Replacement options for TO-3 package Mosfet? - Page 1
Forum section: electronics / repair

author: anewmanx
author_rank: Contributor
timestamp: April 18, 2019, 03:08:33 am
content: Greetings. Both power mosfets in an HP 6034A that I purchased were faulty. Gargantuan TO-3 Mosfets mounted to huge pieces of aluminum and through hole down to the PCB.My goodness they are proving difficult to find! At least at a reasonable value. Deeply obsolete technology I suppose. They were part number 5080-2017 and I have seen online that this part was replaced by 1855-0547.This website seems to have information on the one that replaced the original:https://aerobasegroup.com/part-number/1855-0547_01-275-8265The problem is I haven't found them for less than $16 each. That's entirely too expensive, especially if after replacing them there is another unknown problem that causes them to break again.Would I do best to retrofit a new mounting/cooling solution and use a modern TO-220 MOSFET and perhaps add a small fan aimed directly at their heat sinks?Does anyone have a suggested MOSFET that would make a nice match for the previous ones?Thank you for any advice you may have to offer.

author: CJay
author_rank: Super Contributor
timestamp: April 18, 2019, 05:06:28 am
content: ISTR there's already an old thread here which discusses TO-3 MOSFETs in HP gear and their replacements but for some reason the IRF250 comes to mind

author: anewmanx
author_rank: Contributor
timestamp: April 18, 2019, 05:13:23 am
content: I did a brief search but came up empty. Perhaps you are correct. Thank you.I had looked at the data sheet for the IRF250 before but noticed the drain to source breakdown voltages were very different and the gate threshold max voltage was 4v vs the 6v on the original part. I wasn’t sure if these details were problems as ive never worked with mosfets before... just regular transistors.

author: CJay
author_rank: Super Contributor
timestamp: April 18, 2019, 11:00:18 am
content: It may well be unsuitable, it's just a part number I've associated with HP repairs.I don't know if it is or isn't suitable for your repair

author: Gyro
author_rank: Super Contributor
timestamp: April 18, 2019, 12:52:25 pm
content: You can normally fit a TO-220 or, preferably, TO-3P package to a TO-3 heatsink, using one of the mounting holes for the drain (tab) connection, and feeding the source and gate connections through the pin holes (lose the middle pin).That depends on all the other electrical, thermal and SOA parameters being acceptable of course.

author: Psi
author_rank: Super Contributor
timestamp: April 18, 2019, 12:58:05 pm
content: Get a dead TO-3 cut the top off, scoop out the innards and belt sand it into a flat block of metal.Now you have a perfect TO-3 sized heatsink block.Drill an extra hole somewhere in the center and mount your good TO-220 fet onto it.Solder some wires to the two pins and the caseThen you can mount the whole thing into anything like you would a real TO-3Will it work? i have no idea, but its worth a try

author: Gyro
author_rank: Super Contributor
timestamp: April 18, 2019, 01:16:05 pm
content: No, TO-3 packages aren'tthatbig... and you'd introduce another thermal interface. Direct mounting is usually fine.

author: CJay
author_rank: Super Contributor
timestamp: April 18, 2019, 04:32:20 pm
content: TO-3P packages were, I was taught, intended to be a cost reduced plastic alternative, the body length and hole placing are too much of a coincidence surely?Interesting info here about HP PSU's using TO-3 partshttps://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1981-08.pdf

author: anewmanx
author_rank: Contributor
timestamp: April 18, 2019, 11:29:40 pm
content: Okay, I *think* I have found a suitable replacement for a retrofit situation.https://www.arrow.com/en/products/ixtp12n50p/ixyshttps://www.tme.eu/Document/6b560747274731bd099dcada7fe7c013/IXTA12N50P-DTE.pdfThe price is definitely right. Under $3 shipped per transistor so I can afford to screw a few up without weeping.If someone who knows more than me could take a peek at the specs and ensure I'm not missing any glaring incompatibility compared to the specs of the previous TO-3 mosfets I would appreciate it. I am aware that I'm going to have to do some machining to the existing heatsink blocks in order to make this work, but that does not pose a problem for me.Would it be prudent to add a small fan aimed directly at these mosfets? Perhaps a single 60mm san ace DC fan of some sort, power it using something similar to what these guys were discussing:http://www.dslreports.com/forum/r16890272-Build-a-120v-AC-to-12v-DC-adapterI suppose a simpler temporary situation would be a wall plugin 12v power supply.

author: anewmanx
author_rank: Contributor
timestamp: April 19, 2019, 05:19:51 am
content: I am on the fence a bit.It seems from what I'm reading that the IRF450 might make a good replacement already in TO-3 package.http://www.irf.com/product-info/datasheets/data/jantx2n6770.pdfDoes anyone have a recommendation? These can be found fairly cheaply on eBay from Hong Kong and if they are fakes buyer protection will get me a refund. They've sold 6 with no negative feedback so far, all three times sold in pairs so likely for power supply repairs like this one.

author: CJay
author_rank: Super Contributor
timestamp: April 19, 2019, 11:35:07 am
content: I'd have tried an IRFP450 because I've used them many times in similar age PSUs and it's still available from various reputable places.I'm normally a great fan of the cheap Chinese parts and sellers but I'd be wary of Chinese TO-3 transistors in something where there will be irreplaceable parts (switching transformer) which could possibly be damaged if they fail.

author: Gyro
author_rank: Super Contributor
timestamp: April 19, 2019, 11:51:16 am
content: The only thing to remember with IRFP450s is the insulated mounting hole - don't forget to connect the Drain pin (preferably before you snip it off!).I wouldn't trust ebay parts either, it's one thing being able to get a refund, collateral damage is another matter!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 19, 2019, 01:15:06 pm
content: Quote from: CJay on April 18, 2019, 04:32:20 pmI thought TO-218 was the supposed equivalent?I don't know where TO-3P comes from exactly, but it's metric, not imperial, and the hole size isn't nearly compatible either. I always figured it was more coincidental than intentional.In any case, I would take a close look at the surrounding circuit and adapt it to a modern MOSFET if possible, and hack in a TO-220 or better, as best I can. Mechanical modification may be necessary.That assumes there's little value inrestoringthe equipment, versus repairing it. In that case, you'd be better off locating a spare original part, or equivalent, I think.Tim

author: coromonadalix
author_rank: Super Contributor
timestamp: April 19, 2019, 02:30:54 pm
content: I use arf power mosfets at my job 45$ a piece, 2 of them on a board lollArf 449 (in EOL) replaced by arf 463, with 1 resistor to change in the feedback loop.

author: David Hess
author_rank: Super Contributor
timestamp: April 19, 2019, 06:49:47 pm
content: Usually a package like TO-220, TO-218, TO-3P, or TO-247 can be adapted to fit using one of the existing TO-3 mounting holes.I suspect the TO-3P was just intended to support the same die sizes as the TO-3. The standard TO-220 does not but the enhanced TO-220 packages might get close.

author: anewmanx
author_rank: Contributor
timestamp: April 19, 2019, 08:40:08 pm
content: Quote from: Gyro on April 19, 2019, 11:51:16 amWell, on a plus note I know exactly what will blow up since both the current ones failed drain to gate hahaha. Lots of smoke! I blew 6-7 resistors, 2 transistors, 2 diodes, etc. The entire drive circuit back to the transformer. Everything from there back is fine.If I do give the IRF450's a chance, what would be a good way of bench testing them before install? Or is it one of those situations where you can only tell under load if its going to blow?I'm repairing this for use so I'm not too concerned about using original parts.

author: slbender
author_rank: Regular Contributor
timestamp: May 28, 2019, 03:27:42 am
content: So what happened???I kind of remember having a box or several boxes of samples of TO-3 Mosfets, maybe they were Irf-240's? in my closet. I could send you a pair if they are close match to your original parts. I think they were rated at 200 volts, 20 amps or more, maybe I have both mosfet polarities, I would have to search a bit to see.LMKSteven