# Question about PWM inverter output

## Main Question or Discussion Point

So, this is what I know so far, please correct me if I'm wrong.

The amplitude of the output of a PWM inverter depends on how long the sinusoidal modulating signal is greater than the carrier.

If you subtract the output (scaled down) from a reference signal in order to get some error and use this error as the modulating signal then having a reference higher than your measured output will create a larger modulating signal and it will bring you to around equilibrium.... unless there's an overshoot

if at any point, your measured voltage is greater than your reference signal, the error will increase, thus increasing your measured output voltage thus further increasing the error.

I'm pretty sure I must be missing something here, but from what I can see, if your measured voltage is greater than your reference, your error will just increase forever. Where is my logic breaking down?

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anorlunda
Mentor
Are you missing that the error signal can be plus or minus. Your description sounds like any genetic negative feedback controller.

CWatters
Homework Helper
Gold Member
If you subtract the output (scaled down) from a reference signal in order to get some error and use this error as the modulating signal then having a reference higher than your measured output will create a larger modulating signal and it will bring you to around equilibrium....
Try putting some numbers in....

Lets say the reference is 5V and the output 4V.
Subtract the output from the reference and you get 5-4 = 1V which is POSITIVE so the output will be increased. All fine so far..

if at any point, your measured voltage is greater than your reference signal, the error will increase, thus increasing your measured output voltage thus further increasing the error.
No that's incorrect.
Lets say the reference is still 5V but the output is 6V.
Subtract the output from the reference and you get 5-6 = -1V which is NEGATIVE so the output will be reduced not increased.

Modulation Signal = Reference + ( Reference - Measured).

Modulation Signal = Reference + ( Reference - Measured).
What do you recommend reading to better understand controller design specifically for PWM inverters? So far, what I've found online seem to assume the reader to know a lot of details and I would like as in depth a description as I can get.

This is control theory - and it applies directly and cleanly to Power Electronics.

Wiki

Do not over-think it.... when you try to consider it all at once then it is hard to separate the theory ( control) from the PWM concept, then from the code / microcontroller (the tool).

Thanks for the words of encouragement. This is definitely very interesting stuff though pretty hard.

So, I have some more questions I'm hoping someone can answer:

Why would you need a current control loop AND a voltage control loop in a PWM inverter? By controlling the voltage across the output filter (simple LC), aren't you by default controlling the current?

Why wouldn't you be able to just have a voltage output sensor, compare the value it reads with your reference, send that signal as your modulating signal which will change the PWM accordingly?

Or, why couldn't you do the same thing with just a current sensor since controlling the current across the capacitor will automatically be controlling the output voltage?

So it looks like you CAN do what I proposed, as is seen on this paper http://www.ijsr.net/archive/v3i8/MDIwMTU5MjE=.pdf

now, my question changes to, what would the block diagram for this look like, I keep getting that I will need to also know output current but in the picture the only sensor is output voltage.