Ugly Tezro power

Elf

Storybook
Feb 4, 2019
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28
Curious, I heard reports of similar voltage regulator issues from an O300 or O350 owner as well. Maybe these are just getting to that age!

With regards to mine, I have replacement parts for that whole block, from the LT switching controller, to the switching MOSFETs, to the capacitors. I suspect more than just a bad capacitor given how one of the MOSFETs seems to be stuck and overheating, so I am hoping that just replacing it all will do the trick.

I ordered the parts some time ago, but with everything going on I have not had the chance to pull the board and put it under the knife (so to speak). It is a big board that I am sure will be annoying to pull, so I am not very keen on it, but it is definitely on the to-do list since I am reluctant to use the machine until the issue is resolved.
 

weblacky

Member
Jan 13, 2020
40
4
8
Seattle, WA
Well, I’m pretty new at PCB level semiconductor repair. A part of me understands what your saying, another part of me proposes a different reason for your observations. So I’m kind of in limbo.

I know yours is not happening consistently, but these MOSFETs are being directly controlled. My brain tells me you should check the signal at the gate of the stuck mosfet before blaming it. Perhaps your controller is commanding it on nearly all the time to compensate for something. I mean, from what I know, more ON time means higher voltage, under normal circumstances. If your controller was trying to raise voltage by the maximum amount of gate signal increase, it would likely look like this, right?

so because the MOSFET hasn’t shorted, I think it might be working fine and it’s being told to do this?

I've not heard of a sticking semiconductor that didn’t exploded shortly afterwards and could reset, so I’m having trouble accepting a semiconductor failure. I would assume the controller is overcompensating (aren’t we all) for feedback voltage being low and cannot run enough to make voltage rise.

But the only way to know for sure would be to scope out the signal and a couple points and see, without equipment to evaluate the cap insitu (which I hope to have in the near future) we wouldn’t know without a minimum of cap replacement (using hot tweezers) and see.

Also those caps are extremely high quality so I’m even more disappointed, I looked them up. Panasonic SVP SMD, 2000 hr service life, low ESR, high ripple tolerable. They still make them!
 

Elf

Storybook
Feb 4, 2019
379
98
28
Ah, I did not mean "stuck on" in the sense that the MOSFET itself was shorted, but "stuck on" in the sense that it may be turned on at an inappropriate time by the controller.

If you look at the photo, there are two MOSFETs (FDD6670A, FDD6612A) that appear to be in a push-pull configuration, with the source of one connected to the drain of the other, and that source/drain connection also leads to the inductor.

This actually lines up pretty well with what is provided as the "typical application" in the LTC3728 datasheet, where you can note the push pull arrangement a bit more easily:
LTC3728 typical application.png


If the cycles of TG2 and BG2 gate drives overlap, for example, then this could cause the MOSFETs to overheat. Oddly in my thermal image above, it was only the lower side MOSFET which was significantly overheating. In that case especially, it is unlikely that trying to prop up too low a voltage (too high a PWM duty cycle on the switching output) would be the cause of overheating of that low side MOSFET. The voltage rail itself showed very little ripple, though I do not have a scope trace of it handy.

Unfortunately the events were so intermittent that I gave up trying to catch one with any more than a multimeter left on. Per Murphy's law, the issue almost never happened when I had the whole unit hooked up on the bench.

Edit: I described this as a "push pull" arrangement due to the half-bridge of MOSFETs, but it is not to be confused with an actual push-pull topology DC-DC converter. It is a synchronous buck converter topology.
 
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weblacky

Member
Jan 13, 2020
40
4
8
Seattle, WA
Yes, I've actually read this datasheet before as user on IrixNet is dealing with a low voltage O350 backplane and it uses a 2-pair mosfet design like this (same chip). I actually offered the suggestion that before he condemns the ICs, that perhaps there is a short on his dragging down the voltage and maxing out current with limiting (as his is extremely low...like 0.188v, instantly), but we never got any farther.

The overlap is an interesting idea, to occur, would the mosfet gate bias threshold need to have drifted/moved? Also isn't the low end mosfet the one that connects to ground? So if high-current was being drawn (ohms law on low voltage) wouldn't the mosfet guarding ground take the actual hammer brunt of the on/off cut-off during load? It's been my personal observation (in another buck DC mosfet pair controller case) that the "lower" (if I'm indicating right) of the pair that connects to ground really heats up on high current draw (due to a dead short after the converter, in my observed case) while the top one was not near as hot. In my observed case, I thought the lower mosfet was bad...but a new mosfet did the same thing...until I actually low-voltage bench PSU short-checked the DC output track of the converter and found the DC converter circuit was just feeding a HUGE dead short (in that case). But nothing was wrong with the converter, feeding a dead short causes low voltage/high amp and the lower mosfet takes on more heat/current.

I guess I blamed the filtering...but as well...it could be an intermittent (IC-based) high-current draw that's pulling down the voltage rail naturally? Hard to fathom but checking the other side of the DC rail isn't a huge problem if it's a real short. If not a static short...then...ughh yeah...unknown right now.
 

Elf

Storybook
Feb 4, 2019
379
98
28
Take a look at shoot through current in a push pull arrangement of switches: if both the high and low switches are on at overlapping intervals, you are shorting the input supply to the ground for a short amount of time. To prevent this there is a dead time given between turning one off and the other on, since the devices can turn off somewhat slowly (relatively speaking). The only thing that does not fit this explanation is I would expect both to be hot.

Also the role of the low side MOSFET in a synchronous buck converter is not that straightforward; it essentially takes the place of the schottky diode you would see in a simple (non-synchronous) buck converter. But this is a simple version of a much deeper explanation that is probably better served by a power electronics textbook :p

It could be that some part of the controller has failed, or that some characteristic of the MOSFET has changed unfavorably, or something else entirely. A short on the output side is possible, but the voltage drops slowly, which would make me put that further down the list of likelihoods.

In any case, lacking the ability to reliably catch it in action to troubleshoot (and being reluctant to cause the issue further), I will just replace the whole section and see how it does!
 

weblacky

Member
Jan 13, 2020
40
4
8
Seattle, WA
Yeah, I very much look forward to your report on those replacements!

My Tezro goes down slowly as well (just does it all the time now), I was only mentioning a similar circuit in a different case. I'm hopeful I can break the log jam with new testing equipment, fingers crossed for end of year, as that's the time I was told to call back the business that was going sell their old equipment to me (but backed out for 6 months due to reorg) and see if they will to now sell me their old/unused tools.
 
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