Hello Everyone.
I've been toying with the Idea of designing my own VESC power stage, and I've been looking at the different Mosfets people use for them. It seems that sofar, comparing different brands/packages to the "Official" Infinion DirectFETs, the ones that Onsemi makes in the SO-8FL package seem to be extremely good, especially when it comes to Price, and Gate charge characteristics. The Infinions are better in terms of absolute lowest RDSon per transistor, but they come at the cost of 200nc Gate charge, and total cost per transistor.
For example, here's the IRF7759 found in the 75/300 https://www.mouser.ca/ProductDetail/Infineon-IR/IRF7759L2TRPBF?qs=Z8%252BeY1k3TIIb7EiywnKs9w%3D%3D
and here's the NTMFS6H801NT1G that offers a slightly higher 80Vds, but cost's less than 1/3rd even at volume. https://www.mouser.ca/ProductDetail/onsemi/NTMFS6H801NT1G?qs=chTDxNqvsynTk%2FxuRuyatw%3D%3D
You do sacrifice a bit of RDSon (1.8 vs 2.3 mOHM) but the smaller package size, lower cost, and substantially lower gate charge (200 vs 64 nC) means you could just use 4 in parallel instead of 3, have a lower total RDS on (0.600 vs 0.575 mOHM) Take up less space (191 vs 127 mm^2) and have less gate capacitance to drive (600 vs 256 nC) all for less cost (164.16 for 18 vs $61.75 for 25 CAD)
You also get slightly better thermal characteristics because the junction to can thermal impedance is actually allot higher than one might expect with the metal can (1.2 vs 0.9 C/W).
So, does anyone else have any thoughts on other transistors on the market and there potential use in a VESC?
The direct Fet is still better in performance. The reason is the ability to cool the FET directly on its surface. We also used the Onsemi FET in prototypes. It's a good mosfet. However, the heat is dissipated into the PCB and you need to cool the PCB. With direct FETs less heat travels into the PCB itself.
That's what I would've assumed as well, but according to the directFET data sheet, the thermal impedance to the PCB is 0.5 C/W while to the can is 1.2 C/W. Whereas the TO-8FL one states Junction to Case thermal impedance of 0.9. This is also reflected in the Maximum power dissipation of 125W ((175Cj-25Cc)*1.2) for the Infinion vs 166W (175Cj-25Cc)*0.9) for the Onsemi. And that isn't even taking into the account the fact that more chips in parallel means the total thermal resistance decreases even further (0.4 C/Wtot vs 0.225 C/Wtot).
However, that's all just claimed values, and thermal performance is easy to fudge the numbers based on the test setup. Are you saying that you've experienced first hand better thermal performance with DirectFETs vs TO-8FL in the same application?
I've used TOLL packaged 150v 6.3mOhm mosfets in my power stages for couple years now. They are slightly larger compared to SO-8FL but also have even better thermal resistance (0.3C/W)
To take the full advantage of that low thermal resistance, you have to use metal core boards which I did as seen below.
I ran some comparisons with 100/250 recently running both at 100A. Of course this is not apple to apple comparison with 150v mosfets having about 3x more RdsOn but it's compensated a bit a by having 4 mosfets in the group vs 3.
Overall I saw the mosfet temperature climb about the same rate on both controllers at 100A although I didn't have any heatsink on my power stage, so I'd think with a heatsink it would be better.
Lower thermal resistance definitely helps but it's just a part of the equation. You'd want to use metal core PCB for the whole thing to make sense and that will likely affect the cost and the complexity of the design.
As always, there are tradeoffs, you just need to choose what makes most sense in your particular case.
NextGen FOC High voltage 144v/34s, 30kw (https://vesc-project.com/node/1477)