Single phase sine wave inverter with only positive interval, because why not?

[mohannad]

First of all I'm aware that single phase motors need full sine wave to make the pulsating field, so let's assume that i don't include them in my talk.

I'm designing a sine wave inverter with push pull topolgy and I thought: why do we have to design the inverters with the full (+ and -) swing?
Many loads don't care about positive or negative swing, they rectify it anyway  .



Why would I want do that? well.. becuase ,comparing with push-pull, we need only one mosfet to operate the device and we don't need a center tap so we can use the full winding of the primary coil and get double the power we got before (less cost and higher power).

So what stops me from doing that? (I need to power a switched mode power supply , tv ,and a couple of leds and i don't think any of them care about the full swing)

( If my question is too naive, you can move it the the beginner section  . )

[mohannad]

Isn't that how smps forward converter works?

[Zero999]

Yes, transformers don't work at DC.

You could do it by putting AC on the primary and rectifying it back to DC on the secondary. The output voltage can them be changed by varying the duty cycle but then it's not a proper inverter, simply a DC-DC converter with the output voltage changing in a sinusoidal manner.

[Seekonk]

I have a camp and have lots of inverters.  Most were defective when I bought them and I just pulled out the H bridge.  I run most of my consumer electronics on 140V DC. So you don't really need AC for most things.  The exception is when the supply has a voltage doubler.  Many wall warts work on as little as 60V DC from my solar panel buss.  So, you could just make a basic boost converter.  Old inverts are just so cheap it isn't worth bothering searching for an appropriate transformer.

[mohannad]

Of course they don't transfer dc. What I meant is to provide recitified sine pwm to the primary like the wave in the photo, is that still "seen" dc by the transformer?

[mohannad]

I've thought about doing it that way and i think it's a pretty good idea.

[mohannad]

Can I use any of these toplogies with the 50 hz transformer?

[Zero999]

Of course they don't transfer dc. What I meant is to provide recitified sine pwm to the primary like the wave in the photo, is that still "seen" dc by the transformer?

I know what you mean and that will power a variety of appliances but what's the point? You might as well use a DC-DC converter. It's much simpler and easier to implement.

[mohannad]

I guess i'll do, I just wanted see opinions about the idea from experts.

[NiHaoMike]

My senior design project includes a small grid tie inverter, and I made it half wave to keep the control circuits simple. When multiple units are paralleled (e.g. a solar array), half of them can be connected in opposite polarity so the DC cancels out. The problem is that in quantity, it's cheaper to just add the parts to make it full wave than it is to double the size of the capacitors in order to handle the increased ripple current.

[mohannad]

wow! I think it's more complicated than it seems ,especially the paralleling

[David Hess]

As the other posters mentioned, the transformer will saturate because of the DC content of the signal.  Forward converters take special steps to reset the flux between cycles; they are preferred to push-pull converters in lower power applications because they require fewer active devices.  It is possible to air gap a transformer so it does not saturate but this is not economical at high power levels.

So what stops me from doing that? (I need to power a switched mode power supply , tv ,and a couple of leds and i don't think any of them care about the full swing)

Most switching power supplies can operate on 240 to 340 volts DC making the inverter design much simpler but there are safety issues with this.

[ebclr]

Transformer need polarity inversion, otherwise, the core will saturate magnetically

https://en.wikipedia.org/wiki/Saturation_(magnetic)

[mohannad]

As the other posters mentioned, the transformer will saturate because of the DC content of the signal. 

Transformer need polarity inversion, otherwise, the core will saturate magnetically

https://en.wikipedia.org/wiki/Saturation_(magnetic)

Thank you all, I don't know how I missed that. maybe because school taught me more about the eqivalent circuit than the actual physics action inside the transformer.