Posted by chapmjw on January 30, 2007, 12:54 pm
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:
http://rocky.digikey.com/WebLib/ST%20Micro/Web%20Data/
STB19NB20-1,%20STP19NB20_FP.pdf
Any help is greatly appreciated
Jim
Posted by Fred Bartoli on January 30, 2007, 1:26 pm
chapmjw a écrit :
> 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:
> http://rocky.digikey.com/WebLib/ST%20Micro/Web%20Data/
> STB19NB20-1,%20STP19NB20_FP.pdf
> Any help is greatly appreciated
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).
--
Thanks,
Fred.
Posted by linnix on January 30, 2007, 1:27 pm
> 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.
That's why you always need to test the theory.
> 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.
If your supply is stable at 24V, the resistance across the MOSFET is 2
ohm, not .15 ohm. R(mosfet) = V / I - R(ele)
> 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
With 2 ohms, the junction temperature would be close to 300 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 .
Sound right, with the temperature gradient.
> Data sheet for the mosfet is
here:http://rocky.digikey.com/WebLib/ST%20Micro/Web%20Data/
> STB19NB20-1,%20STP19NB20_FP.pdf
> Any help is greatly appreciated
> Jim
You can't just drive this with a constantly on MOSFET.
Posted by Joerg on January 30, 2007, 6:31 pm
linnix wrote:
>
>>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.
>
>
> That's why you always need to test the theory.
>
>
>>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.
>
>
> If your supply is stable at 24V, the resistance across the MOSFET is 2
> ohm, not .15 ohm. R(mosfet) = V / I - R(ele)
>
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.
>
>>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
>
>
> With 2 ohms, the junction temperature would be close to 300 C.
>
Without a heat sink it's probably just a few seconds until smoke signals
will emerge and some bits and pieces fly off ;-)
>
>>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 .
>
>
> Sound right, with the temperature gradient.
>
>
>>Data sheet for the mosfet is
here:http://rocky.digikey.com/WebLib/ST%20Micro/Web%20Data/
>>STB19NB20-1,%20STP19NB20_FP.pdf
>>Any help is greatly appreciated
>>Jim
>
>
> You can't just drive this with a constantly on MOSFET.
>
Why not? A FET with an Rdson of 0.15ohms looks alright here. If it's fed
the proper gate voltage, that is.
--
Regards, Joerg
http://www.analogconsultants.com
Posted by Ban on January 30, 2007, 2:16 pm
chapmjw wrote:
> 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:
> http://rocky.digikey.com/WebLib/ST%20Micro/Web%20Data/
> STB19NB20-1,%20STP19NB20_FP.pdf
> Any help is greatly appreciated
> Jim
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.
--
ciao Ban
Apricale, Italy
