Tuesday, June 23, 2015

Prototype Friendly SMD Logic-Level MOSFETs

Surface Mount

Recently I received several request for recommendations for good logic-level MOSFETs to use for prototyping. I hit the search engine at Digikey and looked for both N-FET and P-FET transistors in 8-SOIC packages without a thermal pad. This package is friendly to hand-soldering, allowing you to solder wires directly to the pins and mount it "dead-bug" style if needed for quick prototyping.

These transistor look like a good bang for the buck at a little over $1 each in quantity 10. 

N-FET, 20V, 27A, Rds = 2.45 mOhm (Vgs=4.5V), $10.36 for 10.

P-FET, 30V, 20A, Rds = 4.6mOhm (Vgs=10V),  $10.91 for 10.

These should have plenty of performance margin for applications of a few amps (watch your wire size and current rating). Once you get above a couple of amps your wiring and PCB layout become more critical but these devices can handle the current. A couple of details about these transistors. 

The N-FET is a great part for low voltage systems with an Rds of 2.75 mOhms at Vgs=2.5V! So this would work well the Raspberry Pi's 3.3V logic (or your favorite low voltage microcontroller). But be sure to keep the gate voltage below the 12V limit (not a problem when you are driving it from a logic output).

The P-FET is a 30V part because the 20V parts had a maximum gate voltage rating of 12V which would necessitate a Zener diode to protect the gate if used for high-side switching in many "12V" systems. Many "12V" battery based applications operate at up to 14V and automotive "12V" inputs are usually specified up to 16V. This P-FET can tolerate a Vgs up to 20 volts so should be fine for high-side switching of "12V" circuits.

Remember to shop around for inventory and price if you are going to be placing a large order. I love the search engine at Digikey.com but they do not always have the best price on every part. My second stop when shopping for project parts is usually Mouser.com. I need more practice with the search engine at Mouser but I am getting better.


Someone requested similar options for through-hole logic-level MOSFETs for use with breadboards so I went back to the Digikey search engine and found these parts.

This was the lowest Rds (100 mOhm at Fgs=5V) N-FET in a 4-pin DIP package ($1 for 1). At the rated 2.5 amps it would be dissipating 0.63 Watts which sounds like a challenge on a breadboard but feasible on a PCB with some extra copper for spreading the heat. 

You can also use TO-220 or other SIP type packages on a breadboard. The Digikey search engine found this TO-251A 30V N-FET with an Rds of 3.2 mOhms at Vgs=4.5V, $0.81 for 1. The power dissipation should be very low until you have well exceeded the current carrying capacity of your breadboard (0.08W at 5 amps).

Super low Rds in through-hole P-FET is much tougher. They get pricey fast. This is the best bang for the buck P-FET I could find, -30V part, Rds = 10 mOhms at Vgs=-5V, $2.90 for 1. (0.25 Watts at 5 amps). 

Generally at TO-220 can dissipate around 1W without a heat-sink in free-air without damage.

I hope this information is helpful with your projects!


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