Heatsinking the ThermalTraks
The OnSemi ThermalTrak transistors are in the nice big T0-264 plastic package. This package appears to have significantly more metal heatsink contact area, which should provide a much better Theta C-S than the TO3P or TO-247 packages. This is nice.
But does anybody here know of a decent source for TO-264 mica or other type insulators?
Thanks!
Bob
The OnSemi ThermalTrak transistors are in the nice big T0-264 plastic package. This package appears to have significantly more metal heatsink contact area, which should provide a much better Theta C-S than the TO3P or TO-247 packages. This is nice.
But does anybody here know of a decent source for TO-264 mica or other type insulators?
Thanks!
Bob
AndrewT said:
Thanks, Andrew.
What material are your sticky-back versions made of (e.g., aluminum oxide, silicon rubber, etc.) and how does its thermal resistance compare to mica with grease?
Regards,
Bob
BTW, I was thinking that the ThermalTrak diode would allow for an excellent opportunity for doing an experiment for comparing thermal resistance of various insulation approaches (including no insulation), by heating up the ThermalTrak BJT with bias and measuring the junction temperature by measuring the change in the junction drop of the ThermalTrak diode.
Re: ThermalTrak Bias Circuit
Just out of curiosity, have to tried running a sim with lower value bypass/coupling caps for the "base spreader" circuit transistors.
You've got a pair of nfb loops there enclosing the driver and pre driver BJT's, which you might be able to make use of at higher frequencies by using smaller caps. That could provide a lower drive impedance for the output transistors.
Cheers,
Glen
Bob Cordell said:
Just out of curiosity, have to tried running a sim with lower value bypass/coupling caps for the "base spreader" circuit transistors.
You've got a pair of nfb loops there enclosing the driver and pre driver BJT's, which you might be able to make use of at higher frequencies by using smaller caps. That could provide a lower drive impedance for the output transistors.
Cheers,
Glen
I use the technique of rotating the greased interface as the bolt is tightened up. The rotation is limited to about +-20degrees in a oscillatory motion to expell the excess componund and more importantly squeeze out any trapped air bubbles.anatech said:
If this is done with To247 mica on a To264 package there is a high risk of misalignment of the insulator since it is "hidden" from view.
Has anyone found a supplier of To264 sized mica in the UK?
Re: Re: ThermalTrak Bias Circuit
Hi Glen,
You have a very good point here. The circuit I showed was mainly for simulation illustrative purposes to keep things simple. There are actually a number of different ways that the bypassing/local compensation can be done. The brute-force way I chose to illustrate is particularly convenient because of the fact that for simulation I feed the "center point" from a voltage source. In a real amplifier, the signal feed comes from both ends of the spreader with the upper and lower current source transistors replaced with the VAS transistors.
I did a couple of simulations with hard drive at high frequencies where I actually saw some difference (improvement) when I used LARGER capacitors. In a real amplifier, with this brute-force arrangement, I'd probably use 10 uF electrolytics, possibly bypassed by 0.1 uF caps. However, there are also more elegant approaches to compensating/bypassing this arrangement, such as putting collector-base capacitors around the spreader transistors while deliberately increasing somewhat the source impedance to their bases. This pretty much amounts to local Miller effect compensation.
In any of the bypassing/compensation approaches for this scheme, where we are considering the local closed-loop effect on impedance at the output of the driver transistors, we need to distinguish between common-mode impedance and differential mode impedance. In this context, I would more clearly define it as signal-mode impedance and bias-spreading-mode impedance. The local feedback in this bias scheme will, I believe, primarily affect the bias-spreading-mode impedance. However, I must admit that I was not consciously trying to manipulate those impedances here and have not given it a whole lot of thought.
Cheers,
Bob
G.Kleinschmidt said:
Hi Glen,
You have a very good point here. The circuit I showed was mainly for simulation illustrative purposes to keep things simple. There are actually a number of different ways that the bypassing/local compensation can be done. The brute-force way I chose to illustrate is particularly convenient because of the fact that for simulation I feed the "center point" from a voltage source. In a real amplifier, the signal feed comes from both ends of the spreader with the upper and lower current source transistors replaced with the VAS transistors.
I did a couple of simulations with hard drive at high frequencies where I actually saw some difference (improvement) when I used LARGER capacitors. In a real amplifier, with this brute-force arrangement, I'd probably use 10 uF electrolytics, possibly bypassed by 0.1 uF caps. However, there are also more elegant approaches to compensating/bypassing this arrangement, such as putting collector-base capacitors around the spreader transistors while deliberately increasing somewhat the source impedance to their bases. This pretty much amounts to local Miller effect compensation.
In any of the bypassing/compensation approaches for this scheme, where we are considering the local closed-loop effect on impedance at the output of the driver transistors, we need to distinguish between common-mode impedance and differential mode impedance. In this context, I would more clearly define it as signal-mode impedance and bias-spreading-mode impedance. The local feedback in this bias scheme will, I believe, primarily affect the bias-spreading-mode impedance. However, I must admit that I was not consciously trying to manipulate those impedances here and have not given it a whole lot of thought.
Cheers,
Bob
Bob Cordell said:
Can't this be said about any other temperature sensor scheme, in that the Vbe transitor or sensing diode(s), used to sense temperature, are operated at a meager current wrt the current in the output transistor(s). In other words, this issue with the TermalTrak diodes does not appear to differ from any other scheme.
pooge said:
Hi Pooge,
Yes and no. The conventional schemes do operate the Vbe multiplier transistor at a similarly low current, say 5-10 mA, but those transistors probably have fairly small junction area. The thermalTrak diode appears fairly large if it has the same junction drop as the big BJT at one-fourth the current. In other words, I don't think a 2N3904 would be nearly as large, comparatively junction-wise, as 1/4 of a 200-Watt BJT. On the other hand, these generalizations are not always that valid, since the saturation current of the junction is such a big function of the particular doping profile.
Cheers,
Bob
bob cordell, with the use of the termal trac device the overall bias scheme can be tailored to a narrower spec. what overall improvement in sound quality as a percentage could we expect from this tighter control. on semi data sheet stresses a smaller parts count at assemble for mass produced amps. is this a valid product for diy builders in the search for high end performance. heat issues in bjt devices have always been a concern.
Hi tryonziess,
The thermal track devices make possible some circuit designs that were not easily possible before. There are many things we can now do that depend on a accurate reading of junction temperature.
An improvement as a percentage isn't helpful or relevant as it depends on your physical implementation of a circuit. Before now we have not had any method to get a fast accurate adjustment of bias current dependent on die temperature. Before this we had to read the average heat sink temperature or device case and somehow try and track the bias current to this. We had very loose control of bias current in other words. There is no way you would get into a car and attempt to drive it with a system that sloppy on you speed would you?
-Chris
The thermal track devices make possible some circuit designs that were not easily possible before. There are many things we can now do that depend on a accurate reading of junction temperature.
An improvement as a percentage isn't helpful or relevant as it depends on your physical implementation of a circuit. Before now we have not had any method to get a fast accurate adjustment of bias current dependent on die temperature. Before this we had to read the average heat sink temperature or device case and somehow try and track the bias current to this. We had very loose control of bias current in other words. There is no way you would get into a car and attempt to drive it with a system that sloppy on you speed would you?
-Chris
anatech, when we were all younger it seems. that is one of the truly great things about this hobby. you can build a truly exceptional amp/preamp for quite a small sum. to buy the same quality commercially would be well foolish. i ran across this forum six months ago a built a few chip amps one that alex88 designed and i got hooked. i know very little of the math involved but can assemble quite well. thanks for the suggestion. if i find a rather simple discete bjt design i am going to try it.
tad
tad
I think we are all in danger of going off the deep end on this whole 'thermal track' thing.
1. Thermal track is a way for On to make money. There are no hot shot audiophiles in On who are on our side trying to help us build better amps. The product manager that signed off the development budget on this one had one thing in mind: How can I improve my product line's performance? One of the apps guys probably put a justification in as well to help him convince the divisional VP of the profit potential.
2. There are many, many great bipoloar designs out there with impeccable distortion performance (hot and cold) that do not use thermal track. Some of these desigfns have been around for 25 or 30 years.
3. To test the performance of bias compensation, measure the distortion straight after switching on. Then run the amp at a high output level in order to get the output devices really hot. Measure the distortion again. From this you can gauge the effectiveness of your Vbe comp circuit. Alternatively, if your design runs the outputs 'rich' (i.e. c. 80-100mA per output pair), a delta cold to hot of 20mA probably indicates good Vbe control - and well controlled output distortion.
4. Above comments do not apply to guru's (some who write interesting books) who like to run their outputs in true class 'B'. They deserve to have Vbe comp problems that manifest as distortion problems.
1. Thermal track is a way for On to make money. There are no hot shot audiophiles in On who are on our side trying to help us build better amps. The product manager that signed off the development budget on this one had one thing in mind: How can I improve my product line's performance? One of the apps guys probably put a justification in as well to help him convince the divisional VP of the profit potential.
2. There are many, many great bipoloar designs out there with impeccable distortion performance (hot and cold) that do not use thermal track. Some of these desigfns have been around for 25 or 30 years.
3. To test the performance of bias compensation, measure the distortion straight after switching on. Then run the amp at a high output level in order to get the output devices really hot. Measure the distortion again. From this you can gauge the effectiveness of your Vbe comp circuit. Alternatively, if your design runs the outputs 'rich' (i.e. c. 80-100mA per output pair), a delta cold to hot of 20mA probably indicates good Vbe control - and well controlled output distortion.
4. Above comments do not apply to guru's (some who write interesting books) who like to run their outputs in true class 'B'. They deserve to have Vbe comp problems that manifest as distortion problems.
bonsai, that is one of the problems i am having. to determine what the final result will be. i wish to constuct a truly fine discrete amplifier with mid range output. because humans differ in their approach i find it hard to know which is a fast and founded design. something with adequate lifespan and impeccable sound. very few comments on the forum ever state the end result in listening experience --just specs. and you know they do not always mean the same thing. i mean leach is a good amp, but how does it sound. the boards are available if the end result is worthwhile. any suggestions on a tested design to proceed with are greatly appreciated. even a good mosfet output stage would be acceptable given current quality of these devices. this forum has opened up a whole knew hobby for me. thanks tad
Hi Bonsai,
The way to increase sales is to create and market superior devices. The thermaltrack parts do have some very good features and I know exactly how this solves a problem that I had before. This will improve the performance of an older design I was working on (I hope).
If you can't see the need for them, use some of the other recent, excellent devices On Semi has come out with. Device parameters are now closer batch to batch than they ever have been before. Individual device performance is improved as well. Having using some of them recently I can attest to these improvements.
Go make some stuff!
Hi Tad,
There are many good and bad designs out there. Each good design may do a number of things very well, and some a little less well. It comes down to the design and construction that commits the least errors. There are a number of designs that have been developed here and all you have to do is hunt them down and read their threads.
One thing more Tad. Please, please, please use your caps key when appropriate. Reading your posts will be much easier.
-Chris
The way to increase sales is to create and market superior devices. The thermaltrack parts do have some very good features and I know exactly how this solves a problem that I had before. This will improve the performance of an older design I was working on (I hope).
If you can't see the need for them, use some of the other recent, excellent devices On Semi has come out with. Device parameters are now closer batch to batch than they ever have been before. Individual device performance is improved as well. Having using some of them recently I can attest to these improvements.
Go make some stuff!
Hi Tad,
There are many good and bad designs out there. Each good design may do a number of things very well, and some a little less well. It comes down to the design and construction that commits the least errors. There are a number of designs that have been developed here and all you have to do is hunt them down and read their threads.
One thing more Tad. Please, please, please use your caps key when appropriate. Reading your posts will be much easier.
-Chris
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