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Transconductance of Mosfets at low currents
Transconductance of Mosfets at low currents
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Old 15th January 2018, 01:24 PM   #11
john_ellis is offline john_ellis  United Kingdom
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Interesting results- thanks for the info.
I have not made any systematic measurements - perhaps another item you may want to add to your "to do" list (or not). I have used simple aluminium angle to clamp TO-92's to a standard heatsink, and have not overheated as a result.
Typical Rth j-amb is abnout 200K/W. My guess is that because plastic is such a poor conductor that I would not run them at more than 2x (thus I expect 100K/W Rth j-c) , but to check I would also have to measure the performance. You can "easily" measure the Vbe for a device with and without a heatclip to check the temperature rise (with a high impedance probe/amplifier/multimeter of course).
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Old 15th January 2018, 08:48 PM   #12
MarcelvdG is offline MarcelvdG  Netherlands
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Originally Posted by gwolf View Post
I have now measurements with IRFD110 (similar to IRF510), with Ugs=12V and up to 200mA, showing that it definitely does not follow square law in this region:
Attachment 656892
The gm charts are shown negative for better visibility. The horizontal axis is moved by using AC coupling, the Vgs=VTH point is located at -0.4V The theoretical charts are made with K=0.35, which is much smaller than K=0.56 (1.5S at 2A).
I will repeat these measurements with TO92 devices, expecting that they behave differently.
Could you please explain what your measurement set-up is? An IRF510 biased at a Vgs of 12 V would draw a lot more than 200 mA when it is biased outside the triode region (Vds > Vgs - Vth), as amplifying devices normally are.

Is the transconductance proportional to drain current for Id << 10 mA, like it should be in weak inversion?
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Old 16th January 2018, 02:40 PM   #13
gwolf is offline gwolf  Austria
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Sorry for the misspelling. It should read Vds=15V, not Vgs. This is the setup:

Kennlinie anzeigen.JPG

Gm as a function of Ids (for K=0.56, which is the datasheet value):


For small currents it is not totally linear but definitely more so than the theoretical Gm for square law. But the difference (e.g. 0.1S instead of 0.15S at 20mA) is not dramatic. Actually IRFD110/IRF510 is still a small device. Unfortunately there is much noise in the calculation due to the differentiation.
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Old 16th January 2018, 06:32 PM   #14
gwolf is offline gwolf  Austria
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I have now made a test with ZTX653 (case e-line, Ptot=1W @ 25C, 5.7mW/K degradation) at 0.96W (80mA, 12V). Measured Vbe:
- free air (20C): 370mV
- with small heatclip (10x15mm): 450mV
- mounted with the flat side on a big heatsink: 586mV
This results in a cooling (assuming that the 2mV/K are valid in the hole temperature range) by
- heatclip: (450-370mV)/2mV/K = 40K
- big heatsink: 108K

With the heatclip the limit is 1W+5.7mW*40K=1.23W and with an (unrealistically) big heatsink 1.61W.
These are not big improvements, although the surface of the case is much cooler with the heatclip. You are right, the plastic is the real problem. Therefore we must look for cases with an exposed metallic area like SOT223 or rely on cooling through the leads of SMD devices.
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Old 16th January 2018, 06:47 PM   #15
cumbb is offline cumbb
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I would use a transen-casing as small as possible. Sounds much cleaner and so on;-)
Test and tell;-)
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