# MOSFET Power Calculation

Discussion in 'General Electronics Discussion' started by Irvan, Nov 5, 2017.

1. ### Irvan

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Nov 5, 2017
Hi I have a project. Need help to find MOSFET avg power? How do we find it? In the picture the gate voltage(avg) is 4.5v. The gate current Ig(avg) is 62.8 micA. Just these values provide avg . in 0.0000308wats. Its not the right answer. Any help?

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2. ### Harald KappModeratorModerator

12,449
2,981
Nov 17, 2011
You "poll" was useless, therefore I deleted it.
Your consideration does not take into account the power dissipation from drain current. You need to add Vds*Ids. This term is usually much higher than the term from the gate drive (unless the switching frequency is very high where dynamic losses can exeed static loss).

As you seem to be using LTSPICE:

1. do atransient simulation
2. plot the power dissipation of the MOSFET by pressing the ALT key and left klicking on the M;OSFET. The cursor will change to a thermometer symbol and an expression for the power dissipation will be plotted in the trace window.
3. move the mouse tp the trace window, press the CTRL key and left klick on the expression generated in step 2. Now awindow will pop up showing average power dissipation and integral energy for the time window displayed.

3. ### Irvan

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Nov 5, 2017
Vds = Vsw?

Last edited by a moderator: Nov 5, 2017
4. ### Harald KappModeratorModerator

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Nov 17, 2011
Vds = drain-source voltage. In your case this is Vsw.

5. ### Irvan

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Nov 5, 2017
Can I ask something? What do you mean by dynamic and static losses

6. ### Audioguru

3,616
762
Sep 24, 2016
The IRFS4010 Mosfet you are using is not rated for the 4.5V gate voltage you have, instead it is rated for a 10V gate voltage so some will work poorly and get very hot. You cannot order one that works with a 4.5V gate voltage but some might work but you might not find one. Use a "logic level" Mosfet (they are rated for a 4.5V gate voltage).

Why are you measuring the very small average power of the gate? The dissipation of a Mosfet is its very high drain to source current and the voltage from drain to source when it is turned on.

7. ### Irvan

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Nov 5, 2017
I want to calculate the gate driver average power consumption. How do I do it?

8. ### Audioguru

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762
Sep 24, 2016
The gate draws no DC current (static) because it is a capacitor. Its current is charging and discharging that capacitor (dynamic) so you need to know its capacitance and the speed and frequency it is charged and discharged.

9. ### Irvan

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Nov 5, 2017
I would paste the question:
"What is the average power consumption of the gate driver (in watts)?"
What does it mean?

10. ### Irvan

6
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Nov 5, 2017
I gotta submit i an hour or I am gonna loose A+ `_

11. ### Harald KappModeratorModerator

12,449
2,981
Nov 17, 2011
Your original question was for the MOSFET average power which led us in the wrong direction. The gate driver power can be evaluated by the same methos as stated in my post #2. In the trace window simply remove the part which calculates Vds*Ids from the equation for total power dissipation. Then evaluate the average power.

Static loss is the loss when the transistor is permanently on, no switching occurs. As in this case there is no gate current, static loss is obviously determined by Pstatic = Vds*Ids.
Dynamic loss includes additional losses du eto switching. The gate capacitance needs to be scharged and discharged for the transistor to turn on and off which adds losses (to be supplied by the gate driver), plus there are several other capacitances as shown e.g. here.