Note that the design is prone to the load being > removed or applied at will -- can't expect folks > to power the cradle down while inserting/removing > (I mention this only in that it might be a contributing > factor to the failure). > > Converter (SOIC8) gets nice and toasty! > > Voltmeter shows recirculating diode is shorted. > This would be consistent with the overheating as > the output of the converter would just be shorted > through the diode! > > Is this a common failure (or am I making some bad > assumptions)? I have seen diodes (and inductor) fail when output is shorted eg. because of shorted cable from too much flexing. -ek
From: D Yuniskis on 13 Apr 2010 09:18 E wrote: > "D Yuniskis" <not.going.to.be(a)seen.com> kirjoitti > viestiss�:hq075u$fr6$1(a)speranza.aioe.org... >> Hi, >> >> I've got an external SATA-USB cradle that gave up >> the ghost. >> >> 12VDC supply (used by the spindle motor) which is >> then dropped down to 5V (for the logic) via a >> buck converter (I am guessing this based on visually >> examining the foil traces -- I'll have to drag out >> the stereoscope to read the markings on the device). >> >> Note that the design is prone to the load being >> removed or applied at will -- can't expect folks >> to power the cradle down while inserting/removing >> (I mention this only in that it might be a contributing >> factor to the failure). >> >> Converter (SOIC8) gets nice and toasty! >> >> Voltmeter shows recirculating diode is shorted. >> This would be consistent with the overheating as >> the output of the converter would just be shorted >> through the diode! >> >> Is this a common failure (or am I making some bad >> assumptions)? > > I have seen diodes (and inductor) fail when output is shorted eg. because > of shorted cable from too much flexing. How did they fail? (open??) In this case, it would *probably* be hard for the output to have been shorted "mechanically" (I think SATA drives are pseudo-hot-swap) as the connector and cable never are "flexed", etc. (connector is stationary, drive mates *to* it -- sort of like SCA except different connector body). Of course, that's not to say that the drive -- *any* drive that happened to have been plugged into the gizmo (since the whole point of this device is to be able to easily plug/unplug *any* SATA drive) -- didn't have a *fault* which manifested as a short! It really looks like a cheap design. Nothing that protects the device from a defective drive (e.g., the 12V supply is just fed directly through from the wall wart; the 5V supply doesn't *appear* to have any current limiting, etc.)
From: Hammy on 13 Apr 2010 09:42 On Tue, 13 Apr 2010 06:18:26 -0700, D Yuniskis <not.going.to.be(a)seen.com> wrote: > >It really looks like a cheap design. Nothing that protects >the device from a defective drive (e.g., the 12V supply is >just fed directly through from the wall wart; the 5V supply >doesn't *appear* to have any current limiting, etc.) The controller you mentioned (FP6101) does have current limiting. It seems to be a Chinese or Taiwanese controller which may be the problem. If their web site is any indication of the reliability of their products. http://www.micro-bridge.com/FP6101.asp If all they have is ceramic input caps that may be causing the problem. The chip might not be failing from an over voltage transient due to some integrated protection? But the diode might not be so robust from OV. "Ceramic Input Capacitors Can Cause Over voltage Transients" This is I think what Joerg was referring to. http://cds.linear.com/docs/Application%20Note/an88f.pdf You may also want to scope for ringing on the diode under various line load conditions it may need a snubber. Or its possible unstable loop, saturating inductor.
From: whit3rd on 13 Apr 2010 15:32
On Apr 12, 3:39 pm, D Yuniskis <not.going.to...(a)seen.com> wrote: > I've got an external SATA-USB cradle that gave up > the ghost. > > 12VDC supply (used by the spindle motor) which is > then dropped down to 5V (for the logic) via a > buck converter ... > Voltmeter shows recirculating diode is shorted. Yes, that's a common failure mode. Use an ultrafast recovery diode (they're cheap enough), unless the markings indicate this was a Schottky type (more expensive). SATA only includes modern-ish drives, never going to require more than 2A @5V, but that's enough to put some thermal stress on the rectifier; it may need to heat-sink to the printed wiring board. |