To be honest I find interesting, that you would use such big mosfet in your design. As far as I understand you are not switching it at high frequency, because it works like a chopper, so gate capacitance and switching losses are less significant. In my designs, where I have to do similar tasks, I usually use multiple smaller fet in parallel. For that 6 dollar Infineon, you could get 8-10 Power SO8 FET, which can switch much higher current, you just have to make sure that you know how to parallel FETs. Not to mention, because you use higher quantities, you get a lot better discounts at the suppliers.
What is the RDSon on those 8 or 10 SO8 also there will be longer copper traces to connect them all and if you look at my replay above that is quite significant part of the power loss.
I use the FET as a switch mostly so I'm interested in the smallest loss so small RDSon.
I did had in my initial design (just prototype) 4 smaller FET that cost almost the same while they where just 40V not 60V
Yes that discount for larger number is a good thing but I do this in low volume anyway so saving even 1$ at 100 units will save me 100$ not even worth the manual placement of all those SO8 Not to mention I have no space for more than 4x SO8 in my design.
Say, you have 40mm space for the FETs, you can put 7 of those SO8 packages there, plus some spacing. IRFH7085 (conveniently selected for you) has about 2,3Ohm Rdson if you drive it with 15V. A pcb trace 20mm wide 40mm long with 35um plating has half a milliohm resistance, lets calculate with half of this, because the current distribution. That is 0,57mOhm DC resistance. But yes, I get that manual placement is not fun.
I see that you used the open hardware symbol. Is there schematic somewhere? This stuff is quite interesting, even that I dont have a solar system.
All the space I have there is at most 30mm for FETs. I looked at your recommended FET
http://www.irf.com/product-info/datasheets/data/irfh7085pbf.pdf that has 2.6mohm typical so say I can somehow fit 6 but probably realistic just 5 is possible to have some space between them.
So 2.6/5 is 0.52 mohm that is not big improvement over 0.66 mohm typical on the IPT007N06N and where is the cost gain that is still over 1$ each in quantities so it will be more expensive than IPT007N06N or at least there will be no gain. Sorry I looked at CAD $ still you need exactly 4 to replace one IPT007N06N that costs $4 US at 500pcs where IRFH7085 cost 0.76 X 4 = 3$ but more parts so you need a larger PCB to fit them that will add to cost also longer PCB trace that will negate the use of 4 or even 5 smaller FETs to replace the IPT007N06N
For the new design I have even less space is just enough to fit two parallel IPT007N06 on the output and of course much higher current 120A.
The entire SBMS100 will be using 6 of those IPT007N06N so if I need to replace with the smaller ones I will need at least 24 of them.
The SBMS 25 and SBMS40 will actually use something similar a smaller FET but that will need to be in a DPak case so that it can fit on the same footprint of the IPT007N06N probably IPD034N06N3 or similar I will decide when I get closer to the components order.
Yes is open source the schematic for the current model is available you can find the link on youtube in the description of the SBMS4080 HW video or in the latest updates on the first kickstarter project just search for Solar BMS on Kickstarter.
Just noticed you are from Romania
Salut.