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Determining Mosfet Max operating Current rating

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rocky79

Member level 5
Hello all,

I am using a Mosfet S0IC-8 Package that is rated up to 7.5 Amps. This Mosfet is used in a simple load switching circuit. The mosfet is mounted on a FR4 board using the datasheet recommended PCB pads (not using 1SQI pad).
The mosfet part #fpcbp160. With the Mosfet mounted on the PCB with no dedicated heatsinking pad will a 3.5A Max operating Ids current be on the safe side? Assuming Ambient temperature =25C

Thank you

It depends on the Rds on resistance of the device

Your problem is not so simple. The SOIC-8 package current limit is only linked with the lead to soldered PCB connection. The MOSFET max current rating is defined by its manufacturer. Its definition may be (or may not be) dependent upon e.g. pulse frequency and duty cycle.
The prudent advice is to choose the minimum rated current for a defined ambient temperature. You can connect more MOSFETs in parallel to get the required maximum current, or, select a MOSFET type and package that meets your needs.

Some devices allow overdrives but tend to die after some time. It is advisable to use overrated devices and structures for safety.

Here is where I got that 3.5 A max rating from:

Assuming operating junction temperature is 125C the device is rated to a max of 150c, Rteta(JA)=183.5C/W ( from the datasheet)
Pwr (max)=(Tj-Ta)/Rteta(JA)
=(125C-25C)/183.5
=0.54Watts
Since I am running the mosfet at Vgs=5v then Rds=36mOhm
So if I have 3.5Amps going through the mosfet. The powe dissipation is P=(I^2)xR=0.441 w which is less that Pwr(max)

I hope this makes sense, otherwise feel free to correct me.

Thank you

rocky79,
25 deg. for ambient temperature in your assumption, seems unrealistic. Ambient temperature increases during on time of the circuit specially if it is in a box or enclosure. For continuous and reliable operation I recommend you to use a mosfet with a lower rds or use 2 parallel.

rocky79,
25 deg. for ambient temperature in your assumption, seems unrealistic. Ambient temperature increases during on time of the circuit specially if it is in a box or enclosure. For continuous and reliable operation I recommend you to use a mosfet with a lower rds or use 2 parallel.

goodboy_pl,
That's true, if I use 40C as ambient temperature I would still get a 460mW as max power. So I am still within range.

please note that the current and power ratings of power mosfets specially smd ones is depending on mounting conditions. using power pad , via holes near the device and large copper planes reduce thermal resistance. without this precautions junction temperature will rise and this will increase rds(on) up to 2 times. as u can see in infineon or vishay siliconix web sites, there are several high performance solutions for your project.
BTW in comparison with current technologies, 36 mohm device is not state of the art.

BEST

rocky79

rocky79

Points: 2
please note that the current and power ratings of power mosfets specially smd ones is depending on mounting conditions. using power pad , via holes near the device and large copper planes reduce thermal resistance. without this precautions junction temperature will rise and this will increase rds(on) up to 2 times. as u can see in infineon or vishay siliconix web sites, there are several high performance solutions for your project.
BTW in comparison with current technologies, 36 mohm device is not state of the art.

BEST

I appreciate your responce. Thank you

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