Turning a MOSFET on and off

[johnerrington]

This topic raised an interesting question; how much resistance should you include in the gate of a MOSFET to protect the arduino output pin?

Dimmer PWM Led using Arduino Nano Every

Hi everyone, I just made this simple led dimmer using Arduino Nano Every and IRF520N mosfets. I tried on breadboard and I never had any error or problem so I decided to make a real pcb as I need to use them in my house. I've just soldered every component on one of them and after a bunch of minutes the led strips (3x 1m 12v white led strip) start flicker and randomly one of them switch off. I share with you my schematic and code (the RTC module on them is used to make them dimming following th…

@Paul_B

the FET has more or less infinite input resistance but nevertheless substantial capacitance which matters when driven with AC (i.e., PWM).

Call it 500pF at Vds = 5V; which has a reactance of 10k for 31kHz fast PWM;

however at at PWM=1:256 we turn on and off (Once) in 125nS during which we need to transfer 2nC - a current of 2/125 = 16mA

IN PRINCIPLE if the voltage change at the output pin from 0V to 5V or 5V to 0V was instantaneous we "should" limit the current to 40mA or less; and

The effective internal resistance of an Arduino pin is something like 45 Ohms. @Paul_B

which would give an instantaneous current of around 100mA.

HOWEVER - taking these values R=50 ohms, C=500pF; if we assume a rise time of 20nS the maximum current would be 70mA; and only exceed 40mA for 20ns.

(I havent been able to find a figure for the rise time of an arduio output pin)

In this case I dont believe a gate resistor would be needed. However it would seem to be good practice to have a gate resistor in the region of 200 ohms.

[DrDiettrich]

@johnerrington

have a gate resistor in the region of 200 ohms.

If it's about protection of the Arduino pin then use a resistor of at least 150 Ohm (< 40 mA @ 5V). If it's about fast switching then select the right MOSFET and eventually add a bipolar driver chip.

[Paul_B]

@johnerrington

Call it 500pF at Vds = 5V; which has a reactance of 10k for 31kHz fast PWM;

Not how I read that graph!

What is interesting, is that it is a (not at all useful) varactor!

[Smajdalf]

There are discussions about this topic for years. Look for example at this topic:

Myth Busters 3 – Myth: “You must have a gate resistor”

I see it all the time, especially on this site, those “in the know” telling people they must always use a gate resistor when driving a MOSFET from an Arduino. The reason they give is that you need to do this to prevent more than 40mA being sourced by the ATmega328. This has always seemed very odd to me. I mean, it's been painfully obvious to me that those “in the know” have never tried to verify this, because in my experience it’s simply just not been true. The really troubling thing is, who st…

In short my opinion:

1. The often quoted absolute maimum 40 mA is for DC current. There is no transient rating provided.
2. The pins are designed to drive capacitive loads - for example inputs of other CMOS circuits are also FET Gates!
3. Often quite heavy capacitive load is driven by Arduino pins and noone complains - piezzo speaker, a lot of inputs with long wires and so on. The interesting thing - people are worried about signal integrity at this case, not well-being of the pin.
4. From this I believe driving a small MOSFET without Gate resistor is perfectly safe. It is comparable to driving a few CMOS inputs. It is TBD what is "small MOSFET".
5. When driving a huge MOSFET its Gate capacitance may overload the pin. However from my (limited) tests and accidents the drivers (of old AVRs) are very rugged and survive a lot of abuse including short circuit to power rails, PWMing large capacitors and so on. If you use a huge MOSFET it is probably because you intend to control a heavy load. It is already stressed because the relatively weak Arduino pin will turn it on/off slowly and so it stays in the linear region for long time. Adding a Gate resistor makes this even worse. While damage of the pin driver by the transient is theoretical, damage of the power MOSFET by a slow turn on/off is very real danger. Assuming 50 Ohm Arduino driver resistance adding a "safety" 200 Ohm Gate resistor increases turn on time (and so switching losses) of the MOSFET 5 times!
6. The Gate resistor is not entirely useless - you may actually want to slow down turn on/off of the MOSFET. Most likely to reduce noise (i.e. on power rails). For this there is no easy way to determine the optimal value and it could be quite large. For example in one of my projects I turn on LEDs quickly but use 1M resistor to turn the MOSFET off. The LEDs slowly fade out this way and it is simpler and more smooth than PWM.

[GoForSmoke]

Logic level FETs making 12V PWM didn't get gate resistors and no pins died, but maybe I didn't run them long enough. 220R would be safer and I have plenty 220R's!

Youtube's AddOhms is a member here:

MOSFETs and How to Use Them | AddOhms #11

[DrDiettrich]

@Smajdalf

The pins are designed to drive capacitive loads - for example inputs of other CMOS circuits are also FET Gates!

There is a difference between the capacitance of logic gates and power MOSFETs. Logic transistors also can have higher current limiting On resistance than Arduino outputs.

If it works for some time, under certain conditions, who can know when if will start failing?

[GoForSmoke]

I've been told and shown that AVR pins are much more sturdy than ARM pins.

AVRs are also cheap enough to test to destruction as standalone chips.

[johnerrington]

Thanks for all your replies. .. and the link, which is very interesting.

It seems to boil down to this:

• for "hobby" circuits the gate resistor is not needed - because the energy transferred in turning the FET on & off - even with normal PWM up to 31kHz - is too low to damage the arduino.

• If you are using the arduino to drive a BIG MOSFET with FAST PWM (why would you?) you need to do the design properly, and probably use a driver IC.

@Smajdalf

While damage of the pin driver by the transient is theoretical, damage of the power MOSFET by a slow turn on/off is very real danger. Assuming 50 Ohm Arduino driver resistance adding a "safety" 200 Ohm Gate resistor increases turn on time (and so switching losses) of the MOSFET 5 times!

@DrDiettrich

If it works for some time, under certain conditions, who can know when if will start failing?

Alas this is true for all of us!

[DrDiettrich]

@Smajdalf

increases turn on time (and so switching losses)

No such losses with inductive loads like motors or solenoids. Remain pure resistive loads like heating elements or LEDs.

[Smajdalf]

Inductive load has no switch on loss only if it starts at zero current. This won't be true for fast PWM. In either case it is more than compensated by the switch off loss which is much higher than for resistive load.

@DrDiettrich

There is a difference between the capacitance of logic gates and power MOSFETs.

What difference do you have in mind and why do you think it makes safer to drive the logic gate?

[GoForSmoke]

I have MOSFETs with an L in the name. They can switch 10A easily and gate full open by 2V. That's going to take a lot less capacity fillled.

I need to look at an AVR datasheet.... I don't recall having to include 50 Ohm Arduino driver resistance into any Arduino calculations in the past 12 years.