# Calculation of temperature rise in a mosfet

Discussion in 'General Electronics Chat' started by ashok666, Sep 11, 2015.

1. ### ashok666 Thread Starter New Member

Sep 11, 2015
5
0
I am using a mosfet for switching my heater on/off. The heater is 32V 250W. My mosfet is IRFS3206.
The required temperature that is to be maintained by the heater is 40C. How will I check whether this mosfet is enough or I should change this.?
Please explain me in generic way ..
This is what i did. Rds(on)=2.4mohm. the load current is 16A(P/V). So P=0.6W
Rja=62C/W . Tja=62*0.6=37C . Now considering ambient as 40C. Tj=77C.
Is this method rite?
Thanking you..

Last edited: Sep 11, 2015
2. ### #12 Expert

Nov 30, 2010
18,076
9,680
Start by posting a datasheet of the mosfet, or some way to find it.

Sep 11, 2015
5
0
File size:
442.5 KB
Views:
2
4. ### #12 Expert

Nov 30, 2010
18,076
9,680
Rds on = o.oo3 Ohm worst case.
Load current is 250W/32V = 7.8125 amps
P=7.8125 [squared] x 0.003 = 0.1831 watts on the mosfet
x 62C/W = 11.35 C rise in temperature.

One of us needs a new calculator.

I forgot to square the current.

Last edited: Sep 11, 2015
5. ### ashok666 Thread Starter New Member

Sep 11, 2015
5
0
sorry.. the current calculation was by mistake..
still P=(7.8*7.8)*0.0030 =.18W
On multiplying with 62 it comes to 11.3 C
So 11C rise in temperature rite?

Jan 18, 2008
7,067
1,514
7. ### #12 Expert

Nov 30, 2010
18,076
9,680
OK. Now we both have it right at 11.3 C temperature rise.
You caught my mistake in post #4 after I caught your mistake in post #1.

If this is just a, "switch on" and leave it, we're done. If you're switching this at kilohertz or more, there is much more to consider.

8. ### ashok666 Thread Starter New Member

Sep 11, 2015
5
0
Yeah.. 1 more thing.. how do we check whether 11C rise can be taken by my mosfet??

I would like to know more.. What happens if my switching frequency is in khz?

9. ### #12 Expert

Nov 30, 2010
18,076
9,680
Page 1: Operating Junction and Storage Temperature Range = -55C to +175 C

Then the mosfet spends time between "off" and "on" and that causes more heat. It would take a few large paragraphs to explain the whole thing.

10. ### ashok666 Thread Starter New Member

Sep 11, 2015
5
0
If you can share me some material it would have been good...

11. ### GopherT AAC Fanatic!

Nov 23, 2012
7,983
6,777
So, as the Mosfet is turning off or on, At some point in that switching process the resistance is all values between infinity and 0.003 ohms. That means each time the Mosfet turns on or off, it covers the whole range of: Power = amps * voltage drop. You have to look at how fast that on-to-off transition is (See Datasheet) and how much power is dissipated by the device. The power transferred to the mosfet is a combination of the number of on/off cycles each second and power is transferred to the mosfet at each transition.

The time it takes to switch is a function of...
- how quickly your MOSFET driver switches
- the current driving capabilities of that driver
- the series resistance of the connection between driver and Mosfet gate
- capacitance of the gate (see datasheet)

Last edited: Sep 11, 2015
ashok666, #12 and Roderick Young like this.
12. ### peter taylor Member

Apr 1, 2013
106
5
The gate needs to be driven through a low resistance, 15 ohm, and high voltage, above 12v, in order to quickly charge and discharge the gate capacitance. This will insure the drain-source is switched fully on and fully off as quickly as possible, and spending less time transitioning through the ohmic region.

MOS grows fat on a rolling stone