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Below is the SPWM I produced with 50 Hz sine wave and 5 kHz carrier signal. I will use the SPWM signal to drive the MOSFETs in my inverter.

As it is known, duty and period are constantly changing in SPWM. Since the period is constantly changing, I cannot make a precise calculation about the frequency of the signal I produce.

What is my frequency when I produce SPWM? Or in other words what is my switching frequency when I connect this SPWM to the MOSFETs?

enter image description here

Source: My own study

enter image description here Source: My own study

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    \$\begingroup\$ The frecuency of the carrier shown is 500 Hz, that is the switching frecuency. \$\endgroup\$
    – Bravale
    Commented Mar 27 at 15:08
  • \$\begingroup\$ yes i will update it but my real question is why ı can not pick a pulse from the produced spwm and measure the period of it and calculate the swithing frequency ? this is we do in normal pwm. I now that duty is changing since it is spwm but how do ı know the carrier frequency is my switching frequency ? and why ı can not apply the the general calculation to find the frequency on spwm ? @Bravale \$\endgroup\$
    – Mhan
    Commented Mar 27 at 15:22
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    \$\begingroup\$ you can calculate that way, but the average in an output wave period will give you the carrier frecuency. \$\endgroup\$
    – Bravale
    Commented Mar 27 at 15:34
  • \$\begingroup\$ "Since the period is constantly changing..."? Please define 'period'. \$\endgroup\$
    – HarryH
    Commented Mar 27 at 15:52
  • \$\begingroup\$ I mean the width of the varying pulses of spwm is not same for each pulses so the frequency becomes different when i calculate it by general way. @HarryH \$\endgroup\$
    – Mhan
    Commented Mar 27 at 16:01

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You don't need to calculate the frequency, you already know it. 5 kHz, that's the frequency you are putting in.

Frequency is in fact the least important part of the waveform going to your MOSFETs. Rise and fall times, min and max high and low times, are more important to the operation of your power stage than the simple frequency.

The mean switching frequency is one factor that will go into the design of your output filter.

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  • \$\begingroup\$ I want to understand how does the carrier frequency becomes the switching frequency ? what we do generally in normal pwm is to measure the width of one pulse and calculate the frequency but since our duty and period is changing , this doesnt work on spwm, why ? so measuring the width of one pulse of spwm and then calculating the frequency (1/T) doesnt give us the correct answer, why ? @Neil_UK \$\endgroup\$
    – Mhan
    Commented Mar 27 at 15:31
  • \$\begingroup\$ @Mhan I confess I really don't understand how looking at the diagram, seeing the sawtooth 5 kHz wave crossing the sinewave and producing the output, doesn't scream at you that the output frequency is also 5 kHz? I also don't understand how you don't understand that the fact that the sinewave changes the width of the pulses means you then can't use the width of each pulse in a simple way to compute the frequency. 'Frequency' generally means average frequency over a long enough period. In this case, many periods of the modulating sinewave, or exactly one if you can synchronise the measurement. \$\endgroup\$
    – Neil_UK
    Commented Mar 27 at 16:48

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