Electronics > Projects, Designs, and Technical Stuff
My constant current load - 90W , 3A 30V
Spikee:
I made some upgrades , the one nxp fet has been replaced by two BUZ 11 whit gate , drain , source connected together.
some pictures:
setup:
https://dl.dropbox.com/u/100819959/2012-11-28%2023.00.45.jpg
Display:
https://dl.dropbox.com/u/100819959/2012-11-28%2023.01.06.jpg
Heatsink:
https://dl.dropbox.com/u/100819959/2012-11-28%2023.01.17.jpg
Power supply (Agilent U8001A) and mosfet temp. measurement (Rigol DM3058) :
https://dl.dropbox.com/u/100819959/2012-11-28%2023.03.05.jpg
Some more tests:
15V 3A - 45W fan at 5V:
duration: 10 minutes
heatsink temp : 50 deg. C
mosfet (2) temp : 55 deg. C
room temp: 21 deg. C
heatsink Rth (j-a) ~ 0.73 C/W
20V 3A - 60W fan at 5V:
duration: 15 minutes
heatsink temp : 59.5 deg. C
mosfet (2) temp : 65 deg. C (still self balancing)
room temp: 21 deg. C
heatsink Rth (j-a) ~ 0.73 C/W
Mosfet (2) failed at 60W 70 deg c. on package 60 deg c. heatsink. Because the the second fet failed the first one also did.
The balancing was not good , mosfet (2) was 70 deg. c while mosfet (1) was 60 deg. c .
I could ad balance resistors but i deoubt that it will fix much.
The best solution would be one big TO264 package or so.
I will need some help to find the good one.
gr.spikee
peter.mitchell:
did you use some thermal paste/grease or thermal pad? that would really help/hinder, also, make sure you tighten them on sufficiently, bigger packages (to-247 and to-264) will help a bit, but 60W should be fine with a TO-220, provided the heatsink is cool enough and thermal conductivity is high enough.
mrflibble:
--- Quote from: Spikee on November 28, 2012, 10:10:00 pm ---I made some upgrades , the one nxp fet has been replaced by two BUZ 11 whit gate , drain , source connected together.
--- End quote ---
You can parallel those mosfets, but connecting it all together like that is not the best plan. ;-)
At the very least use seperate gate resistors. See for example: http://www.irf.com/technical-info/appnotes/an-941.pdf
Anyways, maybe parallelling is more trouble than it's worth. If you have enough cooling + a decent thermal contact then 1 device should be able to survive that 60 W.
blackdog:
Hi Spikee
You are making mistake's...
Place the 2 powerfets in the middle of your heatsink, thats the place were your vancontroler sensor is.
Mount the powrfets on the other side of the heatsink, that is turn is 90C.
NOT on the edge were you coupled the heatsinkparts togetter...
One on each side.
If you do it this way your heatsink wil be ~0.73c/Watt
Be shure the Fet is mounted flat en use a good compound!
Use a small Source resistor say 0.1 Ohm, use a current of 1A en measurethe voltagedrop over these resistors.
If the voltage is within 15% its OK, other pair some fets or use a little higher source resistor.
Mount the gateresistor direct on the Fet.
Keep those wires short!
Kind regarts,
Blackdog
PS (Dutch Mode)
Als je ook op het forum van "Circuits Online" zit kan ik je wat beter helpen.
Engels is nog steeds knap lastig voor mij :-)
Spikee:
The mosfets were mounted this way: bolt - plastic washer - mosfet - thermal grease - mica plate - thermal grease - heatsink .
I read in some study that it was the best way. After a second look i saw that i could move mosfet (2) , thats probably the reason that one got so much hotter than the other one.
The mosfets i have laying around:
5x MTP3055 - 50V 12A ~48W
7X IRF510 - 100V 5.6A ~43W
30x IRLB8721 - 30V 44A ~ 65W @ 25 deg. C 33W @ 100 deg. C (logic level)
@blackdog ,
Ik zit ook op Circuits online ( Spikee ) maar heb daar momenteel geen topic lopen.
I will try to match a pair of those MTP3055 . What amount of gate resistor should i be looking for ?
And i will mount the mosfets according to your post.
Thanks for all the info and feeback.
Spikee
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