What happened to power BJTs? A question and rant, in one! - Page 1
Forum section: electronics / projects

author: SharpEars
author_rank: Unknown
timestamp: November 23, 2021, 03:01:33 am
content: Go looking for a power transistor (BJT) these days and they are nowhere to be found. What happened to them? Have they completely been replaced by power MOSFETs and IGBTs? What if I need part for linear mode operation at relatively low voltages (say <24v), crazy high current demands, say >=75 A, and I want it all at DC, the bane of MOSFETs and their damn SOAs (Safe Operating Areas) that only go up [down?] to 10 ms (maybe 100 ms, if you get lucky), never quite reaching DC, at least for the majority of them.As the sizes of their guts have shrunk and RDS(on) has gotten lower and lower, MOSFETs have completely lost the ability to "not-switch" - to be useful for linear mode (aka, triode mode) operation. Ask the question of, "Well what should I use then for linear mode?" and you get the answer: "Use a power BJT!" Right, use a power BJT... What power BJT? Companies stopped making them. Look at Texas Instruments, a company once famous for producing power BJTs. Go to their web-site and find me the section for Power BJTs! What happened to those wonderful TO-3 cans of yesteryear with their great thermal conductivity and the ease with which heat could be removed from their all metal bodies? Now we have TO-220s, TO-264 for the more beefy ones, and other manufacturer specific oddball packages, none of which are made of metal. You get a thin metal tab on the back, and you like it - can't even begin to compete with TO-3! What happened to all those TO-3 cans, as a side note??Anyways, you see where I am going with this. Please do comment and tell me the error of my ways.

author: oPossum
author_rank: Super Contributor
timestamp: November 23, 2021, 03:11:16 am
content: https://www.onsemi.com/products/discrete-power-modules/audio-transistorshttps://www.onsemi.com/products/discrete-power-modules/darlington-transistorshttps://www.onsemi.com/products/discrete-power-modules/power-modules/igbt-modules#products=fjIyNTE0MX52YWx1ZX4xfklORFVTVFJJQUx+

author: james_s
author_rank: Super Contributor
timestamp: November 23, 2021, 03:41:01 am
content: There are a lot less of them around than there used to be and much of this is just due to the way modern equipment is designed. When was the last time you opened up a newish consumer or even industrial device and saw a bunch of power BJTs? Almost everything now uses switchmode topologies, power supplies, motor drivers, even audio amplifiers. There aren't many applications for the old linear power transistors so there aren't many parts on the market.A problem I've encountered is finding high voltage BJTs to replace the horizontal output transistor in CRT displays. A lot of them were 1500V or so with not much gain. I've wondered if a mosfet or IGBT would work with some minor modifications to the drive.

author: EPAIII
author_rank: Super Contributor
timestamp: November 23, 2021, 04:07:11 am
content: Where have you been looking? In less than two minutes I called up Digi-Key and found over 8000 BJTs that were rated for 1 A or more.https://www.digikey.com/en/products/filter/transistors-bipolar-bjt-single/276?s=N4IgjCBcoExgDBaIDGUBmBDANgZwKYA0IA9lANrgAEAgiMWAHQy33jMCsrDjAzN%2By50eAdgFMAHAJbCQMRgBZpjIW3kA2ZWNn8dzAbxUHG2toamyllxmAEL913W3tXnR65uuni9i24CcAqrEHDZBDmyhMMEgoU4h7pGMnknesYx%2BxClZETkx6skCaSKJxJkggbII4jJsYK4MMWDZ4Glg5TDw0g1yMTAtMOW8XTq1xLwtCiPOYyAKMRzTWUsg6sWzEisSPQgrYJ2sALrEAA4ALlAgAKoAdgCWZwDy6ACy%2BJi4AK4ATvggAL7-YgwCggE4kADu%2BG%2BVDO30wN1wd1wZxI3xAh3%2BQAJust one that I remember the part number for was the 2N 3055 (115W rating). They had them:https://www.digikey.com/en/products/filter/transistors-bipolar-bjt-single/276?s=N4IgjCBcoExgDBaIDGUBmBDANgZwKYA0IA9lANrgAEAgiMWAHQy33jMCsrDjAzN%2By50eAdgFMAHAJbCQMRgBZpjIW3kA2ZWNn8dzAbxUHG2toamyllxmAEL913W3tXnR65uuni9i24CcAqrEHDZBDmyhMMEgoU4h7pGMnknesYx%2BxClZETkx6skCaSKJxJkggbII4jJsYK4MMWDZ4Glg5TDw0g1yMTAtMOW8XTq1xLwtCiPOYyAKMRzTWUsg6sWzEisSPQgrYJ2sALrEAA4ALlAgAKoAdgCWZwDy6ACy%2BJi4AK4ATvggAL7-YgwCggE4kADu%2BG%2BVDO30wN1wd1wZxI3xAh3%2BQAI am sure other actual electronic suppliers will have similar stocks. But as for Amazon or some fly-by-night internet only sources, I really can't say.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 23, 2021, 05:43:48 am
content: You're still welcome to try, with MOSFETs -- the lower voltage ratings aren't so extreme on density, so they tend to be free of 2nd breakdown. If you need the assurance from the manufacturer, yeah, you'll have to look around a bit. If you're willing to do qualifying testing, you'll likely find more than a few.I forget what I saw for low voltage parts, last time I was looking. TO-220s seemed to be cheaper per watt than anything else. Nexperia PHP33NQ20T,127 was one.There's a surprising abundance of cheap DC SOA parts for high voltages too, even despite the record-high power densities of modern SuperJunction types.And there's always ye olde IRFPxxx, not usually rated for DC at least on modern datasheets but they're hard to blow up. Maybe cheap enough to be worth the qualifying test.As for BJTs, kinda who really cares anymore, there's not much that's 1. fast enough to need linear operation (vs. single stage class D), 2. slow enough to still use power transistors (not RF types), and 3. powerful enough to need discrete transistors (vs. amplifier ICs). The main motivation I think would be noise level, so as to save on the ripple that a plain class D would have, or the complexity of a hybrid solution (perhaps a class D plus smaller linear amp to buck its ripple, or some linear multiband or bilinear modulated method, like what are sometimes used for RF power amps at higher efficiency). So, if you don't need that -- strongly consider something class D, the improvement in efficiency and size are well worth it.But yes, even so, there's still a handful of types out there -- the old generic types from TIP31 and 2N3055, to D44H11 to MJ[E|W|L]xxxxx (15xxx, 21xxx, etc.), to a few onsemi, Sanken and other parts that are the most capable and well renowned in audio amps: 2SC5200, 2SC3519, etc.Tim

author: David Hess
author_rank: Super Contributor
timestamp: November 23, 2021, 06:00:33 am
content: Except for ring emitter transistors intended for audio applications, TO-3 transistors are all legacy parts, but the old classic 30 amp 200 watt MJ802/MJ4502 were still in production last time I checked for a good price. The 50 amp 300 watt 2N5684/2N5686 may still be available but it was 2 to 4 times more expensive last time I checked.The newest power bipolars are ring emitter transistors, like the "old" D44H11/D45H11 or D44VH10/D45VH10 which are excellent general purpose high performance TO-220 power transistors, with high Ft, hfe, and safe operating area, but newer parts are intended for audio applications, so they have higher voltage ratings commensurate with 8 ohm loads; you will not find low voltage ones. The TO-3 MJ15003/MJ15004 top out at 20 amps and 250 watts but at a very reasonable price. Inexplicably the TO-264 MJL4281A/MJL4302A are only 15 amps and 230 watts, but honestly I would prefer the more modern package. Ring emitter transistors work great for regulators but a little more attention has to be paid to stability because they are so much faster.https://www.onsemi.com/products/discrete-power-modules/audio-transistorsNote that if you are going for power dissipation, bipolar transistors lose their area advantage over MOSFETs because that only depends on chip area. Bipolar transistors have an area advantage, and therefor cost advantage, at higher voltages, although less so now.

author: John B
author_rank: Frequent Contributor
timestamp: November 23, 2021, 09:18:29 am
content: What about TIP35 and TIP36? I consider them "jellybean" for class AB audio amps. They come in TO-247 packages.

author: TheMG
author_rank: Frequent Contributor
timestamp: November 23, 2021, 02:44:09 pm
content: The downfall of TO3 package is it doesn't really lend itself well to modern manufacturing techniques and thermal design.They require a lot more manual labor for one, since it must be mounted to the heatsink first, then either soldered to a PCB, point to point wiring, or socketed. Two fasteners instead of one for a TO247, etc. That's a lot of added cost in manufacturing a product that uses them.I would also argue that it is more difficult to dissipate large amounts of heat from a TO3 than something like a TO247 particularly in a high density forced air cooled design. Reason being that you need a flat surface on both sides of the heatsink for mounting and connecting a TO3, this means you can't have fins immediately below the device, where they would be most effective. At the very least this means the heatsink needs to be physically larger for the same amount of cooling.Practically speaking, you can dissipate just as much power from a TO247 as with a TO3, if not more. It's just a lot less forgiving so you need to pay attention to surface flatness, thermal interface choice and application, mounting torque, etc. But otherwise no problem.

author: mag_therm
author_rank: Frequent Contributor
timestamp: November 23, 2021, 04:41:10 pm
content: Look at Silicon Carbide BJT. For example the GeneSic family of which GA08JT17-247 is a mid range.Fig 1 shows output characteristics in linear mode, Fig 15 SOA.I have not tried them as linear amplifiers, but used them in inverters where they transitioned through linear mode.The base current is about same as old BJT but Vbe ( called Vgs) forward is about 3 V so you need a lot more drive power .

author: james_s
author_rank: Super Contributor
timestamp: November 23, 2021, 06:18:12 pm
content: Quote from: mag_therm on November 23, 2021, 04:41:10 pmThose look like interesting parts, however the GA08JT17-247 (and the entire related family) is listed as obsolete and no longer manufactured.

author: mag_therm
author_rank: Frequent Contributor
timestamp: November 23, 2021, 06:55:25 pm
content: I am seeing a shopping cart on right side.Clicking down them, not all are available However GA10JT12-263 are available at Digi Key.I'd suggest to just buy a pair initially unless you find doc of use as linear devices.Edit> You might have clicked on the "Obsolete " TabCurrent ones are on the other tabs, mainly the left one "Junction Transistor"

author: rsjsouza
author_rank: Super Contributor
timestamp: November 23, 2021, 11:59:32 pm
content: Cool transistors! Pity they seem to become rarer than hen's teeth.

author: Nusa
author_rank: Super Contributor
timestamp: November 24, 2021, 12:32:11 am
content: Although hens teeth aren't actually that rare.https://www.sciencedaily.com/releases/2006/02/060223083601.htm

author: aheid
author_rank: Regular Contributor
timestamp: November 25, 2021, 06:21:04 pm
content: Quote from: SharpEars on November 23, 2021, 03:01:33 amI ran across the Linear FET series from Infineon some time ago. At least their SOA show DC and has that extra kink, which gives some hope it's somewhat legit. The IPB017N10N5LF SOA shows 100A at 20V DC. I'm just a n00b though so might be missing something.:https://www.infineon.com/cms/en/product/power/mosfet/n-channel/optimos-and-strongirfet-latest-family-selection-guide/optimos-linear-fet-100v-150v-200v-/