Transistor Equivelants, what's the defining factor? - Page 1
Forum section: electronics / beginners

author: ParsonsUnknown
author_rank: Contributor
timestamp: October 03, 2017, 12:19:03 pm
content: Hi everyone, ran into a problem sourcing transistors for a switch mode PSU I'm attempting to repair. Now the original switching trannies are no longer available, eBay apprantly have a whole plethora of them, but I simply refuse to buy discretes from eBay. Anyway, I've been sourcing websites and I can't find any of the recommended equivelants. I can get close, but no actual like for like replacement without the cost going from a couple of quid, to daft money.My SMPS has 2 2SC2625's, here's the spec:Type - n-p-nCollector-Emitter Voltage: 400 VCollector-Base Voltage: 450 VEmitter-Base Voltage: 7 VCollector Current: 10 ACollector Dissipation - 80 WDC Current Gain (hfe) - 10Operating and Storage Junction Temperature Range -55 to +150 °CPackage - TO-3PNow the nearest I could find is a BUL89, has a few minor differences and it's a different package, here are the specs:Transistor Type NPNMaximum Collector Emitter Voltage 400 VMaximum Emitter Base Voltage 9 VMaximum DC Collector Current 12 AMaximum Power Dissipation 110 WMinimum DC Current Gain 10Package Type TO-220What's the best and obviously safest defining factor when going for replacements or near as damnits? I'm thinking max Amps and hfe, with the other ratings either around or higher. Or am I totally wrong. I'd sooner ask, instead of blowing the bloody thing upIf you have any suggestions or recommendations, I would greatly appreciate them and any help or advice

author: Gyro
author_rank: Super Contributor
timestamp: October 03, 2017, 12:52:16 pm
content: A wise decision not to go to ebay for a a high voltage, high power transistor.The only thing you appear to be missing on the substitute you found is package size: TO220 on a TO3P footprint is going to mean lead bending. The TO220 also has a lower heatsink coupling area.Doing a quick search on RS Components, I see the BUV48A is available in TO-247 for not too silly a price and higher voltage and current ratings.... Vce and Vcb 1000V, Ic 15A Min hfe 8 (would be a bit higher at 10A) and Dissipation 125W. They do also have your BUL89 by the way.RS are pretty good for this sort of thing because they have no minimum order or delivery charge (last time I checked anyway).Before you push the button on either, you should closely compare the datasheets for things like Ft and switching characteristics as this is for SMPS use, oh and pinout! (I haven't gone that far!).P.S.You're presumably replacing the existing one on a whim. If it blew then there is probably a reason, involving other components on the Primary side. I don't know your level of experience, but delving into the Primary side of SMPSs is not for beginners. You would do well to provide more information and ask for guidance if the reason for replacement isn't clear-cut.

author: ParsonsUnknown
author_rank: Contributor
timestamp: October 03, 2017, 01:14:41 pm
content: It blew because I made a stupid mistake, I took out the PWM controller and feedback/sense board by mistake as a few resistors had burned out, tried to power up and the 4 amp quick blow did its thing, I reconnected the board and she still blew fuses. Tested all the parts and only the 2SC2625's read as short circuit. The reason for the strip down was to replace the front A & V meter bulbs and to add a potential divider to the fan voltage, so I could leave it running all the time at half the dB.I can't seem to find a T0-3P within the specs, but then again, they're only switching, not actually providing gain, at least I don't think. My skill level has always good to above average, I'm a former broadcast engineer and have been teaching Amatuer Radio at all 3 levels for 4 years and an electronics enthusiast since I was a kid. Had a life changing event six years ago and I'm struggling to be at my former level, trying hard because I've had to start all over again, but I'm making really stupid mistakes, brain and hands don't seem to communicate as well these days.

author: Benta
author_rank: Super Contributor
timestamp: October 03, 2017, 01:15:29 pm
content: As it's an SMPS, you're missing the most important: switching characteristics and RBSOA. Also, does the original have a reverse diode built in?

author: ParsonsUnknown
author_rank: Contributor
timestamp: October 03, 2017, 01:19:32 pm
content: Very true Benta, then again I've been aiming for products within the SMPS realm. There's plenty of diode protection on board, the switching trannies are surrounded by FR107's, and the other side of the transfomer has a U30D20C across it. But points well taken

author: ParsonsUnknown
author_rank: Contributor
timestamp: October 03, 2017, 01:32:51 pm
content: Just on RS, I've been cross referencing between them, Farnell & Mouser. Think I may have found a winner, the FJA13009TU it's a T0-3P so no fannying about with pins, plus it's based on the same technology employed within the 2SC2625. Similar switching speeds and application is identical and, at £1.14 per unit it's cheaper than a bottle of pop, lol. What do you reckon?

author: Gyro
author_rank: Super Contributor
timestamp: October 03, 2017, 04:00:49 pm
content: I haven't been able to turn up a datasheet for the 2SC2625but if you have a good datasheet match for DC, AC and SOA (as Benta said) then you shouldn't have an issue.Sorry, was looking for 2sc625Yes, it looks a pretty good match (my personal opinion anyway). I was going to say a fraction slower but it depends which manufacturer's 2SC2625 datasheet you compare it against. SOA looks very similar. Headline hFE is a little lower, but closer when you allow for different Ic parameters.

author: ParsonsUnknown
author_rank: Contributor
timestamp: October 03, 2017, 05:57:12 pm
content: According to the specs I've seen it's the closest I'll get. The alleged replacement NTE2308P is not a good match, so I'm optimistic about my purchase from RS. I'll let you know how I get on after installation, I'm also changing the PWM controller too, going to remove and put a turned pin holder in, then hopefully no boom! It was severely daft of me to assume that the board behind the lamps (now leds) wasn't of critical importance, I won't make the same mistake twice.However there is a silver lining to all this, I've learned a lot about SMPS's, in a short space of time. Bloody good fun as well. From now on I'm going to stick with linear psu's.

author: ParsonsUnknown
author_rank: Contributor
timestamp: October 04, 2017, 02:28:51 pm
content: The operation was a success. Trannies fit like a glove and are working well, the better PWM chip has done it a power of good, no pun.Thanks for the help guys 👍

author: Bubba
author_rank: Newbie
timestamp: October 05, 2017, 10:16:56 pm
content: 75 cents is expensive?http://www.futurlec.com/cgi-bin/search/search.cgi

author: Gyro
author_rank: Super Contributor
timestamp: October 06, 2017, 08:38:29 am
content: Well recovered!