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Buck converter failures

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?

Reply to
D Yuniskis
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I haven't seen many of those diodes fail.

Huh? Did the 12V fail or the 5V? I can't imagine the logic needing a whole amp.

Usually a Schottky, plenty fast.

--
Regards, Joerg

http://www.analogconsultants.com/

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Reply to
Joerg

Well, here's one! :> Some research has turned up lots of failures -- though I can't see *how* those failures were caused.

12V supply from wall wart. My comment was that the diode would see at least 12V across it. Current through it would (average) be the 5V load of the disk drive *plus* the logic in the USB-SATA converter itself.

It seems pretty commonplace for a drive to use a significant fraction of an amp at 5V (like 0.75A). Even if the SATA bridge draws 50mA, you're still close to an amp.

Existing diode is 3A 40V. *Seems* plenty big enough which leaves me wondering if there is some other "usage issue" that causes the failure. (?)

Reply to
D Yuniskis

That ought to do. The only thing that can happen is hot-plugging the wallwart which causes a ringing. Worse if not much capacitance at the

12V side. But I assume at 40V the switcher chip would have died as well.

What size package is that diode? Hoping the 3A is a continuous rating.

My hunch is a voltage spike. Most of the diode that I've seen after over-current failures were open.

--
Regards, Joerg

http://www.analogconsultants.com/

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Reply to
Joerg

No idea what sort of abuse this saw before it ended up in my lap (broken). With the plethora of different wall warts out there -- many with the same barrel power connectors -- who knows *what* may have been plugged in, here! (though it seems like this device has a bad reputation so it could just be a sh*tty design!)

It's an SS34. Most of the data sheets show it as a DO214 package. But, this is *much* smaller. I'm guessing

2mm x 5mm? (I'll have to pull it to measure it. The PCB is recessed within the enclosure so hard to get at it even with a dial caliper).

I'll start looking for something a bit bigger (after measuring the pad locations and checking clearance around it). There's not much else that *could* go wrong (unless someone plugged in a faulty drive? I don;t think the little converter has over current protection (?))

(FP6101 -- I'll go dig up a datasheet on it)

Thx,

--don

Reply to
D Yuniskis

Make and model so we don't buy one?

Reply to
miso

"D Yuniskis" kirjoitti viestissä:hq075u$fr6$ snipped-for-privacy@speranza.aioe.org...

I have seen diodes (and inductor) fail when output is shorted eg. because of shorted cable from too much flexing.

-ek

Reply to
E

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.)

Reply to
D Yuniskis

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.

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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.

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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.

Reply to
Hammy

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.

Reply to
whit3rd

Of course, the drive's power consumption will drive the

*average* current through the diode -- no idea what the ripple will be.

That would be hard. There doesn't appear to be much unused real estate to exploit. E.g., moving to a larger package may prove to be a problem in itself. Maybe try to go to a thru-hole part more heavily derated and let it "fly"?

Reply to
D Yuniskis

Well, it has "short circuit" protection but I don't see any way to limit the current (e.g.,setting it to only deliver 2A)

I've not been able to find anything that wasn't written in chinese. I'll see if I can trick google into a translation.

I think the chip is designed for < 20V. Diode is 40. Of course, as you say, the chip might be able to tolerate stresses as long as it hasn't turned the FET on...

Cool! Actually, from eyeballing the foils on this board, there is no way to avoid the scenario outlined in the AN. The power switch has nothing on the upstream side -- besides the wall wart. I.e., turning power on after connecting the wall wart is just the same as turning power on *before* attaching the wall wart.

The only 'lytics I see are on the output of the converter.

So, tacking something across the input "barrel connector" would be a win -- as well as finding a heftier diode.

Have to replace the diode before I can even power it up.

But, that would have toasted the load already, right? (since the load expects to see 5V, not the 12V supplied).

Reply to
D Yuniskis

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