MOSFET gate connected directly to microcontroller pin

[eem2am]

hello,

We have a normally-closed solonoid valve which we are using to hold open a pipe in a domestic water supply pipe.

we are holding this solonoid valve on by PWM'ing a MOSFET in series with the solonoid valve's coil. (obviously we also have a free-wheeling diode)
The supply voltage to the solonoid valve is 36VDC.

SOLONOID VALVE:
**broken link removed**

The MOSFET is as follows:-
**broken link removed**

-the PWM is being supplied by this 5V microcontroller:-
https://www.edaboard.com/newthread.php?do=newthread&f=47

The mosfet has a mximum VGS threshold of 4V so i presume that the 5V microcontroller is sufficient to hold this FET well ON?

Also, the microcontroller output pin is connected directly to the gate of the FET , with no series resistor used.
-Do you not think that this will fry the microcontroller by making it deliver high peak pulses of current into the FET gate when turning the FET on?

-after all, the FET is being PWM'd at 27% at a few KHz.
(the PWM is 40% to start off with, then when the valve opens, the PWM isreduced to 27%)

page 259 of the above datasheet of the microcontroller says each pin can deliver max of 25mA but does not say if short overloads are allowed.

 

[Electro_Nic]

Propably adding a series resistance would help not finding out...
I can't see the microcontroller link you've put /.. however, if the output series impedance of the I/O is very low, the RC constant of the gate will be very fast. And this will pull more current on your I/O.
If you put a series resitance, make sure the RC constant is not effing with your PWM function, but limiting the current to about 20mA.. With a 50Ohm resistor, (2nF of cap at the gate) this gives a 26.6mA peek during CdV/dt transition from 0 to 4V.
So maybe a 75Ohm resistance would be perfect...
I guess the best way is to put a scope on a 1Ohm series resitantce and seeing how much current your MOS needs...

 

[srizbf]

the microcontroller cannot supply the required gate current.

a gate driver is necessary.

 

[FvM]

The linked MOSFET is a 75A, high Vgs threshold type. Seems like you are joking...

With a logic level FET of reasonable output current range however, it would be no problem to drive a kHz PWM from a microprocessor directly. Of course using a gate resistor. The limited driver current turns into a slower switching and somewhat higher losses. For the said application, it's a matter of a few more mW, I guess. But can be calculated exactly, if you like to.

 

[mvs sarma]

while Gate drive capability of port pin is one aspect, we have to consider protection to the microcontroller in case of accident to MosFET (like shot between Gate and drain etc)). under such cases the Microcontroller would be just fried.

 

[FvM]

we have to consider protection to the microcontroller in case of accident to MosFET

A cheap controller board most likely won't be repaired, why protecting one part against failure of another (unless it's a safety issue)?

 

[eem2am]

FvM:
quote
The linked MOSFET is a 75A, high Vgs threshold type. Seems like you are joking...
unquote
....though the datasheet says it has a 4.4V Vgs(th), so the microcontroller's 5V output should be fine to switch it on?

 

[FvM]

so the microcontroller's 5V output should be fine to switch it on?

Not particularly fine, it would still turn it on, with a multiple of the specified Rdson. It's waste of money, energy, achievable switching speed, whatsoever.

 

[Tahmid]

Hi,
It would turn on but it's capabilities would be very limited. The MOSFET can carry nowhere near the full rated power and the RDSon would be much greater than that at full drive at VGS = 8 to 10v.
The MOSFET here would barely turn on without you being able to get full output from it.
Hope this helps.
Tahmid.

 

[bogdanc]

The MOS you are using is overkill for this application. As the valve only draws 6W, the current at 36V is rather small.
I suggest using a lower power transistor that can be directly driven by logic voltage, eg 5V.
One suggestion that comes into mind: IRLML0060trpbf
Datasheet here: http://www.irf.com/product-info/datasheets/data/irlml0060pbf.pdf