Pages: [1] 2 3   Go Down
Author Topic: My mosfet is heating, can't understand why  (Read 3429 times)
0 Members and 1 Guest are viewing this topic.
Offline Offline
Newbie
*
Karma: 0
Posts: 32
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Hi guys,

I've created a circuit in order to activate a load (a little refrigerator) via arduino.
The circuit it's very simple :

Arduino digital pin 2, ground, 5V pins are connected to this mosfet module : http://tinkerkit.com/en/Modules/T010020 that is using this mosfet : http://pdf1.alldatasheet.com/datasheet-pdf/view/227583/IRF/IRFS3806PBF.html
The arduino will provide a Gate power of 5 V, that should be necessary to grant a passage of 10 Amps (i will need no more than 4,2 tough)

To the module it's connected also a 15 v DC power supply , that will power the load.
The load is a little refrigerator : 12 V DC required Voltage, 50 Watt of power. (yes i know that it's less than the power supply, but I tried anyway to use that power supply smiley )

The circuit works, the problem is that the mosfet it's overheating.

What do you think it's the problem? I think it could be the fact that im using 15 v instead or 12 v, or maybe could be an inrush of current in switching on the load ?

Thank you in advance.
Logged

Manchester (England England)
Offline Offline
Brattain Member
*****
Karma: 603
Posts: 33402
Solder is electric glue
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset

It could just be that is how hot it gets when switching the load. Using a higher voltage will give you more current, do you know how much? Then take that current, square it and multiply by the value of the on resistance of the FET. What do you get? It could be you need a heat sink.
« Last Edit: September 30, 2012, 05:11:14 pm by Grumpy_Mike » Logged

Offline Offline
Newbie
*
Karma: 0
Posts: 32
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Here's my calculations :

load : 12 V, 50 W -> resistance = 2,88 ohm (looks strange?)
         current at 12 V = 4.16 Amp
                    at 15 V = 5.20 Amp

Mosfet has a 15 mOhm resistance so, Power in the mosfet at 15 V = 0,4 Watt (datasheet say maximum it can dissipate = 70 W).


Tomorrow (it's night in my country now) if I find time I open the refrigerator and try to see if more things are written inside.

(as usual, thanks for your time)
Logged

Miramar Beach, Florida
Offline Offline
Faraday Member
**
Karma: 140
Posts: 5870
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

That TinkerKit page says that module uses an IRF520. Is your module different?

edit: That would make a BIG difference. The mosfet you have on the link above has a Rds = 15.8 mOhm. The IRF520 has a Rds = .27 Ohm = 270 mOhm
« Last Edit: September 29, 2012, 06:01:45 pm by SurferTim » Logged

Offline Offline
Newbie
*
Karma: 0
Posts: 32
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Uhm you right they told it's the 520... but on mine module's fet it's clearly written :
"IRFS3806
 P208J
5G 7I"
Logged

Miramar Beach, Florida
Offline Offline
Faraday Member
**
Karma: 140
Posts: 5870
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Have you measured the voltage across the mosfet when it is on? Maybe it is not on all the way?
Logged

Left Coast, CA (USA)
Offline Offline
Brattain Member
*****
Karma: 361
Posts: 17263
Measurement changes behavior
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

I would never try to use 5 volt gate drive for a standard power mosfet. Either change to a logic level power mosfet or add a needed gate driver circuit to provide a full +10vdc of gate voltage.
Logged

Anaheim CA.
Offline Offline
Faraday Member
**
Karma: 46
Posts: 2865
...
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

I... think that a piece of 2 mm aluminum 100 mm on a side and drilled such that the device is mounted in the center, you might make it a channel (U shape but flat in the center for the Mosfet and a good Mica washer with a copious dollop of elephant snot on either side would work well. The transistor and the PCB (Heat-Sink) in the drawing is WAY too small. I don't remember the supply voltage but if it is under 15  - 18V... I again would recommend a "High Side" switch, a PNP transistor emitter from the +HV supply  a 1K resistor from the PNP base to the to the + HV supply, a 1K resistor from the PNP collector to the gate of the moset and a 1K resistor from the base of the PNP to the collector of the NPN and 22K resistor from the collector of the NPN to ground a 1K resistor from the base of the NPN to the Arduino output... and set the pin hh to turm on the mosfet. I do hope that is understandable,  IT is the esiest way to go but it requires a bread-board of some kind, Or you could remove the Fet from the PCB and use it to wire the new mosfet on the heat-sink which may well be too big But it is better to have too much... Much Better than having too little and if it is too much, they'res a lot of room for other power devices... Too. and there are very inexpensive "High Side" driver IC's that can be had from the 'bay or from Farnel or Arrow... You'll have no problems with either solution and I am partial to the discrete because I have some "Perfect" PCB's for that and other small jobs AND I have All the parts, the driver Ic's are of limited use a handful of small parts have many uses.

Doc
« Last Edit: September 29, 2012, 10:10:56 pm by Docedison » Logged

--> WA7EMS <--
“The solution of every problem is another problem.” -Johann Wolfgang von Goethe
I do answer technical questions PM'd to me with whatever is in my clipboard

Miramar Beach, Florida
Offline Offline
Faraday Member
**
Karma: 140
Posts: 5870
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

It is already on a circuit board with a "signal amp", according to the webpage. It is Arduino pin compatible.
http://tinkerkit.com/en/Modules/T010020

The question is whether it is working right or not. If the voltage across the mosfet is more than 100mv, then I would check the gate-source voltage to insure the "signal amp" is working. I'll guess that is the other IC on the circuit board.

Logged

Anaheim CA.
Offline Offline
Faraday Member
**
Karma: 46
Posts: 2865
...
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Or strip the mosfet from the board and use it to connect to a larger Mosfet or a larger heat-sink (Preferred)... Been there and done that... Just once, My employer was watching.. With a fire extinguisher in his hands as a "Joke"...

Doc
Logged

--> WA7EMS <--
“The solution of every problem is another problem.” -Johann Wolfgang von Goethe
I do answer technical questions PM'd to me with whatever is in my clipboard

Offline Offline
Newbie
*
Karma: 0
Posts: 32
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

So, today i've tried to power the mosfet with a 9V battery. I've taken some measure :
(dV_x stands for difference of voltage at the sides of x)
dV_supply = 14.8 V
dV_load = 14,3 V

It was not heating at the beginning, i let him go some minute and after that he started heating (much less than last time tough).
I guess the problem was an insufficient gate voltage...It's strange that the datasheet of the mosfet i think it's in (reading his name) says it should work.
Logged

Manchester (England England)
Offline Offline
Brattain Member
*****
Karma: 603
Posts: 33402
Solder is electric glue
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset

Quote
It's strange that the datasheet of the mosfet i think it's in (reading his name) says it should work.
What makes you say that?
The data sheet you posted on the first post quite clearly says it requires 10V to fully turn it on, in the Rds(on) line.

You are making the classic beginner mistake of thinking the Vgs(th) threshold of 4.0V is only the point where the FET starts to turn on, not when it is on fully.
Logged

Offline Offline
Newbie
*
Karma: 0
Posts: 32
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Quote
What makes you say that?
The data sheet you posted on the first post quite clearly says it requires 10V to fully turn it on, in the Rds(on) line.

You are making the classic beginner mistake of thinking the Vgs(th) threshold of 4.0V is only the point where the FET starts to turn on, not when it is on fully.

In the RD(on) line the datasheet say you need 10 volt on the gate to let 25 Amps pass through.
I just need 4-5 Amps, so I've looked for fig 1 pag 3, in which one you may see that 5 Volt allows 5 amps passing in without entering in the saturation area, therefore having the little resistance you expect from a conductor.
I did not look the gate threshold voltage, because I've learned (thanks to you smiley ) that doesn't mean the fet it's fully on.

Thank you in advance
Logged

Offline Offline
Edison Member
*
Karma: 116
Posts: 2205
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Quote
I would never try to use 5 volt gate drive for a standard power mosfet.

That will depend on the current you are trying to switch, among others.
Logged

Manchester (England England)
Offline Offline
Brattain Member
*****
Karma: 603
Posts: 33402
Solder is electric glue
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset

Quote
I just need 4-5 Amps, so I've looked for fig 1 pag 3, in which one you may see that 5 Volt allows 5 amps passing in without entering in the saturation area, therefore having the little resistance you expect from a conductor.

No that is wrong.

In that mode your FET is not fully on. It is only with a fully on FET you get minimum heating. The fact that it says you can get 40A at 10V is besides the point, you need that FET to be on, the current will be limited by your load. Yes 5V will allow 5A to flow but what will the resistance be at that gate voltage. It will be high and that is why your FET is overheating.
The 40A at 10V is also mythical in that at 40A you will be burning off more heat than the device can stand. Such things are just spec talk.
So to stop your FET from overheating give the gate 10V, then it will run the coolest it can. However even at that you might still need a heat sink.
Logged

Pages: [1] 2 3   Go Up
Jump to: