[SOLVED] Try to get familiar with modulation strategies in DC/AC inverters

Dear all,

I have first experiences in the design of SMPS and the control of such DC/DC converters. Both current mode control and voltage mode control are familiar to me.

Now, I really want to expand my experience and knowledge and get familiar with DC/AC inverters. Of course, I want to start with simulations before I design my first inverter. I already know different topologies (2 level, 3 level and multilevel) from both my university lectures and different literature. That is why I took the control of such inverters as my next step. I want to start with a simple two level topology (H-bridge) and first want to realize a proper gate signal. If we really think of the easiest modulation method, we just use a high frequency carrier signal (usually a sawtooth) and compare it to a 50 Hz frequency sine wave. The output of the comparator is a pulse pattern whose fundamental wave also has 50 Hz.

Okay, the basic principle is clear. My question is, however, how to obtain the values for my high frequency carrier signal. Let us just assume a sine wave signal with an amplitude of 1 and a frequency of 50 Hz.
My questions are:
(1) What maximum/minimum amplitude does my sawtooth need to be? Would it be +-1 like the sine wave?
(2) Is the phase angle between the sine wave and the sawtooth of any interest? How do I know which value my sawtooth needs to be when the sine wave crosses its zero point?

I would like to simulate that but don't know the answers of my two questions.

If you have some good literature about that (Books, Application Notes, Designer's Tipps), please let me know.


Kindly,
al3ko

Really, nobody?

Inverters are a frequent topic at this board. A search will turn up a great many threads.

I have a hunch you are talking about class D operation of the transistors/mosfets in your H-bridge. Is this the case? Because you would be more likely to get replies if you tell which of many inverter topologies you are using.

Dear BradtheRad,

BradtheRad said:

Inverters are a frequent topic at this board. A search will turn up a great many threads.

Click to expand...

I already tried the search but apparently not good enough. I have found many questions about the design of inverters but not the general aspects of control.

BradtheRad said:

I have a hunch you are talking about class D operation of the transistors/mosfets in your H-bridge. Is this the case? Because you would be more likely to get replies if you tell which of many inverter topologies you are using.

Click to expand...

I have nothing special in mind. I'm just thinking of a regular H-Bridge as you can see here
https://www.google.de/imgres?um=1&h...&w=310&h=190&ei=EOsBUJ7oDYbUsgb8_cCqAQ&zoom=1

How do I modulate a sine wave with a high frequency carrier sawtooth?

https://pcbheaven.com/wikipages/PWM_Modulation/

If you look at the last picture in the above link, you see the sawtooth, the sine wave and the pulse pattern. My question basically is if I need to pay attention with the amplitude of my sawtooth and the phase angle between sine wave and sawtooth.

I hope my misunderstanding is clearer now.

(1) What maximum/minimum amplitude does my sawtooth need to be? Would it be +-1 like the sine wave?

Click to expand...

The sine wave should have 75 to 95 percent of the amplitude of your sawtooth/triangle, from what I can tell.

Notice that if they are close in amplitude, you will get a long solid 'On' time around the peaks. This is not necessarily disastrous, however.

What matters is the amplitude when they reach the inputs of the comparator. It may be convenient to use a potentiometer to attenuate one or the other waveform, so that you obtain the proper pulse train.

(2) Is the phase angle between the sine wave and the sawtooth of any interest?

Click to expand...

Phase angle is unimportant, as long as you do not have to be in sync with another waveform for some reason.

How do I know which value my sawtooth needs to be when the sine wave crosses its zero point?

Click to expand...

The waveforms must be symmetrical above and below a centerline value. The centerline will be zero if you use true AC. It will be 6V if you use 12V pulsed DC. It depends on what circuit topology you use to drive your H-bridge.

You want only an AC waveform going to the transformer. Nevertheless component mismatches may turn up somewhere later, which would introduce a DC component at the transformer. So you want to provide the ability to make adjustments in your waveforms.

------------------------------

As a simple demonstration how to combine the sine and triangle waves...

Here's the link to my animated simulation. It will play on your computer.

The falstad.com website will open. Click Okay when asked to allow the connection.

You'll see the moving scope traces showing sine, triangle, and resulting pulse waveform.

You can change values at will. Press ctrl (or alt) key, click on a component, and an edit window comes up.

The frequencies are not necessarily the same as you will use. I made the triangular one an odd value, to show the phase angle has little impact.

https://tinyurl.com/86yqw7h

Dear BradtheRad,

thank you very much for your reply. I think I am worried about unimportant stuff at the moment. I'll start my simulations and play a bit with the values to see their impact. In case I have further questions, I'll come back to that.

Kindly,
al3ko

PS: It is a nice app that you have.

The app was done by Paul Falstad (with help from a book on electronic circuit simulators and human contributors).

I have my own animated simulator that I have been developing. My hope is to make it better than Falstad's, but that will take a while.

Trying to create my own simulator has allowed me to better appreciate what his can do. I often use his as an alternate, and I recommend it as an aid for us to conceptualize what's going through the wires.