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I am trying to select a MOSFET that will work as a logical switch on a 24 V power rail, toggling on/off as signaled by a GPIO on an Arduino. The Arduino pin that will connect to the gate on the FET is 5 V, max current is 40 mA. The power rail current is ~1-2 A.

I'd like the On characteristic to be as close to saturation as possible with very low \$R_{DS}\$. Problem is the FETs I'm finding and I see recommended in similar applications either have:

  • A \$V_{GS}\$ that is low enough to work with the Arduino, but not a high enough \$V_{GSS}\$.
  • High enough \$V_{GSS}\$, but either too high a \$V_{GS}\$, or an \$R_{DS}\$ that is too high given the Arduino output.

As I understand it, although the min or max of the VGS spec may be around 5 V, in order to get the lowest \$R_{DS}\$ spec, you have to run it at much higher than max. One part for example specs min \$V_{GS}\$ as 2.0 V, max at 4.0 V, but the test conditions that result in a 50 mΩ \$R_{DS}\$ are 10 V at 8.4 A, which obviously the Arduino can't do.

Do they just not make FETs with the characteristics I need, or is my approach just wrong?

I am a novice in dealing with transistors, so please correct anything I've written that is incorrect.

Thanks

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    \$\begingroup\$ Do you plan to use the MOSFET as a high-side or low-side switch? In other words, will the MOSFET be between the 24V supply and the load or between the load and ground? \$\endgroup\$ – Dan Laks Jan 3 '18 at 0:22
  • \$\begingroup\$ I was looking at using it on the low side, but I can configure it either way. If for some reason it's advantageous to use it on the high side It's simple enough to do so. \$\endgroup\$ – fiboboxsolid Jan 3 '18 at 15:34
  • \$\begingroup\$ I won't post this as an answer, but for picking transistors I like to use LTSpice's choice when selecting the part - it has a lot of useful information in the parameters, and the exact part number given - so if it works in the simulator, you can just easily then search for it and buy it :) or at least source an alternate with similar characteristics \$\endgroup\$ – DSWG Oct 25 '18 at 0:33
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The main search criteria for your Nch low side switch is 5V Vgs so for low RdsOn est. Vgs>3x Vgs(th) which means Vgs(th) <5V/3 ~ 1.7V max

It is usually called a "logic level" FET drive as opposed to the older styles with Vgs(th) = 2 to 4V threshold and prefer a drive voltage >=9V

Vgss should not be a problem.

If you want a Pch High Side switch then Vgs will end up being 24V and then a different driver is needed such as open collector.

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Vgss is the maximum voltage you are allowed to put on the MOSFET gate (to source). You can put any lower voltage you like on it and the MOSFET should not fail due to excessive Vgs (it might fail for some other reason). +/25V for your example datasheet.

Vgs (at a given maximum Rds(on)) is the voltage you need to put on the gate to have the specified Rds(on) guaranteed at an unrealistic 25 degrees C Tj. That voltage is 10V in your example datasheet.

Vgs(th) is the voltage range at which the MOSFET is more-or-less off. Usually some very small drain current such as the 250uA in your example.


So, assuming a low side switch, you want a MOSFET rated for Vds of more than 24V and with a guaranteed Rds(on) at 5.0 or 4.5V.

If you look in Digikey's parametric search engine you will see a column called:

Drive Voltage (Max Rds On, Min Rds On)

If I select Vds between 30V and 40V (best not to go too high if you want a logic-level input), in-stock, active status and non-bulk packaging, and that are specified at no more than 4.5V, that knocks it down to only 44 pages of parts.

Say your Rds(on) spec is 0.05 ohm and current is 5 to 50A and there's only 856 products left.

Just to pick one inexpensive one more-or-less at random, the PSMN7R5-30YLDX from Nexperia (formerly NXP) is $0.57 one-off and has Rds(on) of 0.0103 ohms maximum at 25C and 4.5V drive (**always check- there are frequently misteaks(sic) in the parameter entry). Stock at only 5,947 might be a bit light in some situations and it's only rated at 30V. Might want to limit voltage and try again.

Again, more or less at random with the 40V limit, we have the Toshiba TPN2R304PL,L1Q $0.90 but an incredible 0.004 ohms at 4.5V drive, 40V and an Id in the 40-100A range depending on which lie you want to believe. 17,000+ in stock.

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  • \$\begingroup\$ Thanks for such a detailed explanation. I'll look at those you suggested and also run a digi search based on the parameters you suggested. \$\endgroup\$ – fiboboxsolid Jan 3 '18 at 15:37

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