Sorry for the lame question, but I am a software developer, and have only a limited understanding of analog circuits.
This test says that you can drive a motor with ODrive continuously at 40A with using only the heatsink, and 75-90A when using fans.
However, when I look at the datasheet, it has the following picture (I have already learned on this site that I cannot just take the V and A from the datasheet and multiply them blindly):
(Note that the test is for the 48V variant of ODrive, and this is used in the 24V variant, but I expect at least the same order of magnitude with both ODrive boards.)
So if I look at 20A (because 40A is driven by a pair - red line), then this MOSFET cannot work in DC mode at all, and even a 10ms pulse can have only 2.5V. If I look at 24V, then it can drive only 0.005A, which I find hard to believe.
I would find it very strange if the datasheet would be wrong, as I can see similar values with the MOSFETs used by VESC for example. So what I can think is that 40A is the average current through all the 3 phases (all 12 MOSFETs), and so they receive a lower load. Another is that a heatsink matters so much, and that would mean that I have to do a thermal analysis for selecting MOSFETS.
So what is a quick rule of thumb selecting MOSFETs? Am I reading this datasheet right? I would need just a little more scientific method than ordering and building stuff and then touching the MOSFETs to check if they are too hot...
Thank you for your answers, it seems that I "complected" the breakdown voltage with the drain-to-source voltage. I was thinking that the more voltage I use as VCC to drive the load, the less current the MOSFET can survive. But now, that you have explained that VDS is the drain-to-source voltage drop, suddenly a lot of things make sense...
So, to check if I understand it now, lets assume that I want to drive a single MOSFET NTMFS4935NT1G to source 40A. Because I am a cheap bastard, I will not use a gate driver, but connect the Gate directly (with a current limiting resistor) to an MCU, which can supply 5V/10mA.
It shows that at VGS = 5V, at 40A there is a voltage drop of around 0.15V only. This is clearly in the ohmic region of the MOSFET, so I guess the power dissipated will be Rds(on) * Id^2.
This one shows that the on-resistance with be around 3.7 mOhm, which is very close to the values which are in tables in front of the PDF. 40^2 * 0.0037 = 5.92W, it needs very beefy cooling.
Finally the last graph shows that on 0.15V voltage drop, the maximum current is around 6A DC only. If I want to do 40A DC, then it goes over the Rds(on) limit, which I guess means that there is no way to cool the package so much to reach that. But at 20A it is below the Rds(on) limit, so maybe I can do 20% PWM or similar. (This result seems to be closer to the measured 20A maximum per MOSFET while driving a motor, which is not DC.) <- here I am not sure what does it mean physically to go above the Rds(on) limit...
Is it a correct reading of the datasheet now? (Ignoring that at 10mA, and with Total Gate Charge = 22nC, it will take several microseconds to switch on the transistor, where I think it is in the linear region, heating like crazy.)