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Old 7th December 2000, 09:59 PM   #11
Eric is offline Eric  United States
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Default Heat sink options

Jon: OK, so I need more heat dissipation. Does all of the heat dissipation need to be on a single heat sink for some reason? Or can I use multiple smaller sinks?

Assuming I use eight TO-3s and put each one on its own heatsink, that 0.15c/w total figure gets divided by the 8 output divices (not 6 as you mentioned above). So that reduces the thermal requirements for each transistor on its own heatsink to something more like 1.2c/w. Is this correct?

Assuming the above is valid, I have found a few alternative configurations at http://www.surplussales.com/Heatsinks/HeatSinkmain.html
(one of the original sites you provided above).

1) On page 1 they have the (HSK)NC-421A double surface heat sink already drilled for a single TO-3. Its rating is 1.1c/w - just about right.

2) On page 2, they have the (HSK)641A, which is rated to dissipate 35 watts. If we have a total of 100 watts to dissipate, spread over 8 output devices, shouldn't each device only need to dissipate 12.5 watts (eventhough its smaller than the one listed above)?

What's your take on these? Each alternative is cheaper than one or two larger sinks, and I can put them side by side on the sides and back of the chasis to keep a "smaller" overall appearance of the final stereo amp.
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Old 8th December 2000, 02:54 AM   #12
Jon T. is offline Jon T.  United States
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If there are 8 devices, the junction/case and case/heatsink resistance drops to .075 C/W, so the total heatsink/ambient resistance you need to shoot for is .175 C/W.

There's no reason you can't use multiple heatsinks. In fact, in this application, I can't see any other way. You simply divide the resistance of each heatsink by the number you are using. The formulas for series and parallel resistance is the same as for electrical resistance.

If you mount each transistor on its own heatsink, you'll only need about 1.4 C/W heatsinks which should be relatively easy to find.

The NC-421A you mentioned is exactly the same as the Wakefield 423, but half as high. The 423 is rated at .67C/w.

I can't seem to get to the site right now, but if I remember correctly, the 641 is a little less efficient 1.3 or something in that ballpark. Their "wattage" figure is a little high -- they assume a greater than 25 degree temperature rise.

I would go for either 8 of the 421's or 4 of the 423's. If you're building a stereo amp, remember, you'll need twice this many. I personally would think the 4 (x2) 423s would be better. Mounting 16 621s on a normal sized chassis would be hard.

Actually, if you used 6 621s or 3 623s per channel, the total heatsink resistance would be .183 and .223 respectively. These would give a 26 and 29 degree rise over ambient, which would probably be fine.

One final thing to remember, the resistance rating on these are for them mounted with the fins vertical and with adequate airflow through the sink. If you mount the fins horizontally, they'll be less efficient, but I don't know by what factor.

-Jon
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Old 8th December 2000, 02:56 PM   #13
Eric is offline Eric  United States
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Exclamation output devices & heatsinks

Jon: I think we have a little misunderstanding: for the a40 amp, there are 4 output devices per channel, for a total of 8 devices for a stereo amp. These 8 total devices need to dissipate a total of about 100w (50w per channel) at idle.

Does this then raise the thermal resistance requirement for each heatsink to 0.7c/w? (half of the 1.4 you mentioned above)

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Old 8th December 2000, 07:26 PM   #14
Jon T. is offline Jon T.  United States
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I'm afraid the dissipation is 100w in each channel in the A40. In a class A push/pull design, you dissipate at least twice as much power as you can deliver into the load.

In general, the formula for calculating the total thermal resistance of the whole mess is:

R = (Rj + Rc)/n + Ra/m

where:
Rj is the junction/case resistance of the transistor
Rc is the case to heatsink resistance of whatever insulator you're using
Ra is the heatsink/ambient resistance of the heatsink (the normally quoted efficiency)
n is the number of output devices
m is the number of heatsinks

Typical values for Rj and Rc are 0.5 and 0.1 respectively, but look on the data sheets for your transistors.

If you find the contribution of Rj and Rc too high, you can always add additional output devices in parallel. Four output devices for 40w class A seems really low to me. Pass's own A75 design uses 24 devices per channel for 75w.

Also, you can probably run the junctions higher than 25C above ambient. Most power transistors can be run with junction temps in the 100-200C range. Just make sure that the power dissipation at that temperature doesn't exceed the safe operating area given on the spec sheet.

-Jon
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Old 8th December 2000, 07:34 PM   #15
Eric is offline Eric  United States
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Talking oops! Your're right!

Ooops, I had calculated the power dissipation accross each of the push/pull output stages for each channel. According to the calculations in the paper on the A75, I calculated the dissipation for each bank at 54w (54w push, 54w pull), so there are a total of 108w per channel, or 216w for the stereo pair - duh! Got lost in my own calculations...

Thanks for pointing out my error - could have lead to a big problem!
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Old 8th December 2000, 08:31 PM   #16
Eric is offline Eric  United States
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Talking One more try

OK, here goes another try putting together what we have gone over:

Following Pass's calculations on dissipated power (A75 amp article), the a40 will need to dissipate 216w in total for stereo operation. We wish to keep thermal rise to 25c. Therefore, the target thermal resistance = 25/216 = 0.115c/w. OK so far?

In the article on the A75 http://www.passlabs.com/projects/a75_2_8.htm, this figure is simply divided by the number of heat sinks to use, and no mention it made of junction/case and case/heatsink loss. If we ignore these losses, I could use 8 heat sinks each with a 0.92w/c thermal resistance (8 * 0.115 = 0.92). This seems pretty easy to accomplish...

However, if we factor in the junction/case and case/heatsink loss for a total of 8 output devices (stereo) and plan on using 8 individual heat sinks (onee per output device), we then get the formula:

(0.5+0.1)/8 devices + cw/8 heatsinks = 0.115 total thermal resistance
cw = (0.115 - 0.075) * 8 or <b>0.32c/w for EACH</b> of the 8 heatsinks?!?!

8 sinks with thermal resistance of 0.32c/w seems REALLY excessive... Am I wrong here? This is like having 8 of those Seal heatsinks!

Any guess why Pass's article ignores losses in thermal resistance? In the last paragraph of the heatsink section, he simply indicates that the case temp rises 10c over the 50c temp of the sink - is this because no additional thermal losses are accounted for?


[Edited by Eric on 12-08-2000 at 03:43 PM]
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Old 8th December 2000, 11:06 PM   #17
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Default Thermal compensation

I haven't spent much time looking at the pass40, but if it has a thermal compensation loop to prevent thermal run away then putting the transtors on too many different heat sinks may nullify this circuit.
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Old 9th December 2000, 10:58 PM   #18
Eric is offline Eric  United States
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Question How to recognize a thermal feedback loop

I'll go back to the plans and re-read them to see if he mentions such a loop. On the plans, the wiring guide for the transistors simply indicates 4 output devices and four resistors that are not mounted on the main amplifier circuit board, so these are the components located near/on the heatsinks. I'll go back over the article to see what I can find. The two pictures of this project on the pass web site show 1 design using 2 heat sinks, and a second design using 4 heat sinks...
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Old 11th December 2000, 07:19 PM   #19
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I've been considering the Leach amp which uses diodes in the heat sink for thermal compensation. Sounds like the design you are looking at doesn't have(need) thermal compensation.

Wade
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