How hot do you get, really ?

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protos

protos

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Joined 2002
#1
There are thousands of postings on heatsink temperatures and dissipations but nobody seems to also add info on actual mosfet temperatures. I get about 45C on my Aleph 5 which is very cool by most temps mentioned here and I was wondering if it can handle a raise in bias. However what worries me is that some of the mosfets when measured directly, on the hottest middle pin, are running at around 90-100C.Is such a difference normal? So how much more could they really take and be reasonably reliable ?
OK guys, what temperatures do you get on directly on your mosfets compared to heatsink temperatures?
 
#2
First off, don't make the junction temperature go above 150 deg C. The middle pin is a reasonable approximation of tab temperature if the wire soldered to it is not drawing away too much heat, otherwise most brands of fet have a notch on each side of the case where you can get at the junction side of the tab with a thermocouple. Use a bit of heatsink past on the thermocouple. Second, look up the thermal resistance of junction to case (Rthj-c or similar) It will tell you how many degrees per watt the junction will run above tab temp. For a TO-247 device the Rthj-c would be in the range of 0.25 to 0.7 or thereabouts. So if you had a device with a Rthj-c of 0.7 deg C per watt then at 10 watts dissipation the junction would be 7 degrees above the tab temp. Hope this is of some use.

GP. - Professional temp-measurer (among other things) of big-slugger switchmodes.
 
#3
Yes, but isn't 150C the absolute maximum beyond which you get break-down ? Are the fets meant to be run so close to max and how does reliability suffer? I mean if you run at 100C and the fet has let's say 20,000 hrs projected life span , what is the life span at 130C?
According to my calculations my heatsinks are dissipating normally. They are 0.33C/W and are dissipating heat from four mosfets at 32V X .55A= 17.6W each so 4 X 17.6=70.4 W let's say 71W. So 71W x .33C=23C above ambient. Since ambient in my place is around 22C , 23+22 = 45 C which is exactly what I'm getting and which proves I think that I've got the mosfets correctly positioned and fitted despite them being on an aluminium extrusion which is then coupled to the heatsinks. Or are there other parameters I forgot?
 
#7
protos said:
Yes, but isn't 150C the absolute maximum beyond which you get break-down ? Are the fets meant to be run so close to max and how does reliability suffer?
Click to expand...

In the RCA Receiving Tube Manual their tubes are rated by two methods - CCS and ICAS, that is "continuous commercial service" and "intermittent commercial &amateur service. This recognises the extra reliability you would get if you rate something conservatively. Conversely, if you run things a bit harder as in ICAS it doesn't mean it is going to quit soon. In fact with solid state stuff it may last only 50 years instead of 350. A diy amp for domestic use (I'm presuming that's what yours is) is similar to ICAS. Lives don't depend on it and the amount of hours logged isn't comparatively great. I don't expect you would ever have a failure in practice unless you made tens of thousands of amps and kept statistics.

GP.
 
#9
A general rule of thumb I have been told several times is for each 10 degrees of temp rise, the expected lifespan of a piece of equipment halves. This figure takes into account effects such as the heat affecting the lifespan of capacitors, increased likelihood of dry solder joint developing etc.

Adrian.
 
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