OnSemi ThermalTrak power transistors
- Thread starter Racingh11
- Start date
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You might know something I don't, but I think this innovation dates from around 2005. See:
https://www.onsemi.com/pub/collateral/and8196-d.pdf
(BTW, the datasheet link in post #1 seems to point to a conventional 3-terminal BJT.)
I'm not quite clear on whether ThermalTraks are still considered "production" parts, but based on my experience, I think you'll have a hard time finding complementary sets.
I'm also not clear on whether the idea ever caught on. The only production amp I've ever found them in (Audio Research HD-220) was decidedly not stable in bias, at least not after replacing its ThermalTrak outputs. But then, that design was not a close match to the application note. And maybe there are matching requirements that I was not in a position to know or to fulfill. All I can say for sure is that I found individual output devices heading for thermal runaway, and the only way I found to tame that was by upping the emitter resistances for the runaways (which I know is a pretty bogus hack).
If you build an amp with ThermalTraks, I'd love to hear how it comes out, especially with respect to bias stability.
Racingh11
Super Member
Thanks for the added info. I re-uploaded the correct data sheet. I guess they could have been around for a while, I only thought it was new because about once a year I look at the OnSemi website for the current lineup of transistors. These were on there and I don't remember seeing them before, but maybe I had seen them and forgot. It seems OnSemi is constantly replacing part numbers. Some of the part numbers I was getting just a few years ago, have been discontinued or replaced with new numbers. KSA1220A and KSC2690A are both being phased out, and I don't see anything like them anymore. They were used as drivers in the amp I designed, so I'll have to keep looking for something.
Racingh11
Super Member
I am looking at the OnSemi website, the KSA1220 is listed as "last shipments", so only having the P-channel transistors is the same as having none at all for what I'm doing. And I would expect the ksc2690 are next for the chopping block. But you are correct, they are still available at the moment.
fred soop
Super Member
Some of the experts on DIY Audio have suggested that these may not be worth the trouble as proper bias can be achieved with close coupling of the sensing device to an output transistor. The design below shows a homemade clamp for a small transistor sandwiched to one of the output transistors in each channel. There is another sensing transistor in series clamped to one of the driver transistors. The resulting circuit tracks extremely well from 10 seconds after a cold start, to running very hot, then suddenly dropping back to idle. Took quite a bit of time to dial in the proper resistor values.
I did see a commercial amplifier with thermaltraks at AXPONAin 2019 but did not note the brand or model.
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I did see a commercial amplifier with thermaltraks at AXPONAin 2019 but did not note the brand or model.
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Racingh11
Super Member
I'm starting to agree with you after reading some of those threads. Not sure if it's worth spending time on.
See if maybe a TTA004b and TTC004b will serve. Last time I needed a transistor in that package, they were the official Toshiba suggestion to replace whatever it was that had failed. I don't recall offhand what that was unfortunately.
Racingh11
Super Member
Perfect, thanks! Not sure how I missed these.