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Old 17th January 2009, 12:50 PM   #511
pooge is offline pooge  United States
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This will not fit a ThermalTrak TO264 package, which has one screw hole. Even if it had two screw holes in accordance with the TO264 standard dimensions, they wouldn't be aligned as shown in that part. Further, at only 21mm wide, that is treading on thin ice when the heat plate is around 19mm wide. Doesn't leave much room for alignment error.

TO264 dimensions
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Old 17th January 2009, 02:16 PM   #512
AndrewT is offline AndrewT  Scotland
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The To264 package maxes out at 26.1x20
The metal plate is only 21x17
A To247 25x18.5 easily covers the part that needs isolating and thermally conductive.
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Old 17th January 2009, 03:07 PM   #513
pooge is offline pooge  United States
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Easily in theory, but not with a comfort level.
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Old 17th January 2009, 04:50 PM   #514
JPV is offline JPV  Belgium
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Quote:
Originally posted by pooge
Easily in theory, but not with a comfort level.
I am not using the hole to attach the transistors It is better to press the range of side by side transistors with a U aluminium profile across them. Bolt between every one or two transistors in the heatsink with a Belleville washer at each bolt and you have uniforme controlable pressure under changing temperature. This is key for reliability.

You have 20 by 21mm between the holes of the pad wich is enough to accomodate the copper plate 17 by 21 with the small side of the plate parallel to the top edge of the transistor so very easy to align in this critical direction, the other is not critical. If you don't feel at ease take two pads, cut the holes and stick them side by side.

Mounting with constant pressure on the plate is the key.

JPV
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Old 17th January 2009, 06:19 PM   #515
pooge is offline pooge  United States
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OK, I see. But it is still a shame that ones specifically made for TO264s are unobtainium.
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Old 18th January 2009, 06:13 PM   #516
JPV is offline JPV  Belgium
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Quote:
Originally posted by pooge
OK, I see. But it is still a shame that ones specifically made for TO264s are unobtainium.
RScomponent is selling Berquist Hi-Flow in sheets of 300x300 mm.

You can cut any pad size out of it.
Cost 33 euros

JPV
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Old 29th January 2009, 01:25 PM   #517
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Hello One and All

I'm sorry that we seem to have reached a bit of an impasse on the use of ThermalTraks, but I think their potential is such that we should press on as best we can. I have had time to do a little more work on it.

Ignoring for the moment the unsolved problem of why the short-term time response of the sense diode does not seem to match up with theory, I have worked out a plan for making the compensation over the long term (100 to 1000 seconds) rather accurate. This just assumes knowledge of the junction-copper thermal resistance and the total copper-heatsink thermal resistance. The scheme is still very much in the realm of theory and SPICE, but it looks promising; with a little tweaking of parameters the bias voltage can be held accurate to a small fraction of a millivolt over the long-term.

I hope to try this for real in the next couple of weeks, and see if the short-term issues can be dealt with empirically.
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Old 29th January 2009, 01:59 PM   #518
SQLGuy is offline SQLGuy  United States
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Thinking about these transistors got me thinking about other temperature sensing technologies. Does anyone make a similar transistor but with an embedded infrared temperature pickup in the case - literally "looking" at the die to read its temperature?

Those optical sensors can read very accurately in milliseconds. If the sensors could be embedded economically enough it would seem to make for a very accurate and responsive closed-loop bias control system.
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Old 29th January 2009, 05:13 PM   #519
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Default A ThermalTrak Bias Spreader

Below is a schematic of the bias spreader I have been investigating for use with ThermalTrak output transistors in a class-AB Triple EF (Locanthi T circuit) output stage. It is essentially a Vbe multiplier with two ThermalTack tracking diodes in series with it.

To first order, The Vbe multiplier is responsible for providing the four Vbe of spread needed by the pre-drivers and drivers, while the tracking diodes are responsible for providing that part of the spread required by the output transistors. The added feature here is R3, which allows control of the temperature compensation slope created by the tracking diodes. The sensitivity to the tracking diode TC can thus be adjusted.

Traditional Vbe-based bias spreaders place the Vbe multiplier transistor or an associated diode on the heat sink to track the temperature of the output transistors. This has at least two problems. The first is the problem addressed by the ThermalTrak power transistors, namely the need to more intimately track the internal temperature of the output devices in a more accurate and timely way, bypassing much of the thermal time delay introduced by the heat sink.

The second is the fact that the conventional arrangement often does not temperature compensate for the driver transistors, which can heat up significantly. Ditto for the predriver transistors in an EF Triple. Triples can be more of a challenge to temperature stabilize because of the larger number of Vbes that are effectively stacked in series.

As a result of driver heat-up, bias currents will drift upward as these transistors warm up. If bias is set after a long warm-up period, it is likely that the amplifier may be under-biased when it is initially turned on. This effect is mitigated somewhat by the fact that the warm-up time constant for the drivers will not be too long because of the smaller thermal inertia of the drivers and their smaller heat sinks (if used). If the drivers are mounted on the main heat sink, their Vbes will tend to track the main heat sink temperature and these mis-tracking effects may be reduced. In a Triple, the pre-drivers are generally left out of the temperature compensation loop, so that mis-tracking there is still quite possible. Fortunately, their dissipation is lower and their thermal time constant tends to be shorter.

This post is continued below where the operation of the ThermalTrak bias spreader shown here will be described.

Cheers,
Bob
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Old 29th January 2009, 05:24 PM   #520
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Default ThermalTrak Bias Spreader continued

If the driver transistors are not mounted on the main heat sink, a useful arrangement that is well-suited to this bias spreader architecture is to mount the pre-drivers, drivers and Vbe multiplier transistor on a single, nearly isothermal, metal bar. The bar also acts as a bit of a heat sink for these components. If all of these devices are at the same temperature, it can be seen that the Vbe multiplier will do a pretty good job of temperature compensating these devices, while the tracking diodes can do a good job tracking the output device temperatures.

As mentioned above, R3 acts to enhance and control the sensitivity to the TC of the tracking diodes. The numbers below show the sensitivity to the tracking diodes as a function of R3. In all cases the spread was adjusted to produce 26 mV across the output emitter resistors. Sensitivity is defined as the change in voltage base-to-base of the output transistors divided by the change in voltage of D1 + D2. Notice that sensitivity is less than unity in a simple arrangement without R3. Also note that in some previous postings there has been some discussion about the need for the sensitivity to be substantially greater than unity. The numbers shown here provide sensitivity values up to about 1.5:1.


R3...... R2...... sensitivity
Infin... 716..... 0.92
20k..... 794... 1.01
10k..... 893... 1.10
7.5k ... 974 ... 1.16
5.0k.... 1190... 1.28
4.0k... 1430... 1.37
3.0k... 2142... 1.52

The current through R3 acts to increase the bias spread by forcing more current through R2. The current through R3 is strongly influenced by the voltage drop across the tracking diodes. As their voltage decreases with increases in temperature, the current through R3 decreases, decreasing the spreader voltage as desired. This is how the sensitivity of the tracking diodes is enhanced. The presence of R3, of course, requires that R2 be made larger for a given output stage bias current.

Because R3 requires that R2 be made larger, the multiplication factor of the Vbe multiplier is reduced somewhat. Ideally, its factor would be 4:1, since the single Vbe of the multiplier is being called upon to compensate for the four pre-driver and driver Vbes. The price paid for increased sensitivity to the tracking diodes is some decrease in sensitivity to the multiplier Vbe. With R3 set for 3k and tracking diode sensitivity at 1.52, sensitivity to the multiplier Vbe has decreased to about 2.57.

This means that the pre-drivers and drivers will be a bit under-compensated, but this is a lot better than not being compensated at all as is often the case in conventional arrangements. If the drivers are mounted closely on either side of the multiplier transistor at the center of the bar, and the pre-drivers are mounted further out on the wings of the metal heat spreader bar, some thermal attenuation experienced by the pre-drivers will improve the degree of compensation. The placement of one or two board-mounted diodes in series with R3 would also mitigate the reduction in sensitivity to the multiplier Vbe. Other similar circuit approaches are also possible.

Cheers,
Bob
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