Sine wave power inverter

[mincing]

When feed back is not used the out put is pure sinewave.
."

However I could not make out how we do it practically.Do we have to implement it in software or hardware.

I observed lot of noise while the current wave drops tozero and below.NO load voltage waveform is a correct sinewave.

I would also like to understand " I was just trying to understand this concept of flux imbalance control

Kindly tell me how to implement the flux balance control.

regards,

Hi,

I am not an expert on C (uNsUrE of my expertise of PBP aswell ) so may be cannot help much with the code (which has not been posted ) but here are some tips.

According to your schematic:

1. Feedback is from one cycle only (not recommended)
2. Driving through inverters means both MOSFETs doing PWM (wasting energy)
3. Use Flux reset when changing phase
4. Be sure that the computed PWM is not eating up cycles and resulting in uneven spread of the modulating PWM

Please report what happens when no feedback is used. Use a lower duty cycle lookup table to ensure that your MOSFETs are not fried due to full drive. Or use a lower input supply to the Bridge.

[sougata]

Hi,

I am not qualified enough to explain. So I would suggest you to go through these links.

http://techtrain.microchip.com/webse...MPS_090106.pdf

http://www.intersil.com/data/an/AN1246.pdf

In my software I do this while changing phase.

1. Switch off the High Side MOSFET
2. Wait for the dead time.
3. Switch on both low side (flux reset)
4. Switch off both low side
5. Switch on the other High Side
6. Start PWM on the corresponding low side.

If you are not getting sinewave in your output while you apply feedback then its your correction algo that is not working. I cannot release our code here without permission from pramod but there is certainly an example of a Sinewave Inverter using C which uses a delta lookup scheme to make computations easier. Here is the link

http://rapidshare.com/files/43362188/C_Sinewave.rar

Hope this helps.

[mincing]

Dear Sougata,

It was a great help indeed.


C-sinewave has lots of info.

However I would like to know little back ground.Thewre are three tables used ,one for sine wave,one reference sine wave and one error/delta correction.I was not able to understand how he arrive at those tables and the values in the tables.Little more deep in to back ground would probably a great help.

thanks very much,

[tahmid]

Hi all,
I am facing another problem, that is sending the pulses from micro controller to the mosfet driver as the sinusoidal pulses from pic micro controller to the driver is very low and hence it is difficult to drive the ir2120. I could use Tc 427 but it is not available here. Hope to get your suggestions to overcome the problem.
Hope to post the schematics soon. Thanks mincing for your kind suggestion.
Thanks.

[tahmid]

Hi mincing,
What is your progress? Can you apprise me regarding the sine/square wave smps inverters made in India? Do they use microcontroller?
I have solved my problems and tested my circuit and got clean/pure sine wave output at 50khz freq. I have tested 2 types of inverter- single stage smps modified sine wave inverter and dual stage pure sine wave inverter(software pwm-ed). For sine wave generation, I have used 8 pin pic 12F675. Hope to help you and others in this respect.
Thanks.

[mincing]

Hi mincing,
What is your progress? Can you apprise me regarding the sine/square wave smps inverters made in India? Do they use microcontroller?
I have solved my problems and tested my circuit and got clean/pure sine wave output at 50khz freq. I have tested 2 types of inverter- single stage smps modified sine wave inverter and dual stage pure sine wave inverter(software pwm-ed). For sine wave generation, I have used 8 pin pic 12F675. Hope to help you and others in this respect.
Thanks.

They use microc controllers .And most companies that supply sine wave inverters use dsp chips.

However looking forward to your support and cooperation all through.
Thanks!

[mincing]

Hi all,
I am facing another problem, that is sending the pulses from micro controller to the mosfet driver as the sinusoidal pulses from pic micro controller to the driver is very low and hence it is difficult to drive the ir2120. I could use Tc 427 but it is not available here. Hope to get your suggestions to overcome the problem.
Hope to post the schematics soon. Thanks mincing for your kind suggestion.
Thanks.

Hai Tahmid,

You can use simple transistor ampli. ckts for boosting up the signals from pic to suit the levels(say 10V) of IR 2110 and feed.

This will solve the problem of low level signals.
Thanks!

[mincing]

Also I would like to know what is the nominal DC volatge used.in that scheme.

[sougata]

Hi Mincing,

I am sorry I need to put some time and scratch my head to explain the C code. Currently I am a bit involved in some projects (pulling my hair almost always ) But here is a crude explanation.

For a simple proportional control system the PWM for the sinewave for the current pointer is looked up and the previous PWM is used with the feedback to determine the error. Thus before the current PWM is made through it is corrected for errors by proportionally modifying it.

Since for your system the max PWM is 255 (8bits) you cannot scale it up any more so your maximum gain is 1.0. Means your hardware should be able to step up the voltage and provide the current at maximum load, minimum permitted input supply. Now what we found in using the 16F is that the multiplication routines were not fast enough (18F has got a hardware multiply) to be calculated on the fly and still leave room for other tasks such as AC Restore Sense (F/V permit), Batt-low sense (+ Indication), PC communication, Overload (Thermal/Power) and Fault sense. So what we did was implement a pseudo hardware PWM. We used the compare in 16 bits and used the High Byte from the sinetable and lowbyte from the error computation. Now we find that PWM results are unsatisfactory. So we decided for lookups table that contains some precomputed results for current and voltage. This speeded up the whole thing but we lost resolution. However for a domestic inverter it was acceptable as the regulation were within specs. I am sorry I cannot reveal any more details.

So far you have not posted any codes and thus I cannot help you on your code.
We (Me and Pramod) are planning for a dsPIC based inverter for the next season and if that happens as a reality then we may release the old code to public. No promises however at this moment.

Best of Luck

[mincing]

Hope you are fine.

Thios time I have come up with a weird question.What is the meaning of Sine PWM being 25KHz.

Say we consder 32 samples it gives ,20ms/32=625uS.
===>frequency of each sample is only 1.6KHz .

Then ecverybody talks of 25 KHz .Could you please explain that a little.

Thanks,

[sougata]

Hi,

You are using a lookup table to modulate a carrier frequency (The PWM). In analogue world this is AM. But the pronciple here is of a class D amplifier. That is your switches are either off/on and not working in the linear region. You can use any frequency thats twice (nyquist) the output frequency mulitplied by the resolution (steps) of the lookup table. Your output transformer and parallel resonant capacitor forms a filter to integrate the modulated PWM into a sinewave. Normally the PWM is kept higher than 20KHz to reduce audible noise in the transformer. While keeping it lower has a certain advantage of lower switching loss.