Suggest logic-level mosfet for 4A switching load

I need to control a motor with PWM that will draw 4A and my TIP110 transistors are only spec'd to 2A. I could switch to TIP120 but if I'm going to buy parts I heard that logic level mosfets are "better" (is this true?). I want to run the PWM frequency very high, like 10kHz or something, and I want to run right off my Arduino pins. Can anyone suggest a part for this?

Totally true.

10 kHz is not an especially high frequency for MOSFET's (but it's getting up there). I like the STP27N3LH5 because it has a low gate charge, is logic-level, has an on-resistance of 30 milliohms or less thus will only dissipate 480mW at 4A.

You'll need a series resistor between the Arduino pins and the gate terminal -- I suggest 220 ohms.

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Thanks a bundle. I'll add it to my digikey cart. A part like that is good to have around.

Since it's my thread and I can hijack it if I want to, I now have a different question about darlington transistors.

TIP120/110 are NPN darlingtons so they are hooked up between the load and ground. I wired an LED (with series resistor) across the collector and emitter so that the LED will be on when the transistor is off, and it will be off when the transistor is on. As you know it's recommended to add a 'flyback' diode to inductive loads. Three things:

  1. The TIP120 schematic shows a diode like this. Does that mean I already have a flyback diode and I can do without an external flyback diode?

  2. If not 1, then will my LEDs this serve that purpose with a 2kohm resistor in series, or should I parallel another normal diode?

  3. If neither 1 or 3, will the LEDs burn out from the high-voltage reverse bias spikes?

  1. The TIP120 schematic shows a diode like this. Does that mean I already have a flyback diode and I can do without an external flyback diode?

The TIP120 schematic shows a diode but going the other way. Your LED is going to have the anode near the collector and the cathode near GND.

A "flyback diode" generally refers to a diode connected across the motor terminals, with anode at the collector and cathode at the other motor terminal.

I don't think an LED will burn out with a 2k resistor in series.

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I drew a picture.

You are right. The TIP120 diode goes the opposite way of my LED.

Also, the TIP120 diode cannot act like a flyback diode because it doesn't connect to the other side of the motor coil.

What kind of diode should I use for a flyback diode? Does it need to be as schottky or really high voltage or anything?

The diode should be at least as high a voltage as your motor supply (with a little extra for headroom). More important is its current rating -- ensure it is rated for about the same as the motor current (it won't carry that much on average, but it doesn't hurt to be safe).

Something like an RL205-TP diode should be OK. It's rated for 600V (I hope that's enough!) and 2A steady state operation so it will handle the 4A peaks of your motor. If you want to be safer and just get a 4A diode then the NUR460 should do the trick.

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