Help with smd voltage regulator ID - Page 1
Forum section: electronics / beginners

author: jerryk
author_rank: Regular Contributor
timestamp: November 23, 2021, 06:21:22 pm
content: I am repairing a bms board that has an SOT23-3 voltage regulator that outputs 3.3 volts. It should be able to take input voltages at least to 15V as a guess. The markings vary but always start with HU.I have seen them with the following markings: HUHJ, HU75, HUG3. The board I'm working on has two of them with HUG3 markings and one is failed short circuit.Any help on this ID is greatly appreciated.Jerry

author: Refrigerator
author_rank: Super Contributor
timestamp: November 24, 2021, 12:12:56 pm
content: Almost any 3.3V regulator in that same package with that same pinout will work.Just make sure it can handle the input voltage.Finding a part from the engraved numbers is nearly impossible so you're better off just finding an equivalent.The numbers might not even be part numbers but rather batch numbers, or production codes, each manufacturer has their own numbering scheme.

author: Manul
author_rank: Super Contributor
timestamp: November 24, 2021, 04:16:31 pm
content: It is likely a MCP1703T-3302E/CB. It is far from impossible to find with practice, but yes, sometimes it is easier to just use another part.

author: Zero999
author_rank: Super Contributor
timestamp: November 24, 2021, 04:37:51 pm
content: 78L03/78L33.

author: jerryk
author_rank: Regular Contributor
timestamp: November 24, 2021, 07:57:48 pm
content: Thanks for the input and suggestions. The MCP1703T-3302E/CB that @Manul suggested seems like it is the part or close enough. The only thing that seems off is the stabilizing cap(C6) is .1u and not 1u like the datasheet called for. Now the problem is supply related. I actually ordered 3 of them a couple of years ago so I thought I might have them in inventory. As luck would have it Mouser sent the wrong part and now I cannot find them at any of the suppliers (Mouser, Digikey, Arrow) that I have dealt with. They are available on Aliexpress but who knows what I'll get there.I am curious as the what would cause it to fail like it did? I attached some pictures of the board and the burn marks it left. Another oddity was the Kapton tape around the mosfets. Why? I have used it to hold the underside mosfets when reflowing the top of the board but then I remove it. Maybe cooling is not an issue here?Again - Thanks for the helpJerry

author: Manul
author_rank: Super Contributor
timestamp: November 24, 2021, 09:04:17 pm
content: Implying that it is MCP1703, it has overcurrent and overtemperature protection, in my opinion the most likely scenario for failure is input overvoltage. MCP1703 would be 16V absolute maximum. The layout is quite bad, look at burnt regulator: it's input bypass cap is who-knows-where, maybe quite far, thin long trace (read - significant inductance). Output cap C6 is not sufficient for stability at all conditions. I perfectly imagine, that regulator input might experience voltage transients in some situations. Long traces and ceramic caps are a receipt for ringing. Can be risky if margin is low. You said it is up to 15V? So almost no margin. It might not be a coincidence, that the failed regulator is the one with bad layout. The other one in the picture seems to have better physical arrangement.Generally speaking, proper bypassing and choosing a part with higher input voltage (more margin) would likely prevent this from happening again. Of course, very high working temperature might also cause failure long term. So check that it is not running near limits.

author: jerryk
author_rank: Regular Contributor
timestamp: November 25, 2021, 07:43:24 pm
content: Below is a clear scan of the bms board. As you can see there is no input bypass cap for the defective regulator. The Input of the voltage regulator U2B comes from the positive power plane that connects directly to positive terminal of the battery packs. I have always wondered about this power scheme on the bms board from this company but just blindly assumed that because it is from a well respected battery manufacturer that it has been designed correctly. It sure doesn't match the datasheet application notes of any of the possible voltage regulators that I have looked at, thus my quest for an exact ID. I may call their tech support and see if they will confirm what it is.The load on this regulator seems small enough as in a couple of parallel smd LED's and it also powers some of transistors that are part of the gate driver schemes for the charge/discharge mosfets.I'm ordering some MCP1754ST-3302E/CB since I cannot find a reputable source for the MCP1703. It has a lower current rating but maybe it will let me do some further checking of the health of other parts namely the microcontroller.If anyone knows where to find any MCP1703T-3302E/CB that would be great. Otherwise, for just for the mere punishment I may get some from Aliexpress.As always - Thanks for the help.Jerry

author: Manul
author_rank: Super Contributor
timestamp: November 25, 2021, 08:28:51 pm
content: Linear voltage regulators which are not "low dropout" can often work without bypassing, but most enginneers will put a cap anyway. And if part identification was correct, this one is low dropout, so it needs a properly sized bypass cap.But of course putting a cap is not a whole story, because there could still be voltage transients on the input while connecting and disconneting battery for example. In fact, ceramic bypass cap could make it worse in some cases. Maybe this is why they haven't put it there? Who knows.Solution could be using higher input voltage regulator. Or you could cut the input trace, add small series resistor (maybe 5 - 10 Ohms) and a ceramic bypass cap near regulator. Or add electrolytic cap, they reduce and absorb transients and ringing quite well.

author: jerryk
author_rank: Regular Contributor
timestamp: November 25, 2021, 08:58:28 pm
content: On a previous repair attempt I replaced that same regulator with a MCP1799T-3302H/TT which has an input max of 45V but the downside was the low current rating of 80mv and it's quiescent current was much higher than suspected original part MCP1703. I never used that board because there was a glitch in the microcontroller on two of the cell balancing circuits.ESD is a real possibility in that it's an aircraft application and the master solenoid coils can generate significant flyback. The owner says his is protected with a flyback diode.Thanks - Jerry

author: Manul
author_rank: Super Contributor
timestamp: November 25, 2021, 09:41:08 pm
content: I think you should either use another manufacturer board (this one is quite objectively crap), or modify it according to good practices. Systematically failing parts are not confidence inspiring, not to mention in avionics.

author: jerryk
author_rank: Regular Contributor
timestamp: November 25, 2021, 10:55:04 pm
content: I do believe in the company that makes these batteries as they are a significant weight savings, especially for light aircraft. I have seen their designs evolve rapidly over time and hope they get the kinks out. In the mean time I have circuits to learn from and I get plenty of good use out of the ones that fail. They never get returned for aircraft use.I sure appreciate the informative replies.Jerry