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Topic: [sorted] Tesla member but never used a MOSFET: clarity please? (Read 4679 times) previous topic - next topic

nickgammon

I did a page about driving stuff with MOSFETs: http://www.gammon.com.au/motors.

It shows current sources and sinks, plus calculations for heat dissipation etc.
Please post technical questions on the forum, not by personal message. Thanks!

More info: http://www.gammon.com.au/electronics

KeithRB

In other words the transfer parameter for bipolars is the unitless Ic/Ib, while the Fet's is Id/Vg, which has the unit of mho or sieman - the reciprocal of ohms.

krupski

Last visit to my preferred supplier I picked up a couple of IRL520N logic level mosfets. I've never used a mosfet before.

If I want to use one connected to an Arduino digital output does it need a current limiting resistor on the base oops I mean gate?
Most everyone will tell you you need a series resistor to absorb the "current spike caused by charging and discharging the gate capacitance (and fighting the Miller effect)".

However, that is completely wrong. The mosfet drivers in the AVR chip are basically voltage controlled resistors (i.e. mosfets!). The way to damage them is to overheat them. But the current spike caused by driving a mosfet gate is SO short in duration that you will never even begin to warm the output drivers, let alone damage them.

Also, a series resistor will slow down the switching time of the mosfet you are controlling your load with and keep IT in the linear region longer, resulting in a higher power dissipation in the mosfet.

What resistor to use? 0 ohms.
Gentlemen may prefer Blondes, but Real Men prefer Redheads!

pito

Quote
However, that is completely wrong.
If you work in electronics for some time you are maybe aware of something called "good engineering practice".

If Atmel claims the max i/o current is 20mA, then the good practice is to limit the current to be less than 20mA under any conditions.

The input capacitance of the fet is 450pF, so you may (can?, able to?) calculate easily what is the max charge/discharge current for a certain speed of a falling/rising signal edge  ;)

Paul__B

The input capacitance of the fet is 450pF, so you may (can?, able to?) calculate easily what is the max charge/discharge current for a certain speed of a falling/rising signal edge  ;)
And further on that, it entirely depends on what you are doing with the FET.

If you are switching it on or off every ten seconds or so, it may well not be critical.  If on the other hand you are performing PWM with it, than it absolutely may be drawing these charge/ discharge currents for a significant proportion of the time.

MarkT

You can overload a pin by vaporizing the metalization like a fuse, not just by
overheating an output transistor.  If the datasheet says absolute maximum 40mA,
that's all you have to go on (without clarification from the manufacturer).

There is also a mechnism of slow degradation by overcurrent in ICs called
electromigration. 

Moral - use those gate resistors.  If you want more current to the MOSFET gate
use a MOSFET driver.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

charliesixpack

It is probable that the designer of the off chip driver included some means of transition rate control making a series resistor unnecessary.  The only reason for considering limiting the transition rate is to prevent spikes above VDD or below GND on inductive transmission lines with capacitive loads at the end.  The waveform at the far end of a transmission line can get ugly.

polymorph

Quote
Actually, it should not be a 10k gate to source, but Arduino output to ground. You do not want a potential divider - however minor - between the Arduino and the FET.
I've said that before, and inexplicably had people jump down my throat for it.
Steve Greenfield AE7HD
Drawing Schematics: tinyurl.com/23mo9pf - tinyurl.com/o97ysyx - https://tinyurl.com/Technote8
Multitasking: forum.arduino.cc/index.php?topic=223286.0
gammon.com.au/blink - gammon.com.au/serial - gammon.com.au/interrupts

krupski

If you work in electronics for some time you are maybe aware of something called "good engineering practice".

If Atmel claims the max i/o current is 20mA, then the good practice is to limit the current to be less than 20mA under any conditions.

The input capacitance of the fet is 450pF, so you may (can?, able to?) calculate easily what is the max charge/discharge current for a certain speed of a falling/rising signal edge  ;)
I think 32 years as an EE counts... sorta....

Good engineering practice also includes NOT using components that are un-necessary (and who's fraction of a cent cost builds up over a large number of widgets).

For that matter, extra resistors also consume board real estate.  Cost and space are reasons not to use them commercially, the simple fact that they are not necessary is the reason not to use them on a hobby board.

I will admit that if you are using an Arduino (or any other MCU) to directly drive the gate of a mosfet AT VERY HIGH SPEEDS (like tens of megahertz) so that the R/C charge and discharge times are a substantial part of the total time, THEN yes you would want to limit the current.  But when you are turning a motor or LED array on and off several times per second to thousands of times per second, the microsecond range overcurrents will NOT heat the output drivers and will NOT hurt them in any way.

Gentlemen may prefer Blondes, but Real Men prefer Redheads!

krupski

You can overload a pin by vaporizing the metalization like a fuse, not just by
overheating an output transistor.  If the datasheet says absolute maximum 40mA,
that's all you have to go on (without clarification from the manufacturer).

There is also a mechnism of slow degradation by overcurrent in ICs called
electromigration.  

Moral - use those gate resistors.  If you want more current to the MOSFET gate
use a MOSFET driver.
Found this link on Mr. Gammon's site: link


Gentlemen may prefer Blondes, but Real Men prefer Redheads!

nickgammon

I've said that before, and inexplicably had people jump down my throat for it.
Are you saying this is wrong?



I've posted that a few times without adverse comment.

(edit) Image updated in view of comments below.
Please post technical questions on the forum, not by personal message. Thanks!

More info: http://www.gammon.com.au/electronics

nickgammon

Found this link on Mr. Gammon's site: link
Ah yes, 12 pages of interesting reading there. :)
Please post technical questions on the forum, not by personal message. Thanks!

More info: http://www.gammon.com.au/electronics

CrossRoads

I have 32 MOSFETs on a board with only the pulldown resistor, driving them from 74hc595. Lack of series resistor does not seem to bother the shift register. No long term data to offer up tho.
http://www.crossroadsfencing.com/BobuinoRev17/

Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

Paul__B

Are you saying this is wrong?
Not "wrong" and generally workable, just not best design.

Best design should be the goal.  (Ignoring the other argument re series resistor.)

polymorph

I'm saying that when I've suggested that the 10k pulldown should be on the Arduino side of the current limiting resistor, MOSFET or BJT, I've had a few long-time members here scorn that suggestion, as if it were a hardship to connect it so.
Steve Greenfield AE7HD
Drawing Schematics: tinyurl.com/23mo9pf - tinyurl.com/o97ysyx - https://tinyurl.com/Technote8
Multitasking: forum.arduino.cc/index.php?topic=223286.0
gammon.com.au/blink - gammon.com.au/serial - gammon.com.au/interrupts

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