How do I find max continuous drain current

Thread Starter

Nutball

Joined Jan 29, 2016
23
I have often wondered this, so I can properly determine what a MOSFET can do by looking at the data sheet. Some, few, data sheets show a continuous drain current very close if not exactly what I'm looking for along with the very high continuous current numbers all other data sheets show. I have a link to a sheet I am currently interested in, it says things like 180A @10v continuous drain or 75A package limitation, but even 75A seems way too high to be a continuous value through those legs, maybe unless you had a big cool heatsink for cooling the body and a big cool heatsink soldered to the legs. I know brushless sensorless ESCs involve some low frequency switching at full throttle (no PWM), and under this condition is what I want to know the max continuous amps. I notice it says in the sheet @10v, what about at 65v for example? I know it can't withstand 180A at 65v for very long at a time, and I still highly doubt it could do the same at 10v 75A, but I don't know much about how they work, I'm just going off of experience with wire gauge heating vs amps, and the number of FETs it takes in an ESC to keep from overheating at xxx amount of amps.

So, please tell me what the value is I'm looking for and how to calculate it, so I can figure it out for myself next time.
http://www.irf.com/product-info/datasheets/data/irfs3207.pdf

thanks
 

AlbertHall

Joined Jun 4, 2014
9,831
This is the absolute max rating. That means anything above this may damage the device so you should leave a safety margin below these numbers. It says continuous drain current with Vgs @ 10V (NOT drain-source voltage). The gate source voltage affects the drain-source resistance and therefore the heat produced by the drain current. Also each line includes the case temperature (Tc) at which the rating applies so to use this spec your heatsink needs to be able to keep the case temperature below this value. Then, by the max current number there is a note number (1). Note 1 says that this maximum current is based on the dissipation in the device and further says the package limit is 75A.

So putting all that together, the absolute maximum current allowed by the package is 75A. How much current is allowed in any particular circumstance is determined by the dissipation and the heat produced, so it depends on the heatsink and the gate-source voltage.

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crutschow

Joined Mar 14, 2008
24,946
Generally you want a max rating well above the desired current to minimize transistor power dissipation and heat sink requirements.
For that you want to look at the maximum Rds(on) value for the MOSFET.
The dissipation is equal to the current times the Rds(on) when operating it as a switch.
 

Thread Starter

Nutball

Joined Jan 29, 2016
23
I guess SM means surface mount because of this PDF, but it doesn't really say what it means http://www.aosmd.com/res/application_notes/mosfets/Power_MOSFET_Basics.pdf

I wish there were some standards for certain components to make things easier instead of having to go through testing by a potential customer since the data sheets don't use the same methods of obtaining values. Of course the resistance values give away what is best for the most part. It seems like IR's ID is based on max junction temp, and AO's is based on package limitations. AO also has an IDSM value I assume means ID when Surface Mounted to a 1sqin FR-4 board with 2oz copper, in a still air environment with 150C max junction temp. That is more like the conditions on an ESC because of the high copper content board and finned heatsink on top. So, it seems like what I'm looking for isn't an easily calculated value, but a value that requires testing or complex math.

The other thing is I think "continuous" should be better defined because the first thing that comes to mind is forever without overheating, not until it reaches a certain temp. Maybe they should say continuous with a duty cycle of...

Thanks for your help so far. Am I correct on these guesses?
 
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