# How to calculate power dissipation for a signal?

#### coinmaster

Joined Dec 24, 2015
502
I want to use SMD mosfets near their dissipation limit without using too many of them in parallel.
I'm trying to understand how to calculate average power dissipation for an AC circuit. Here is a clip from spice. Is the average power dissipation at the center point of the waveform or the top of the waveform?

#### shteii01

Joined Feb 19, 2010
4,647
mosfet datasheet should have a graph for it.
since you can not tell us the mosfet model, that is all the help you are going to get.

#### coinmaster

Joined Dec 24, 2015
502
I didn't ask for mosfet max dissipation specs, I asked how to calculate average power dissipation within a device that is passing a sinusoidal waveform.

#### shteii01

Joined Feb 19, 2010
4,647
I didn't ask for mosfet max dissipation specs, I asked how to calculate average power dissipation within a device that is passing a sinusoidal waveform.
You fail to understand that I am not talking about maximum.

In the datasheet there should be a graph (as in picture) that shows the power curve. Since your are the only one who knows what super secret mosfet you are using, you can take the data point for the x axis, find it on the curve and see what the power dissipation is.

#### DickCappels

Joined Aug 21, 2008
6,170
the answer to your question depends on the type of circuit that produces the sugnal, the nature of the load, and the amount of offset.

#### Picbuster

Joined Dec 2, 2013
990
I want to use SMD mosfets near their dissipation limit without using too many of them in parallel.
I'm trying to understand how to calculate average power dissipation for an AC circuit. Here is a clip from spice. Is the average power dissipation at the center point of the waveform or the top of the waveform?
I want to use SMD mosfets near their dissipation limit without using too many of them in parallel.
I'm trying to understand how to calculate average power dissipation for an AC circuit. Here is a clip from spice. Is the average power dissipation at the center point of the waveform or the top of the waveform?

This is still new to me too, but I recently found an article that might help you. I think you can use a ".meas" command to get average values over a defined time range.

View attachment 154634

http://www.analog.com/en/technical-articles/ltspice-using-meas-and-step-commands-to-calculate-efficiency.html
Power (Sine Wave) = 1/2 * (peak amplitude)^2
This is not enough to do your calculations.
The current is the most important part I^2 x internal resistance at a given gate voltage is a measure for the heat.
The thermal- and electronic resistance are shown in the spec's.
A Fet in saturation 10^-2 Ohm does @ 10Amps 100 x 10^-2 = 1 Watt dissipation
But this is useless information is you don't know the thermal resistance.
That value will allow you to calculate the number of square mm PCB copper needed.

Picbuster