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Old 6th December 2010, 01:38 PM   #1
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Default What determines how high bias can be on F-5?

My F-5 has fairly large heat sinks. 12.25 by 8 by 2.3. They do a great job, the rest of the aluminum chassis helps as well. It is easy to hold on to these as long as you want.

I know that stressing the outputs can be detrimental to their reliability. Is the point at which damage occurs determined by going over the recommennded .59, or is it from the heat of the heat sink going to high? I realize one is connected to the other, but I'm wondering if it is ok to up the bias, as long as the heatsink can be touched for 5 seconds, or is it more to it than whether the heat sink is sufficient to keep itself with in the 5 second rule?

How high is too high, a particular number on the multimeter, or the actual heat of the sink? Where do most of you let yours run and how hot does your sink get?

Thanks,

Russellc
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Old 6th December 2010, 01:55 PM   #2
AndrewT is offline AndrewT  Scotland
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The manufacturer tells us that the maximum Tj for long term reliability is somewhere in the range 125degC to 200degC.
Most plastic packaged output devices are either Tjmax=150degC or Tjmax=175degC

The SOA curves for Tc=25degC show how much current and voltage the device can take and for how long to just bring Tj = Tjmax.

The hotter you run Tc then the lower is the SOA (temperature de-rated SOAR).

If your sink at the contact face is 50degC then Tc~55degC to 65degC.
Draw your temp de-rated SOAR to find the allowable currents/voltages/durations.

raise the device dissipation (increase Ib) and Ts, Tc both rise. New set of de-rated SOAR for the new Tc.
Eventually as SOAR moves down, the output devices pass current for the duration that the speaker demands and you eat into the reliability reserve. Not instant failure, but likely to be a shortened lifetime.
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Old 6th December 2010, 02:37 PM   #3
frags is offline frags  Canada
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Russellc, it doesn't matter how hot or cold your heat sinks, it does matter how high the Tj is. You can have big enough heat sinks and crappy thermal interface between them and transistor - heat sinks will be cold and transistors are hot.
I usually measure the temperature on transistor case (Tc) with IR thermometer then, using formula Tj = Tc + Pt * K, define junction temperature.
Pt - applied power
K - junction to case heat transfer - I'm using 1.0 for transistors like IRFP240
Acceptable junction temperature is less than 100C.
So, for example, if Tc = 60C and Pt=25W, junction temperature Tj will be 85C which are OK.

PS. Mica insulator with 0.05mm thickness will give you 8-10C difference in temperature between heat sink and transistor case.

Last edited by frags; 6th December 2010 at 02:39 PM.
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Old 6th December 2010, 02:45 PM   #4
anilva is offline anilva  India
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My question is similar to the first post. Do not understand the answers well enough. Can they be explained to a casual DIYer?
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Old 6th December 2010, 03:05 PM   #5
frags is offline frags  Canada
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Quote:
Originally Posted by Russellc View Post
but I'm wondering if it is ok to up the bias, as long as the heatsink can be touched for 5 seconds, or is it more to it than whether the heat sink is sufficient to keep itself with in the 5 second rule?
Simple answer is NO, it is not OK. I understand why Mr. Pass introduced this rule of the thumb for touching heat sinks for DIYers but it is not applied for any heat sinking. For each situation you are better to calculate junction temperature to make sure it's acceptable.
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Old 6th December 2010, 05:39 PM   #6
h_a is offline h_a  Europe
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Jeez, if I remember at what power the fets in the old Alephs were cooking in comparison.

Personally, I would not mind running the heatsinks at even 60 degrees or a little more.

In fact many commercial amps do that (and higher) if the heatsinks are internal and there's no risk of the owner to get burned.

Have fun, Hannes

PS: wouldn't hurt if you make sure first that the thermal pads are of high quality for good heat conduction of fet to sink.
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Old 6th December 2010, 09:06 PM   #7
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I hadnt really thought about the juncture not sending the heat to the sink.
The surface was really smooth and the fit is tight. I am using silicone silpads now, and am running the IRF devices.

Sounds like the best bet is to get a thermometer and measure the actual output device temp. Then is seems from what everyone says that it would be safe to up it until the heat of the output device is maximal.



Thanks,
Russellc
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Old 6th December 2010, 09:12 PM   #8
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Quote:
Originally Posted by frags View Post
I understand why Mr. Pass introduced this rule of the thumb for touching heat sinks for DIYers but it is not applied for any heat sinking.

Thanks for the response, but it wasnt Mr pass saying this to DIYers that I am referring to, I believe I saw that in the F-5 owners manual, Speaking to his own commercial F-5s. Doesnt mean the rule holds true for my DIY effort based on what everyone's saying here. If my ouputs crap out, I have spares and several sets of Toshiba outputs. Next time I'll use them and the mica pads with thermal greas instead of silpads.

Westend, the member who cut the aluminum for my case and also used the same heat sinks in his effort, polished the surface to a almost mirror finish
and based on his description to my queries, runs no hotter or cooler than my effort. I need to go buy a suitable thermometer!

thanks again,
Russellc
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Old 6th December 2010, 09:16 PM   #9
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Now that I've thought about it a minute, I still have the same question based on a different set of circumstances. If I am measuring the heat of the output device ITSELF, is it OK to up the bias as long as the heat of the device remains within recommendation?

Where is everyone running their F-5 in terms of bias?

Russellc
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Old 6th December 2010, 09:17 PM   #10
frags is offline frags  Canada
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Quote:
Originally Posted by Russellc View Post
I am using silicone silpads now, and am running the IRF devices.
I would avoid using them for high power applications. The best transfer rate you can get with them for TO-247 is about 1C/W. If you are running 25W through a transistor you will get 25C difference between transistor case and heat sink.
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