Hi guys
Thoroughly enjoyed this very informative thread.
Here are a couple of thoughts I've had. In my professional career, I spent most of my time in development labs, working directly and in parallel with lots of very clever EEs. They, invariably, like me, tended to include a small gate resistor. This thread has got me thinking as to where the gate resistor requirement came from.
Very many of you will never have come across the situation where it was obligatory to attach heat shunts to the legs of transistors, before applying a soldering iron. Yes, when transistors first started to appear in commercial devices, this is what we had to do. Ever hear of the expression "the fastest fuse on three legs"? This is what transistors were commonly called, as they would die, just by looking at them the wrong way. We used to joke that they were only there to protect the main fuse.
In these sort of scenarios, if I was a design engineer, I would use each and every form of insurance I could incorporate. Don't forget that the development of transistors took quite a few years before one could (almost) guarantee many of the stated characteristics and parameters required for the design.
As FETs have always been the 'delicate' flower compared to BJT, due to potential for static damage, I suspect that the need for a gate resistor, might be a "folk memory", passed down by 'generations' of old farts like me, who have never had any need or reason to doubt the accepted wisdom.
Thanks for a great, educational read.
Fof
Hi again,
The original gate resistor requirement came from the oscillation due to there being some inductance in every mosfet source circuit. What we are talking about here though sort of ignores that and we were focusing on what the Arduino pin can handle from a typical Uno pin for example. If the pin is worked too hard the dissipation in the chip could rise too much, and we were also worried about burning out the pin itself due to repeated high currents.
What i found out from the scope pics is first of all that mosfet was acting like a typical mosfet when turning it on. Also, the dissipation comes out to a formula like:
P=K/Tp
where K is some constant based on the driver and Tp is the total period and P is the total power dissipated by the microcontroller chip.
This of course simply means that the frequency is limited because as frequency goes up Tp goes down, and when Tp goes down P goes up. The chip can only handle so much power because of it's surface area, and the local effects are probably more strict because we cant assume that the heat distributes perfectly.
I was thinking maybe a rule of thumb would be, assuming only one pin being used, one-half the power allowed for the whole chip, which i think would then be 0.5 watts because the total power of the chip is about 1 watt max probably at 25 degree C ambient.
For a very rough and crude estimate, this seems to be around 100kHz. That should be proved though by experiment, and of course different mosfets could possibly limit this more. From what i remember from the 1980's, that was a typical limit anyway.
The other interesting thing is that when we add a gate resistor to the Arduino we also slow the mosfet down, which would tend to increase the turn on time. The difference is some of the power would be moved out of the chip then.
With all the variables here it is starting to look like the best way to test for resistor or no resistor is to run it in the application and see if the chip gets hot. If the chip gets hot then either lower the frequency or add a gate resistor. Of course if the efficiency goes down too much it could be a fight to find the right combination.
BTW another application that requires a gate resistor is when connecting mosfets in parallel to obtain a higher current and lower Rds. One gate resistor per device is the common scheme. That is to help with oscillations as well as help to get the mosfets all switching at nearly the same time.