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Help with Mosfet data sheet.

Can one of you EE's please help an ME understand the details of a mosfet data sheet. The values I've calculated are not even close to real world results. My understanding of the data sheet is limited to what I have read on this and other news groups.

Here's what I've done thus far. To test my calculated values to a real world example I did the following.

24V dc precision power supply 1 mosfet STP1920-NB (TO-220 case) 1 Heating Element reads 13.xx ohms

With this connected and powered up, I get a current reading of 1.540 amps.

From the data sheet I get the following info.

RDS(on) .15 ohm Tja 62.5 C/W

From search results online I used the following to predict the rise in

temp on the case. Ambient Temp 27.8 C Power dissipation (1.54 * 1.54) * .15 = .35574 W Rise in temp = Tja * PowerDissipation 62.5 * .35574 = 22.23 C Case temp = ambient + RiseInTemp 27.8 + 22.23 = 50.01 C In about 10 seconds time the actual value I'm reading on the case is almost triple that at 148 C .

Data sheet for the mosfet is here:

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Any help is greatly appreciated Jim

Reply to
chapmjw
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chapmjw a écrit :

Several points:

1) 24V/13.xx = 1.84A and you measure 1.54A This seems to indicate your mosfet isn't saturated. How much gate drive do you have? You can check the source-drain voltage which should be under half a volt. 2) Apart this your calc is almost OK, except the you forgot to correct RDSon for temperature (see curve GC74110 page 5). 
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Thanks,
Fred.
Reply to
Fred Bartoli

That's why you always need to test the theory.

If your supply is stable at 24V, the resistance across the MOSFET is 2 ohm, not .15 ohm. R(mosfet) = V / I - R(ele)

With 2 ohms, the junction temperature would be close to 300 C.

Sound right, with the temperature gradient.

here:

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You can't just drive this with a constantly on MOSFET.

Reply to
linnix

The real world is more like this: 24V supply

-0.15V meter loss

-(14R*1.54A) Heater has pos tempco = 2.29V across FET *1.54A = 3.5W Why does your fet have almost 1.5R ? Your gate voltage is too low, or you have made up a wrong circuit. Your fet must have the Source on gnd, and the load is between +24V and Drain.

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ciao Ban
Apricale, Italy
Reply to
Ban

I would agree. Even if 13.xx would be 13.99 ohms that should result in about 1.7A, provided the 24V are really precise. Either the FET gate-source voltage isn't high enough or maybe gate and source are reversed and Jim is accidentally running it via its body diode, which is slowly frying the thing up.

Without a heat sink it's probably just a few seconds until smoke signals will emerge and some bits and pieces fly off ;-)

here:

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>STB19NB20-1,%20STP19NB20_FP.pdf

Why not? A FET with an Rdson of 0.15ohms looks alright here. If it's fed the proper gate voltage, that is.

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Regards, Joerg

http://www.analogconsultants.com
Reply to
Joerg

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