What mosfet do I use to PWM a DC motor

[tstanford]

Wanting to use a propeller chip to PWM some DC Pumps.  Just wondering if there was a good way to determine which mosfet to use.  Prop uses 3.3v logic.

[con-f-use]

Maximum current at what max voltage? Frequency of the pwm? Usually you can use just about everything. With pumps a snubber might be in order.

[Psi]

A switching freq from 100Hz to 1000Hz will work fine (but may produce some sound from the motor windings, usually that's ok)

Since 100-1000Hz isn't fast (almost every switching fet can do that fine) you can pretty much ignore the mosfet on/off switching speed and gate capacitance.
(If the datasheet says high frequency anywhere in the "Intended applications" section you will be fine)

The critical parameters to check are;

- Mosfet max voltage
Aim for at least twice the voltage you intent to switch. For 12V switching i use 60V mosfets etc..

- Mosfet max continuous current
Again, you want a mosfet that can switch much more current than you plan on switching.  In one of my projects i use a 50A fet to switch 8A.
However it's a bit more complex than that, keeping the current well under the mosfet max current is only important to prevent damage, it's not a licence to put that much current through the mosfet.

If a mosfet has a max current of say 50A, it doesn't mean its always ok to push 20A through it. What matters is the temperature that's produced from the wasted voltage across the fet(due to Rsd on).  This limits the current you can push through it WELL before the max current does.

- Mosfet on resistance (Rds on)
You want this low to reduce the amount of heat wasted in the mosfet. The less wasted heat the smaller mosfet/heatsink you can use. Try to get a mosfet around 30mR  (0.03ohms) or lower
Have a look at the graph for 'gate-source voltage' vs 'Rds on' and see what Rds on is at 3.3V, since that's what you're using.

- Gate threshold
This is very important for you because you have 3.3V logic.
If you intend to drive the mosfet from 3.3V logic then you MUST use a mosfet which can turn fully on from a 3.3v logic level signal and which has a Rdson (at that voltage) which is low enough so the fet doesnt overheat.


- Junction to ambient / Junction to case temperature.
This tells you how hot your mosfet will get for X watts being wasted as heat.
eg, a value of 60degC/W means if your mosfet is burning up 1Watt it will get 60deg hotter than ambient.

If you have a mosfet without a heatsink then 'Junction to ambient' is what you want to use.
If you use a heatsink then you have to combine the 'junction to case' figure with the figure from the heatsink you're using to get the overall value.
(Google for that if needed and Dave has a video on it as well)


Assuming you're using a mosfet without a heatsink, like a freestanding TO220, then you first need to check if a mosfet is suitable by calculating how many watts the mosfet is going to be burning off as heat.

To do that you use the Mosfet on resistance (Rds on) and the current you have flowing to work out the power.
P = I^2 * R

So if your motor used 5A and the Rds on for the mosfet was 20mR (at 3.3V gate voltage)
P = 5^2 * 0.02
P = 25 * 0.02
P = 0.5 Watts.

So, if the 'Junction to ambient' was 60deg/W , then 0.5W of power is 30deg increase in temp. Which is fine, assuming 25deg room temp that's only 55degC.

[tstanford]

So I need a Gate Threshold of 3.3V at 40 mA? (Datasheet says I/O pins can source/sink 40 mA).  I'm having a hard time finding one that can have a decent Id on digikey or mouser. 12V upto 1A for the motor, one hasn't been selected yet.  Would I be fine getting 2.8V at a lower current?