MOSET selection confusion - Page 1
Forum section: electronics / beginners

author: Benjam
author_rank: Regular Contributor
timestamp: October 13, 2021, 07:02:24 am
content: The DC bus voltage of my motor is 48V, and the parameters are as follows:Code:Continuous Current: Adc --> 12.00 | Arms --> 10.00Peak Current: Adc --> 40.00 | Arms --> 30.00Rated Cont Power: Watts --> 350Speed at Rated Power: RPM --> 2400Design Voltage(Rated voltage): Vbus --> 48.0 (Vdc) | Vac --> 34(Vrms)Back EMF at Temp: Vpk / kRPM -->18.5 | Vrms/kRPM --> 13.0Back EMF: Vpk / kRPM --> 19.0 | Vrms/kRPM -> 14.0Motor Constant at 25°C: N-m/√watt --> 0.28 | oz-in/√watt --> 39.5Inductance: Lm --> 0.55Then when I was designing the drive, I needed to choose an NMOS. I saw ON Semiconductor, but when I was reading the datasheet, I was confused. For example: the datasheet of this product FDMS86101DC, the first sentence says:MOSFET-N-Channel,POWERTRENCH, DUALCOOL 56 Shielded Gate 100 V, 60 A, 7.5 m(https://www.onsemi.com/pdf/datasheet/fdms86101dc-d.pdf)It seems that it can meet my needs, but there is a data in the datasheet Id = Drain Current − Continuous TA = 25°C Ratings=14.5A. This data makes me very confused. Doesn't it mean 100V, 60A? Why does 14.5A appear again? Can this product meet my needs?

author: Benjam
author_rank: Regular Contributor
timestamp: October 13, 2021, 07:06:03 am
content: Sorry for the picture above, it is wrong, add it again

author: Le_Bassiste
author_rank: Frequent Contributor
timestamp: October 13, 2021, 08:52:50 am
content: Quote from: Benjam on October 13, 2021, 07:02:24 amthe table calls out footnote 1a. MOSFET current ratings come with a defined set of environmental conditions, where that ratings are valid, i.e., ambient temperature, heatsink size, drive voltage, and so on.

author: Jeroen3
author_rank: Super Contributor
timestamp: October 13, 2021, 09:38:20 am
content: 60 Amp is only possible with liquid nitrogen cooling or a thermostreamer, or a very short duty cycle.The mosfet will dissipate arounds 30 Watts running 60 Amps, which it challenging in this small package.Check Figure 11. Forward Bias Safe Operating Area what you can do with this mosfet.Check also the other pages of the datasheet for great thermal examples.

author: exe
author_rank: Unknown
timestamp: October 13, 2021, 04:03:30 pm
content: Ha-ha, marketing.If space allows, you can look into fets in TO-220 package (or any other through-hole part), those are easier to cool down because there are tons of heatsinks.

author: Faranight
author_rank: Unknown
timestamp: October 13, 2021, 07:01:45 pm
content: While selecting a Schottky diode for a project of mine, I've almost learned the hard way thatchartsin a datasheet are more important than the listed rated values. Because those values are normally specified under given test conditions. In my case, I was looking at forward voltage of 0.45V that was specified under current of 100mA, but I thought I could run the diode at 1A at that voltage. Fortunately someone then pointed me at the voltage/current chart in the datasheet that showed my diode would have a forward voltage of 0.8V at 1A current. Bummer. So yeah, be careful when selecting components by the values listed in the table - those are always given at predetermined test conditions.

author: Jeroen3
author_rank: Super Contributor
timestamp: October 14, 2021, 05:58:42 am
content: Quote from: exe on October 13, 2021, 04:03:30 pmThese mosfets have exposed metal op the top for heatsinking which is actually quite niche.Infineon also has some inverted mosfets for the same thing, you can attach a heatsink with a squeezy thermal interface layer, like thick foil.CanPak iirc, and there are the shared packages like TOLT, ixys has those as well.https://www.infineon.com/dgdl/Infineon+-+Product+Brief+-OptiMOS+in+CanPAK.pdf?fileId=db3a30433d346a2d013d443eeb6e4719

author: magic
author_rank: Super Contributor
timestamp: October 14, 2021, 06:39:15 am
content: The important thing is power dissipation, which is I²·R where I is current and R is RDS(on) of the FET.For 12A it's only about 1W. This is probably doable with SMD parts and no heatsinking besides the PCB itself.The 14A spec likely applies to continuous current with the FET soldered to some particular test board, see footnote 1a.The remaining question is: how long are those 30A peaks.