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In Reply to: Re: Yo John posted by morricab on March 22, 2007 at 03:31:08:
morricab: ""
Are you saying that back EMF can then prevent the transistor from shutting off properly or suffer from secondary breakdown?""Yes. For inductors, it's the stored energy which manifests as a pseduo constant current, and it will continue to supply that current even if the output goes to the opposite polarity. Perhaps it's easier to envision a large capacitor as load..have the amp ramp slowly to the negative rail, then once the load cap is charged, change the output to the positive rail..the cap doesn't move quickly, so the output stage draws lots of current in an attempt to move the cap. The pass devices trying to pull the cap up are from the positive rail, so the devices are dissipating both rail voltages and whatever current being forced..in the case of an inductive load, it's quite easy for the pass devices to move the out to the other rail, however, since an inductor cannot change it's current as quickly, it continues to have current in the same direction for a while. A more controlled current, but the passes still have to hold it for double rail voltage momentarily. (it's easier for most to envision caps rather than inductors, but they are both reactive storage mechanisms.)
If this brings the outputs close to the secondary breakdown, poof. (bipolar discussion). As devices approach this dangerous region, several mechanisms happen..as you raise the C-B voltage, the neutral base region gets thinner (early effect), increasing the gain of the device. Also, the leakage collector to base increases, this leakage is amplified by the device gain.. And thermally, gain is also going up..these conspire to make the output devices "cramp up"..this is secondary breakdown.
..Consider a darlington config output..if nothing is put in to pull the output bases down when this "cramping" happens, the device will clamp on. As this starts, the only way to stop it is to pull the base down hard. If the only thing the base sees is an emitter, then that driver has to go reverse breakdown base-emitter to get any current into the base. (note, I've been using current nomenclature loosely for ease of discussion). That'll probably toast the driver e-b junction.
Follow Ups:
This seems to be primarily a problem with bipolars because if I am not mistaken what you are describing is a form of thermal runaway. FETs tend to have the opposite effect under thermal stress and reduce gain if I am not mistaken. Sorry if I am not talking in quite a technical language, I understand the issues conceptually quite well but the actual jargon I am not so experienced in using. Keep talking away though because I appreciate the lessons. Now if I am also not mistaken many (if not all) BJT amps require extensive thermal tracking to prevent such a situation from getting out of hand. A Class A mosfet amp would not require this thermal tracking as they are more or less self clamping.I find it a relevant topic because I use a big pair of full-range electrostatic speakers. These are about as reactive (mainly capacitive) as a speaker is likely to get. A very large percentage of the signal fed into the speakers is sent right back into the output stage of the amp. Now I am using a tube hybrid amp (with the tubes on the output unusually) without negative feedback. I think the tubes (or the output transformers) are able to dissipate this back EMF without trouble and of course there is no breakdown possible. What I have found though is that a lot of SS amps sound very anemic on these speakers, with my modest 30 watts sounding much more powerful. I am wondering if the speaker is regularly putting the output stage under a lot of stress or if a significant portion of the back EMF is making its way back into the input and being reamplified (as described by Otala as IIM distortion).
morricab: ""
This seems to be primarily a problem with bipolars because if I am not mistaken what you are describing is a form of thermal runaway""While I did say "bipolar discussion", I did neglect to say the word thermal runaway...sorry bout that. I was describing thermal runaway as a result of secondary breakdown in a bjt...
The thermal tracking for most bjt amps has a far slower time constant. The silicon die and the thermal structure have several time constants, ranging from the 100 uSec range for the junction, to the millisecond range for junction to case, to the seconds from case to heatsink. So the thermal tracking schemes typically correct in the second or so timescale. Some schemes can be faster, like the sensefets, or that new device with an onboard sense device.
I could guess as to your amp's reaction to it's load, but that'd be a guess. I'm talking with Dan (and anybody else who's interested) about a test scheme for amps which cover reactive loads. Sent him a first pass jpg (then of course, modded it into a better scheme) for a setup. Rev 2 has a programmable resistive load, as well as a completely independent reactive load control, from capacitive through to inductive. That type of testing regimen is what is needed to get to the bottom of why you hear differences between amps..It may also be the root cause of tube vs solid state sound...who knows..
Hopefully, it can get posted here in a new thread..I fear that I will not be able to do it at AH, as I am "this close" to being banned there because I posted my displeasure with having some of my analysis misrepresented as part of an AH article. Apparently it is allowed to misrepresent the words of another, and there is no rebuttal allowed on an open forum. In my humble opinion, that is an abuse of the term "open discussion".
I'm a bad boy..:-)
Well as long as DB can remain civil I am more than willing to dive in and learn (hopefully contribute a bit as well but I have to think more deeply on these things first). I appreciate a sane discussion for once on this forum.
Ah, yes...now I recall some kind of animosity between you two. I honestly do not know why, nor am I really interested to know why.I agree, I hope civil discourse continues..
Can you post diagrams here?
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