SMD MOSFET for driving medium loads?

Other Hardware General Electronics
1

Whats is your go-to SMD MOSFET for driving medium loads?

I mainly worked with trough hole components so far. For small loads like LED's i generally use a BS170, and for medium and high loads i usually go for something like a IRF3205 which is probably a huge overkill most of the time.

I'm looking for a cheap(ish) SMD replacement these. Most of the time i will want to use these as a on-off switch for low to medium loads, or for PWM. Application examples are:

  • Signaling LED's and small lamps
  • 12/24V DC relays
  • buzzers
  • powering circuits, for example a 2nd Arduino.

Typically i will use them in 5 or 12V circuits, and they will be switched by a 5V arduino (a 3V option would be nice)

So far i found the AO3400A for N channel, and AO3401A for P channel.

  • These are cheap, widely available
  • offers a low (but not to low) Vgs (should work at 3v)
  • a decent Vds max of 30V
  • reasonably low Rds(on) of 32mOhm
  • max drain of 5,7A
  • SOT23 package is small but easily solderable by hand

Are these the best go-to option, or do you prefer others?

For example, it would like to to drive a rather large 200A automotive relay with a coil that uses approx 0,4A at 12v. I would need to add a flyback diode off course, but would a AO3400A be up for controlling such a high inductive load?

2

I'd just do a parametric search for the cheapest device matching my requirements - there are thousands of MOSFETs on the market, changing all the time.

You can get 6-pin dual MOSFETs with one p-channel and one n-channel device - then you only need one device "in-stock" for all these uses.

You need a free-wheel diode to switch an inductive load safely. The diode protects the switch.

3

NMOS: PMV16XN (8A or so, very low RDS, ON), IRLML2502 (5A or so), BSS138 (small signal switching). All these I primarily picked for switching properly at 3.3V VGS.
PMOS: IRLML6401 (3-4A or so); BSS170 (small signal switching).
For details on these MOSFETs refer to the respective data sheets.

4

Be aware that current ratings for MOSFETs are not normally useful. The on-resistance and power dissipation are much more useful.

Usually current maximum is under the assumption of infinite heatsink, ie it is not a practical rating for real-world use, but a theoretical one. If you happen to have water-cooled heatsink it becomes realistic, but that's unlikely!

For a small package you might be limited to, say, 0.1W dissipation, in which case a 50 milliohm device can handle 1.4A, or a 10 milliohm device about 3A. Power = I-squared-R

5

These work well:

AO4419 SOP-8 SMD P-Channel

AO4614 SOP-8 4614 N/P-Channel

AO4484 SOP-8 4484 N-Channel

6

I normally calculate the limits at 250 mW dissipation. I do expect the PCB it's mounted on to help out, especially as I normally place the ground plane on top (also gives much easier connection of the drains - of course no thermal relief there).

7

MarkT:
I'd just do a parametric search for the cheapest device matching my requirements - there are thousands of MOSFETs on the market, changing all the time.

You can get 6-pin dual MOSFETs with one p-channel and one n-channel device - then you only need one device "in-stock" for all these uses.

You need a free-wheel diode to switch an inductive load safely. The diode protects the switch.

Thank you for your reply! The thing is, when doing a parametric search i just have too many options. As i have little experience with SMD MOSFETs it can be hard to chooose.

wvmarle:
NMOS: PMV16XN (8A or so, very low RDS, ON), IRLML2502 (5A or so), BSS138 (small signal switching). All these I primarily picked for switching properly at 3.3V VGS.
PMOS: IRLML6401 (3-4A or so); BSS170 (small signal switching).
For details on these MOSFETs refer to the respective data sheets.

Thanks for your suggestions. The PMV16XN has a ver nice low Rds(on) indeed, but its much more expensive, and has a Vds of only 20V. The other N-fets you mention seem to be more expensive then the dirt cheap AO3400A without offering much better specs. The PMV16XN may be interesting though.

MarkT:
Be aware that current ratings for MOSFETs are not normally useful. The on-resistance and power dissipation are much more useful.

Usually current maximum is under the assumption of infinite heatsink, ie it is not a practical rating for real-world use, but a theoretical one. If you happen to have water-cooled heatsink it becomes realistic, but that's unlikely!

For a small package you might be limited to, say, 0.1W dissipation, in which case a 50 milliohm device can handle 1.4A, or a 10 milliohm device about 3A. Power = I-squared-R

You may have a good point here. For previous trough hole projects is always just grap a overpowered mosfet in TO220, but for these small devices i may need to take dissipation into account.

I'm guessing heat can be a real problem for high frequency PWM application, and i would really need a lower Rds(on) and a fast switching FET.

larryd:
These work well:

AO4419 SOP-8 SMD P-Channel
AO4614 SOP-8 4614 N/P-Channel
AO4484 SOP-8 4484 N-Channel

Those also loook interesting. A little larger package which is not always ideal, but for the P-fet the Rds(on) seems very low. They are not as dirt cheap as the AO3400A however, and it seems like they will not work perfectly on a Vgs of 3V, but 5V seems fine. The larger package seems to allows for more heat dissipation, so this one might be very suitable for higher power applications.

wvmarle:
I normally calculate the limits at 250 mW dissipation. I do expect the PCB it's mounted on to help out, especially as I normally place the ground plane on top (also gives much easier connection of the drains - of course no thermal relief there).

I think you are right, and i need to take the power dissipation into account. If i look at the AO3400A datasheet i only see a maximum rating of approx 1W. Is 1/4th of that a rule of thumb for realistic dissipation?

If so a AO3400A would be able to handle 2.8A which is still plenty for most applications i have in mind, or am i missing something?

All of these suggestions are very useful for me for both current applications and future applications!

8

The thing is, when doing a parametric search i just have too many options.

Define only the parameters that matter to you, sort the results by price. Works for me.

9

superkris:
Thanks for your suggestions. The PMV16XN has a ver nice low Rds(on) indeed, but its much more expensive,
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Really? I find them actually very cheap. USD 0.36 a piece (for 50 pcs) on RS Components; down to a mere USD 0.12 in bulk.

The AO3400A is indeed a bit cheaper even - and is also end of life. RS doesn't offer it; Digikey has them listed as "not for new designs". The datasheet may list the successor - typically better specs for lower price.

By the way, in general a p-MOS is more expensive than an n-MOS, while the RDS, ON for a p-MOS is usually higher than that of an n-MOS.

If i look at the AO3400A datasheet i only see a maximum rating of approx 1W. Is 1/4th of that a rule of thumb for realistic dissipation?

Acceptable power dissipation depends mostly on the package size/type and the way it's mounted.

For an SOT23 in air without any heat sinking 100 mW is safe. Mounted on a PCB that goes up; attached to large copper pour it rises further, and combined with active cooling (blower or so) you may reach 1W. Nonetheless that's a LOT of power for such a tiny package.

10

wvmarle:
Really? I find them actually very cheap. USD 0.36 a piece (for 50 pcs) on RS Components; down to a mere USD 0.12 in bulk.

The AO3400A is indeed a bit cheaper even - and is also end of life. RS doesn't offer it; Digikey has them listed as "not for new designs". The datasheet may list the successor - typically better specs for lower price.

By the way, in general a p-MOS is more expensive than an n-MOS, while the RDS, ON for a p-MOS is usually higher than that of an n-MOS.

0,36 $ is sure is cheap, but its easy to buy the AO3400A at prices below $ 0,02 so there is quite a bit of price gap there.

https://nl.aliexpress.com/wholesale?catId=0&initiative_id=AS_20200211041510&SearchText=AO3400A

11

That first site (lcsc.com) lists the PMV16XN at just USD 0.11 in small quantity. OK it's a bit more but still dirt cheap.I don't think you're going to buy thousands of them!
I bought mine at USD 0.036, by the way. Don't know how they get it so cheap. Hope it's not a mainland-made clone.