Help on old transistor, out of production

[CrashNBurn]

Hello all,
I am trying to repair an old DC/DC Board from an incredibly old laptop. (mostly for fun and sentimental value)
Making a long story short.. the laptop has been used with a wrong psu, it worked fine for a lot of time without a battery. When connected a battery it charged, overheated the battery and the cells leaked badly. At that point has been discovered the psu was 24v instead of 19v.
From that moment the laptop refused to power on.
Today i decided to try fixing it after leaving it in a box for some years.

Gladly the dc/dc board is detachable and comfortable to work on. i soon found a couple mosfets going at the temperature of sun when powering up the board. after a quick check with other mosfets on the board..those got weird values.

point is.. those are some old k2555 ( i guess 2sk2555) which are impossible to find anywhere.
i tried to find some alternatives but my limited knowledge on transistors brought me into an hard stop.
https://alltransistors.com/mosfet/transistor.php?transistor=22478

I tried looking online but i am not sure about a viable alternative for those transistors. Now they are in TO251 package but there is some space allowing also a bigger package.
Can someone kindly help me on this?
Thanks in advance

[RoGeorge]

Search for "cross-reference k2555"
https://duckduckgo.com/?q=cross-reference+k2555+transistor&t=ffab&ia=web
https://alltransistors.com/mosfet/transistor.php?transistor=22478

https://alltransistors.com/mosfet/crsearch.php?&struct=MOSFET&polarity=N&pd=30&uds=30&ugsoff=1&id=12&qg=20&rds=0.046&caps=TP&cf=on
and see if you can find any replacement to buy from that list of the equivalent transistors in the above link.  Check also if the capsule can fit in place of the old one.  Laptops are often very tidy, so a few mm more because of a different type of capsule might be a problem.

[CrashNBurn]

Thank you for your repy, i did the suggested research, but on over 500+ results i didn't had a clue on what to choose.
I refined the search adding VGs value (which should be around 25) and it refined on about 21 results
https://alltransistors.com/mosfet/crsearch.php?struct=MOSFET&polarity=N&pd=30&uds=30&ugs=25&ugsth=0&ugsoff=1&id=12&tj=0&qg=20&fr=0&cd=0&rds=0.046&caps=TP&cf=on
Seems the AP93T03AGH‑HF or AOD402   are the closest.. but a few values are slightly off. my knowledge is not enough to judge if its ok or not..


Maybe I can source those:
https://www.alldatasheet.com/datasheet-pdf/view/485386/AOSMD/D4184.html
What do you think?? they seem to have a lower Coss but not so distant other values an its listed on the replacement on alltransistors

[RoGeorge]

V_GS(th) is important to be the same or less, otherwise it might not switch at all.
R_DS(on) the same, has to be no more than the original K2555.
Then, the max current, and max voltage has to be equal or bigger with the original.

I see there is also a K2555 made by VBsemi, which has V_GS(th) less than 2V.
https://www.vbsemi.com/Package/TO252/K2555-VB.pdf

The one you found (4184) might work, too.  Can't say for sure.  Try whatever you have with low threshold Vgs(th), low Rds(on) and enough current.

[CrashNBurn]

Thanks for the precious hints.

THe VBSemi datasheet seems more complete than the Sanyo one found on Alltransistors.com. At least it has VGS(th) value specified.
Parameters on the 4184 seems close to it. only the VGs is a bit lower (20 instead of 25) but the power supply is 19 so it should work.. i guess.

[wraper]

Don't look in VBsemi datasheets for specs of original parts. It's almost like VBsemi puts part numbers of other manufacturers on random parts they have in stock.

At least it has VGS(th) value specified.

There is a graph in datasheet.

[wraper]

Just checked VBsemi datasheet. As I suspected it's an entirely different part. The only things in common are package, channel type and rated voltage. The rest of specs are different by up to several times.

[CrashNBurn]

Thanks for the feedback, the original datasheet on alltransistors.com (from Sanyo) is missing the VGS(th) value.
anyway i just checked the board (seems to have like 2 channels with 2 mosfets each) swapping the good ones  and the lower VGS  of the found 4184 should not be a problem as i read 15 / 17 v at max. so even if tight ... is still lower than 20v

[wraper]

Thanks for the feedback, the original datasheet on alltransistors.com (from Sanyo) is missing the VGS(th) value.
anyway i just checked the board (seems to have like 2 channels with 2 mosfets each) swapping the good ones  and the lower VGS  of the found 4184 should not be a problem as i read 15 / 17 v at max. so even if tight ... is still lower than 20v

V_GS(th) by itself is a quite pointless spec for most applications (it only tells when MOSFET just starts conducting but it does not tell at what voltage MOSFET is conducting well enough). There is I_D - V_GS graph. What you want is that circuit provides enough gate voltage for driving the gate of replacement mosfet. For 2sk2555 5V is enough. With no other info about the circuit, you should chose other part for which 5V is enough. Gate capacitance, gate charge is another thing that's important. You don't want them much higher as MOSFET will be harder to drive fast. But avoid them to be exceedingly lower either, otherwise they may be unintended consequences. And R_DS(ON) is much more important spec than current rating.

[Doctorandus_P]

Finding suitable replacements is probably not too hard. Mass produced goods are usually cost optimized, so a part with "better" specifications (an a bit more expensive) is not a big issue for repair work. I also don't know how old the laptop is, but improvements on transistors is also being made. In general, I think that power FET's have gone though 6 or so generations now.
Whether Vgs(th) is important in your application is not sure at all. For most power fet's it's between 4 and 6V, and power fets usually are controlled by a 12V to 18V gate voltage (20V max) because a higher gate voltage reduces the on resistance.

For a switching FET I would have a look at gate capacitance and switching speed. In general, faster is better, but it can also be a cause for excessive switching noise. For the rest, it's mostly common sense and what parts are on hand. Switching applications usually are not very critical.

[CrashNBurn]

Finding suitable replacements is probably not too hard. Mass produced goods are usually cost optimized, so a part with "better" specifications (an a bit more expensive) is not a big issue for repair work. I also don't know how old the laptop is, but improvements on transistors is also being made. In general, I think that power FET's have gone though 6 or so generations now.
Whether Vgs(th) is important in your application is not sure at all. For most power fet's it's between 4 and 6V, and power fets usually are controlled by a 12V to 18V gate voltage (20V max) because a higher gate voltage reduces the on resistance.

For a switching FET I would have a look at gate capacitance and switching speed. In general, faster is better, but it can also be a cause for excessive switching noise. For the rest, it's mostly common sense and what parts are on hand. Switching applications usually are not very critical.

Your words definitely make sense. It remember me of some past time quick repairs where I swapped non critical components with others on hand with similar (but not same) values and those things still work today after a huge amout of work time.

Anway.. wanna laugh? the laptop is from 1995. its a Compaq LTE 5100.
I checked the parameters you were pointing out.
Speaking of the FETs i could rapidly source (4184) they have a lower output Capacitance figures (around 220 instead of 480) but higher input (around 1500 instead of 650)
Checking switching speeds.. they seem to be very similar, just a hair slower but really little difference.
They still seem to be one of the best choices.
I am tempted to buy those 4184 and try them.
AP93T03AGH‑HF has closer capacitance figures but I cannot find it anywhere...