3
\$\begingroup\$

I'm currently trying to find an N-channel MOSFET to act as a switch to be controlled by my Arduino, and I've come across two particular MOSFETS so far that seem like they would be a good fit to the application I am trying to do.

The first one is this one: https://components101.com/mosfets/irfz44n-datasheet-pinout-features
The second one is this one: https://www.infineon.com/cms/en/product/power/mosfet/12v-300v-n-channel-power-mosfet/irlz44n/.

For the first MOSFET, in that link it says that the MOSFET will turn on starting at 4V and won't fully turn on until it exceeds 10V, so its recommended to use the second MOSFET I've linked. I'm kinda new to this so I was wondering how do I know that the first MOSFET requires 10V to fully turn on? I looked at the datasheet and while I don't understand most of it, I did see the section labelled as Vgs(TH). The min voltage stated is 2V while the max voltage stated is 4V. Does the 2V not mean anything then? Why does the datasheet state 4V is that max Vgs(TH)? Looking at the absolute maximum ratings, it seems that Vgs should get nowhere near 20V, so how do I know that at 10V it will start to be fully on to use as a switch?

In the link to the first MOSFET, it also actually recommends using the second MOSFET I've linked if I want to use a microcontroller to control the transistor. They've come to that conclusion because the second MOSFET can be fully turned on using the 5V from an Arduino.

\$\endgroup\$
2
  • 1
    \$\begingroup\$ Look for a graph or similar which shows what kind of Id drain current you can get at a given Vgs. Typically you need to be well above the worst-case threshold voltage to get a guaranteed ability to pass substantial current. There are lots of very good FET's available today, though if you refuse surface mount packages your choices will be more limited... \$\endgroup\$ Oct 26, 2020 at 1:46
  • 9
    \$\begingroup\$ I usually look for the lowest Vgs at which Rds is specified. Usually, that is the the lowest voltage I would consider using to drive the gate. For example, the IRFZ44N specified Rds(on) at 10V only. That is a red flag if you are trying to drive it from 5V. The IRLZ44N specifies Rds(on) all the way down to 4V. This is not a 100 percent guarantee of anything, but I generally avoid using MOSFET's for power switching if they don't have an Rds(on) specified for the gate voltage I am using. Vgs(th) is only vaguely helpful. \$\endgroup\$
    – mkeith
    Oct 26, 2020 at 3:50

1 Answer 1

5
\$\begingroup\$

In this specific case the way you can tell that the second one is a better choice is that Rds(on) is specified down to 4V, but in the first one, Rds(on) is specified only at 10V.

In power switching applications I usually try to stick to MOSFET's that have Rds(on) specified at the Vgs I will be using (or lower).

Vgs(th) is only marginally useful in figuring out power switching applications. The reason is that Vgs(th) is usually defined as the gate-source voltage when drain current = 250uA or 1mA or some other low current. In order to switch a high current with low Rds, the voltage will need to be much higher than Vgs(th).

Vgs(th) may be a useful parameter in cases where the drain current you plan to switch is very low, comparable to the drain current used in defining and measuring Vgs(th).

In cases where Rds is not specified at your voltage, you may be able to figure it out or at least constrain the possibilities by studying the curves in the datasheet. But it is easier to find another FET that has guaranteed Rds at or below the voltage you plan to use.

\$\endgroup\$
2
  • \$\begingroup\$ I see, thank you so much! What would be an application where I would generally care about the stated min and max Vgs(th)? I'm not sure what is meant by power switching applications. \$\endgroup\$ Oct 26, 2020 at 16:18
  • 1
    \$\begingroup\$ Some mosfets such as BSS138 or 2N7002 may be used in logic switching applications. In those cases, Vgs(th) may be the appropriate figure to look at. I guess it is all to do with drain current. When the drain current is close to the value specified in Vgs(th) then you should look at Vgs(th). When the drain current is much higher than the current used for Vgs(th) then you should look elsewhere in the dataseet (or test samples). \$\endgroup\$
    – mkeith
    Oct 26, 2020 at 16:28

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.