Khundude,
If I read the F5 thread correctly, there are two IRFP240s each biased at 1.3 A with 24 volt rails. That's 31.2 W per device.
If you look at Coulomb's models. the 4 x 13 comes close to the same total dissipation as an F5. Notice the average temperature, over 50C. With a pair of devices, the devices would be hotter, probably around 115 C junction temperature. While the devices can withstand this temperature, reliability will be reduced. This leads to the recommendation for two 10" sections - significantly over sized.
The numbers for the 5" with a single device dissipating 32W will be similar - doable, but your amp may be a bit unreliable.
Looking at the Conrad MF30-151.5 derated for 20C temperature rise we see that we can expect 22 C rise with 64 watts dissipation. That puts the sinks at 40C in a 28C room. With the thermal resistance from die to case and through the heat sink, we can expect nearly 60C rise from heat sink to junction. This puts you right at the recommended maximum 100C junction temperature, without much room to expand.
So, if you want to use a Conrad MF30-151.5 for your F5, or a pair of 5", have at it. Those of us interested in these sinks either want a bit more margin or have other projects in mind. Neither camp is wrong. Please try to let us enjoy our projects without criticizing our choice simply because it is more than YOU need.
If I read the F5 thread correctly, there are two IRFP240s each biased at 1.3 A with 24 volt rails. That's 31.2 W per device.
If you look at Coulomb's models. the 4 x 13 comes close to the same total dissipation as an F5. Notice the average temperature, over 50C. With a pair of devices, the devices would be hotter, probably around 115 C junction temperature. While the devices can withstand this temperature, reliability will be reduced. This leads to the recommendation for two 10" sections - significantly over sized.
The numbers for the 5" with a single device dissipating 32W will be similar - doable, but your amp may be a bit unreliable.
Looking at the Conrad MF30-151.5 derated for 20C temperature rise we see that we can expect 22 C rise with 64 watts dissipation. That puts the sinks at 40C in a 28C room. With the thermal resistance from die to case and through the heat sink, we can expect nearly 60C rise from heat sink to junction. This puts you right at the recommended maximum 100C junction temperature, without much room to expand.
So, if you want to use a Conrad MF30-151.5 for your F5, or a pair of 5", have at it. Those of us interested in these sinks either want a bit more margin or have other projects in mind. Neither camp is wrong. Please try to let us enjoy our projects without criticizing our choice simply because it is more than YOU need.
Coulomb said:
Thank you, Anthony.
I just wanted to play conservative and didn't want to bail out at the last minute. I certainly hope that I can get back to the game when this GB is finalized. I still have one pair of the R-theta from your last GB. They are really really nice.
Wiki updated. I am interested in 4 HS and 1 large chassis with 4 HS (i.e. a total of 8 HS).
Dan
Dan
BobEllis said:
Once again,
Dudes take a chill pill. I am asking questions and getting different answers and simply trying to clarify the facts. If you call this critizism, I am confused. The only reason why there is a "tone" to my posts is that I was told at first that I would NEED 4-10", and when I questioned that all I got was grief. There are a lot of know-it-alls that are sometimes right and this is dangerous to the humble guy that is just as often right and never believes he knows it all. What worse is when that humble dude asks for help and gets conflicting answers and then is ridiculed for not understanding how all responses can be correct and all different at the same time. Crystal?
Hello all,
I'm interested in this GB, but I haven't had the time yet to digest all 11 pages of this thread to see what exactly I'll need. I'm in on a GB building Roender's RMI-FC100 amp, but haven't sourced any chassis or heatsinks, yet.
This is my first amp project, so I'm leaning heavily on those with more experience. My hunch is that I'll be in the market for a large chassis with four heatsinks, and if anyone with more experience cares to verify, I'll be much obliged. Otherwise you'll have to bear with me as I slowly get up to speed.
Thanks,
Ted
I'm interested in this GB, but I haven't had the time yet to digest all 11 pages of this thread to see what exactly I'll need. I'm in on a GB building Roender's RMI-FC100 amp, but haven't sourced any chassis or heatsinks, yet.
This is my first amp project, so I'm leaning heavily on those with more experience. My hunch is that I'll be in the market for a large chassis with four heatsinks, and if anyone with more experience cares to verify, I'll be much obliged. Otherwise you'll have to bear with me as I slowly get up to speed.
Thanks,
Ted
Well I am trying to build a thermal report specifically for the F5 to help members who are planning on putting F5's into this chassis. I have tried every extrusion R-Theta has from 5" to 10" in length and although I can get some very good Tsink-Avg ratings I cannot get the Tjunction temperature below 116 Deg.
32Watts per device really builds heat and even the biggest heatsinks do not seem to help. I can attest to one fact the thicker the base the lower the Tj. I would really like to know what Mr. Pass plots the Tj at on his F5.
The 66449 10" profile meets or exceeds the specifications of a good heatink for a F5, but at one per transistor it can only achieve a Tj of 122 Deg C.
The Tj on a 5" section of 66449 soars to 191 Deg C. For some reason I thought the dissipated power was 16 Watts per device, but if it is as Bob indicates at 32 Watts per device then Mr. pass must be using Magic heatsinks!
Anthony
There isn't anything magic about maintaining <100C junction temperatures. It's just a number that Nelson Pass threw out several years ago as a target for good reliability and has since become almost gospel around here. I wouldn't be terribly surprised if he exceeds it by a few degrees on the F5. Devices don't necessarily blow up if 100C is exceeded, but the hotter they are the less reliable they may be.
If you want to run hot, be sure to derate your devices appropriately. (data sheet time) Most plastic devices I am familiar with are rated for 150C maximum junction temperature. Exceed that and you should expect failure in short order.
Junction temperature will also be reduced by reducing the thermal resistance of the isolator. Thin Kapton tape helps. The ultimate in low thermal resistance is mounting the devices directly to to the sink, without mica or other isolator. As long as it remains intact, the anodizing will isolate the device. (the big IF) This is not necessarily good practice, and creates the potential of shorting the rails together or having the sink/case at one rail potential above ground.
If you want to run hot, be sure to derate your devices appropriately. (data sheet time) Most plastic devices I am familiar with are rated for 150C maximum junction temperature. Exceed that and you should expect failure in short order.
Junction temperature will also be reduced by reducing the thermal resistance of the isolator. Thin Kapton tape helps. The ultimate in low thermal resistance is mounting the devices directly to to the sink, without mica or other isolator. As long as it remains intact, the anodizing will isolate the device. (the big IF) This is not necessarily good practice, and creates the potential of shorting the rails together or having the sink/case at one rail potential above ground.
argofanatic said:
ok be nice...
Khundude there is not enough interest in a 5" non machined profle to make it worthwhile to offfer one.
Even the 5" machined is slow to show any interest, even after a few members asked for it. I have considered moving it up to a 6" as I have been working on a configuration that would work for an F5 stereo.
Anthony
argofanatic said:
ok be nice...
Khundude there is not enough interest in a 5" non machined profle to make it worthwhile to offfer one.
Even the 5" machined is slow to show any interest, even after a few members asked for it. I have considered moving it up to a 6" as I have been working on a configuration that would work for an F5 stereo.
Anthony
OK some good news for F5 Builders.
As you all know there has been quite some discussion about using these Heat Sinks for an F5 build. The issue is not so much the Tsink-avg but the Tjunction. I spent some time optimizing the parameters by trying different components and this is what I have come up with.
Increase from 5" to 6" in length, hopefully at no additional charge.
Switch from Sil Pad 400 to Sill Pad 2000, much higher W/M-K
Use IRFP250N which has a RèJC of 0.7, an IRFP240PbF has an RèJC of 0.83, I was using 1.1 from an old data sheet from an IRFP140N.
The new IRFP250N has an Input Capacitance of 2190Pf a little higher than the 1400Pf of an IRFP240 device, but as it is not in parallel with other HEXFETs it should be fine.
IRFP250
Sil Pad 2000
I have attached a report that indicates the vital thermal parameters for a 6" Profile with these changes.
The result is a very usable profile for an F5, (1 per transistor). 4 sections to make a Stereo chassis using the small Chassis kit if you are so inclined.
I hope this helps members still on the fence to sign up for our little GB. The option still remains to use the 10" with these changes as well.
Regards
Anthony
As you all know there has been quite some discussion about using these Heat Sinks for an F5 build. The issue is not so much the Tsink-avg but the Tjunction. I spent some time optimizing the parameters by trying different components and this is what I have come up with.
Increase from 5" to 6" in length, hopefully at no additional charge.
Switch from Sil Pad 400 to Sill Pad 2000, much higher W/M-K
Use IRFP250N which has a RèJC of 0.7, an IRFP240PbF has an RèJC of 0.83, I was using 1.1 from an old data sheet from an IRFP140N.
The new IRFP250N has an Input Capacitance of 2190Pf a little higher than the 1400Pf of an IRFP240 device, but as it is not in parallel with other HEXFETs it should be fine.
IRFP250
Sil Pad 2000
I have attached a report that indicates the vital thermal parameters for a 6" Profile with these changes.
The result is a very usable profile for an F5, (1 per transistor). 4 sections to make a Stereo chassis using the small Chassis kit if you are so inclined.
I hope this helps members still on the fence to sign up for our little GB. The option still remains to use the 10" with these changes as well.
Regards
Anthony
AndrewT said:
Yes curious that isn't it. I assume you are talking about Tsink-max & Tcase, I have asked the Engineering department at R-Theta and I have as yet to get a reply. I have tried hundreds of simulations changing all known variables and although they do react to the changes the react exactly the same.
I had assumed that the report was trying to indicate that the hottest measurable spot on the heatsink was the case of the device.
To segway to a somewhat related topic, when looking at the Thermal Imedance and Resistivity of an insulator be aware that they use 50lbf/in squared clamping pressure to express thier typical values. Sometimes wrongly expressed in Psi, which is a measure of pressure, this amount of force is fine on a tabbed device like a TO-220. 50lbf/in squared clamping pressure is considered to be the average for most devices and offers lower more attractive values, most specs for these insulators offer a chart of relative clamping force applied to the device and the resultant values that can be expected from the insulator performance wise.
This is very important as the maximum clamping pressure allowable on a TO-247 device is only 10lbf/in squared, thus all ratings on insulators must be derated.
Anthony
AndrewT said:
I just got this email from R-Theta.
Hello Anthony,
There is a display bug that doesn’t show the proper calculation for T-case. It is on our to-do list to fix. The Tjunction temperature is properly calculated and does include the proper Tcase temperature.
R-Theta Thermal Solutions
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