I'm planning to use this MOSFET as on/off switches for various loads ranging from roughly 100 to 250VDC and 0.1 - 1A. I believe it to be more than adequately sized. I can also add quite a bit of cooling power if needed (although I think even that may be largely unnecessary.) They won't be switching often, they'll completely on or off for several hours at a time between switches.

The drive voltage is listed as: Drive Voltage (Max Rds On, Min Rds On): 10V I'm a bit confused as to how maximum and minimum can be the same value.

Here is the chart that I think is most relevant to my question:

enter image description here

Question 1: The gate doesn't need any specific amount of current, correct?

Question 2: Am I correctly interpreting the plateau in this graph to say that if I use 9V to drive the gate instead of 10V that the FET will operate nearly identically to 10V? I simultaneously think and hope that this is the case.

Question 3: What happens if I apply 12V to the gate? Do I insta-fry it, does it slowly cause damage, is it totally fine?

Bonus: Explain what's going on with both RDS minimum and maximum both being 10V.

The reason I ask is that I can most easily supply 12V or 9V to the gates. Sourcing a perfect 10V is in no way prohibitively difficult, but it would be a bit more of a pain than using 9 or 12, based on what I have on hand. I understand that the truest answer to the title of this post is likely "10V, you dummy" but I'm hoping one of either 9V or 12B will suffice. I don't need perfect precision for my application, I just need to not fry the FETs.

  • \$\begingroup\$ The chart is for a Mosfet that has "typical" specs. The one you buy might have maximum (10V Vgs) specs. \$\endgroup\$
    – Audioguru
    Jun 23, 2022 at 21:20

3 Answers 3

  1. The gate needs enough current to fill its capacitance fast enough to get you out of the linear region before dissipating too much power.

  2. Yes, but I am more likely to look at figure 1 on that same page and conclude that anything over 6.5 V is almost as good as 10 V: enter image description here

  3. Vgs, as specified under Absolute Maximum Ratings, is +/- 30 V, so 12 V would be fine.

Bonus: If you are referring to the parametric listing on Digikey, that is because only the RDS min is listed.

To answer the question in the title: As high a voltage as you have convenient up to 30 V. Maybe a bit less for safety. 12 V would be great.

As for the particular MOSFET for your application, I would be very tempted to find one with a lower RDS. There are many in this range with 1/10th the resistance.

  • 1
    \$\begingroup\$ Thank you so much. If I had asked hours ago I could have avoided speding hours worrying about nothing! As for the hedged way you answered question 1: which datasheet figure can I used to calculate how much current that would be, or, if you're willing to do my homework for me, is something like 50mA likely to be sufficient? \$\endgroup\$ Jun 23, 2022 at 20:13
  • \$\begingroup\$ You want to look at gate resistance and capacitance, then just use the basic RC formula. These are listed under Dynamic Parameters on your datasheet. \$\endgroup\$ Jun 23, 2022 at 20:17
  • \$\begingroup\$ Your biggest problem will likely be total dissipation. Try to find a MOSFET with lower RDS-on, if it's not too late. Otherwise, use a good heatsink! \$\endgroup\$ Jun 23, 2022 at 20:18
  • 1
    \$\begingroup\$ Gotcha. Knowing now that I can use significantly higher voltage, I can easily supply up to 30v or so. I was thinking I needed a step down for each fet! I think I'll be in the clear. Will be heatsinking as well, no big deal. \$\endgroup\$ Jun 23, 2022 at 20:24
  • 2
    \$\begingroup\$ Not 30V or so. The absolute max spec is the value you do not want to exceed or come close to. \$\endgroup\$
    – Kartman
    Jun 24, 2022 at 1:17
  1. Gate current will affect the turn-on and turn-off time. If you're only switching a 1A load infrequently, it's not critical but something to think about. The device will be dissipating more heat inbetween the full-on and full-off regions.
  2. Yes; see also: figs. 1 and 3.
  3. 12V is fine; your VGSmax is +/- 30V.
  4. RDSmin and RDSmax being the same is just an artifact of Digikey's system. On the datasheet, you can see the typical and max values for RDS.

You will need some sort of heatsink since you will be dissipating 2.2W at 1A drain current, which at 65C/W puts you above your max junction temperature at room temperature. Remember that the tab is electrically connected to the drain.

  • \$\begingroup\$ Was very much NOT aware that the tab was live. Thank you for that. I probably would have figured that before calamity struck, but perhaps not. If I get the kind where the tab is made out of the same material as the rest of the case the tab will NOT be live, but I'll get worse dissipation, yes? I want to mount these with simple bolts to a copper pipe heatsink, so not having to insulate each connection seems ideal. \$\endgroup\$ Jun 23, 2022 at 20:20
  • 1
    \$\begingroup\$ I would either get a standalone heatsink that you can easily isolate or a lower RDSon device that doesn't require a heatsink; 1A isn't a big ask. Encapsulated tab packages have worse junction to case Rth (3C/W vs 0.5), and the money you'd spend on an insulating mica spacer and insulated mounting hardware for a non-insulated tab would probably be more than just getting a 100mOhm device. \$\endgroup\$
    – vir
    Jun 23, 2022 at 20:30
  • \$\begingroup\$ Also note that for 250V/1A, the IPC minimum electrical clearance of 3.2mm is greater than the nominal 2.75mm distance between the lead plane and mounting face of a TO220 FULLPAK. Additionally, looking through a few datasheets, I'm not seeing isolation voltages listed so I would be very wary about assuming that you are good to go without any additional precautions. \$\endgroup\$
    – vir
    Jun 23, 2022 at 20:59
  • \$\begingroup\$ I think I'm going to go with an insulated tab, but with much lower RDSon. These won't need to be mounted close to anything in particular, they can be separated in units of several inches rather than several millimeters. My case is (for now) a milk crate. There's plenty of room and airflow to dissipate a couple watts. They'll get bolted to a simple heatsink but that heatsink will be fan cooled with a pretty decent old cpu cooler. They'll be cycled on/off very infrequently. This is what I'm thinking: digikey.com/en/products/detail/infineon-technologies/… \$\endgroup\$ Jun 23, 2022 at 21:22
  • \$\begingroup\$ If anyone is still reading- would it be better to go with a similar fet that had the metal tab and then use heat shrink on it to avoid accidental zaps? Would give me the option to remove the heat shrink later if I decided to make a smaller enclosure with less free airspace. \$\endgroup\$ Jun 24, 2022 at 21:50

The drive voltage parameter is not what manufacturer directly reports, but the Digikey parametric search system. They are not the same value. It's a confusing string that says drive voltage is 10V, but next item in the table is the Rdson(max) itself which is 2.2 ohms at 2A with 10V Vgs which is true.

  1. Gate does not need DC current (there might be leakage of 100 nA which you might ignore). However, to turn FET on or off, you need to push or pull charge in or out the gate to control it. And since it is bad for a FET to spend time in half-on condition where it dissipates most power, the FET must be turned on or off quickly, which means moving the charges quickly, which means momentary currents needed for driving the gate can be surprisingly large. If your maximum resistive load is 250V at 1A, when the FET is halfway turned on, there will be 125V over the FET and 0.5A current passing through it, so at that instant it will dissipating 62.5 watts.

  2. Based on the curve, 9V or 10V does not seem to make a lot of difference.

  3. Datasheet says max Vgs is +/- 30V. Driving it with 12V should be totally fine. All the FET parameters are just measured at 10V, unless otherwise stated.

Bonus) It's a misunderstanding already explained. Besides resistance can't have units of volts


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