I do not know which is the right section of the forum in which to put this question. Feel free to move it if you think this is not correct.

I want a PWM square wave, for example, with duty cycle 35%.
I don't understand how V(rms) is calculated by the voltage probe.
I've a square wave with Vpeak = 9V and Duty Cycle of 35% .. so if I'm not mistaken Vrms = 0.35*9 = 3.15V.

Am I wrong?
Did I make a mistake when creating the PWM circuit?

If my calculation are right .. I noticed that the correct V(rms) value appears in the "yellow box" of the voltage probe during the simulation with the entry V(dc).

(I'm using NI multisim)


Thanks!

 

The RMS voltage of a waveform is different than the Average. 3.15 Volts is the average. To get the RMS value you have to square the waveform, compute the average or mean of the squared waveform over one period, and then take the square root of the average of the squared waveform. In this case the RMS value is 5.3245 Volts.

\( 9^2\;=\;81.0 \)
\( (81.0)(0.35)\;=\;28.35 \)
\( \sqrt{28.35}\;=\;5.3245 \)

RMS stands for Root Mean Square

 

The RMS voltage of a waveform is different than the Average. 3.15 Volts is the average. To get the RMS value you have to square the waveform, compute the average or mean of the squared waveform over one period, and then take the square root of the average of the squared waveform. In this case the RMS value is 5.3245 Volts.

\( 9^2\;=\;81.0 \)
\( (81.0)(0.35)\;=\;28.35 \)
\( \sqrt{28.35}\;=\;5.3245 \)

RMS stands for Root Mean Square

Thanks for the clarification!

I'll just take advantage of this for another curiosity (if it's not a problem).
If I want to drive the Vgs of a MOSFET with a PWM square wave .. what I'm interested in is the Vrms value .. or the average?

 

Here's another way to look at it:
The PWM average voltage as well as the power is obviously proportional to the PWM duty-cycle.
But since the power is proportional to the square of the voltage, not the average, you calculate the RMS voltage as the square-root of the voltage-squared duty-cycle product, or √(9²*0.35) = √28.35 = 5.3245Vrms.

 

If I want to drive the Vgs of a MOSFET with a PWM square wave .. what I'm interested in is the Vrms value .. or the average?

If you are filtering the PWM to get a DC voltage, then its the Vave value.
You use Vrms when you are interested in the heating effect or power generated by the waveform.

 

If you are filtering the PWM to get a DC voltage, then its the Vave value.
You use Vrms when you are interested in the heating effect or power generated by the waveform.

What if I'm interested to pilot the gate of a Mosfet?
(in this case I think I'm interested in filtering the PWM to get a DC voltage .. so Vaverage)

 

What if I'm interested to pilot the gate of a Mosfet?

What does "pilot" mean in this context?
That's not a common word used when referring to MOSFETs.

(in this case I think I'm interested in filtering the PWM to get a DC voltage .. so Vaverage)

As I previously stated.

 

What does "pilot" mean in this context?

With a microcontroller I regulate the voltage [0 ... 3.3V] output at the GPIO with PWM (gate driver).
(sorry if I have misunderstood)

 

(sorry if I have misunderstood)

Nothing to be sorry about.
I assume it's just a minor translation error.

I believe you meant "control" the gate of the MOSFET.
"Pilot" is usually used to refer to the control of a ship or airplane.