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picture1

I'm studying about inverter PWM with textbook.

The similar one with attached figure is in the textbook.

I can understand how the blue chopped square voltage is made.

In the text book, the red curve is the average voltage.

I can understand that.

But if a load is connected, doesn't the load really see the 'chopped square voltage with same amplitude', and not the average voltage (or sinusiodal voltage)?

To convert the 'chopped square voltage with same amplitude' to a sinusiodal voltage (or modified square voltage with different amplitude), is it necessary to use a transformer or an LC filter?

Waveform at the MOSFET gates:

Waveform at the MOSFET gates

Waveform at the output of the transformer:

enter image description here

Waveform after proper filtering using inductors and capacitors at the output of the transformer:

Waveform after proper filtering using inductors and capacitors at the output of the transformer:

From Convert a Square Wave Inverter into a Sine Wave Inverter

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  • \$\begingroup\$ The chopped up blue wave from the three level inverter will be fed via LC filtering to produce the red wave. \$\endgroup\$
    – winny
    Commented Jan 28, 2023 at 11:21
  • \$\begingroup\$ @winny What the load actually feels is square voltage with same amplitude, not average voltage in every discrete time interval. Am I right? If the load actually feels average voltage or sinusiodal wave, lc filter will be not necessary. Am I right? \$\endgroup\$
    – user331400
    Commented Jan 28, 2023 at 11:37
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    \$\begingroup\$ @BriL both you're "Am I right" are answered with "it depends on the load / system"; see my answer that addresses exactly that. \$\endgroup\$ Commented Jan 28, 2023 at 11:42
  • \$\begingroup\$ @Marcus Müller Very thanks for your answer. I'm tryng to understand your answer. I'm struggling to understand due to my little electronic wisdom... \$\endgroup\$
    – user331400
    Commented Jan 28, 2023 at 11:47
  • \$\begingroup\$ @BriL That's why I have an example for every case. But: if you don't understand much of my answer's very thin theory, it might be a sign that you need to go back to basics of signals and systems, before you learn about PWM from a textbook (which I'd guess builds upon textbook knowledge from a book about signals & systems); it's very hard to understand any of this if you don't understand spectra / power spectral density (PSD), and if you want to actually design and understand inverters, these are usually control system (closed-loop, most of them), and there's control theory you'll need as well. \$\endgroup\$ Commented Jan 28, 2023 at 11:48

1 Answer 1

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Well, that depends on the load, right?

If your load inherently has low bandwidth compared to the speed of your switching, then that filtering element might not be necessary – for example, there's ICs that are class-D (-E, -G, whatever: PWM amplification) audio amplifiers for low-power low-cost application, where the speakers themselves are sufficiently slow (as in: actual mass inertia and energy stored in a magnetic field!) that the the average is found.

Other applications cannot accept these fast-switching signals at all: If you build a DC/AC converter for a few meters of cable (think: Uninterruptible power supplies), then your dozens-to-hundreds of kHz switching frequency will convert to electromagnetic interference pretty quickly, and the power supplies at the end of these lines have grid filters that are supposed to keep exactly this kind of noise seeping out of the supply back into the net, and will suddenly get very hot. Probably, they would even refuse to start to work.

Yet other applications honestly don't care. You use a switching frequency much faster than the human eye could perceive, neurochemically, and the whole point of your PWM is to modulate the brightness of some LEDs? Well, go right ahead; the averaging is done by the chemical processes that make up light perception.

And there's applications where digital PWM is actually the desired means of information transport – for example, if you have something that feeds a microcontroller with microphone signals from a microphone far away enough to warrant wanting to make the transmission down the mic line more robust against noise than it would be if it was the very small voltages and currents that a microphone itself would cause in the cabling? Compare with high-frequency sawtooth wave, get PWM, send PWM, receive, compare received signal with a rough midpoint voltage: Congratulations, you just eliminated everything but very strong noise from your signal. Reconstruct original signal, if necessary at all, in software.


A comment on your approach to filtering:

Waveform after proper filtering using inductors and capacitors at the output of the transformer:

Waveform after proper filtering using inductors and capacitors at
the output of the transformer:

emphasis mine

You don't say what you want this signal to be. Interestingly, your oscilloscope curves have nothing to do with your sine PWM example from the top of your question, so this is a lot of guesswork, and trying to read your mind, but, I can say with some sincerity:

You did some filtering but it's not the "proper filtering" you claim to and should be doing.

  • Does that look like a sine? not to me! Also, you must have AC-coupled this, as you've actually lost the average of the signal, which would have been positive, and your AC output is zero-average. So, for some applications this is OK, for others, you might have wanted that actual DC offset that you have in your PWM output to persist! So, again, depends, but if you wanted to generate a sine wave from your PWM, then this was not appropriate filtering.
  • If you wanted to produce a rectangular wave of some adjustable amplitude, it's not appropriate filtering, either. I don't see that approximate square wave that your original PWM signal suggests you might have been trying to produce.

So, take a look at the spectrum your PWM gives you, and take a look at the spectrum you want to generate. The job of your filtering – whatever shape it takes, and wherever it is happening, is to remove all components from the PWM spectrum that should not be there in the output spectrum, and weigh the rest as appropriate.

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  • \$\begingroup\$ Very thanks for your answer. But now I'm not related to signal study. I'm studying inverter related to mainly motors. I might understand human sense limit (ex. led light, 120hz monitor) in parts of your answer. But motors will feel voltage with same amplitude, different time interval not sinusiodal voltage(or real time average voltage). So that seems harmful to motors. So transformer, lc filter seem be necessary. \$\endgroup\$
    – user331400
    Commented Jan 28, 2023 at 12:10
  • \$\begingroup\$ motors don't "feel" things. you need to be a bit more precise in your wording, because, what you think is the case is actually not: Motors are large inductive loads with significant angular mass inertia. So, the part of the spectrum that's at your switching frequency should not matter at all for the turning operation of the motor. It might depend very much for thermal losses. Again, it's a bit hard to discuss this if you're not familiar with a tiny bit of signal theory. \$\endgroup\$ Commented Jan 28, 2023 at 12:12
  • \$\begingroup\$ Sorry, I don't know how to delete comment. I wrote to @ .. \$\endgroup\$
    – user331400
    Commented Jan 28, 2023 at 12:17
  • \$\begingroup\$ Ah. . Very thanks for your teaching. I deleted my duplicated comments. Thank you. \$\endgroup\$
    – user331400
    Commented Jan 28, 2023 at 12:26

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