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There's a google competition going on at the moment called the little box challenge. It's to design a very efficient AC inverter. Basically the inverter is fed a DC voltage of a few hundred volts and the winning design will be chosen by its ability to produce a 2kW (or 2kVA) output in the most electrically efficient manner. There are a few other criteria to be met but that's the basic challenge and the organizers state that an efficiency greater than 95% is a must.

That's a tall order and it got me thinking about it just as an exercise. I've seen plenty of inverter H bridge designs but they all drive PWM to all four MOSFETs meaning there are 4 transistors contributing to switching losses all the time: -

enter image description here

The top diagram is as I normally read about inverter designs but the lower diagram struck me as a means of cutting switching losses by virtually 2.

I've never seen it before so I thought I'd aske here if anyone else had - maybe there's a "problem" that I don't recognize. Anyway, I decided not to enter the competition if anybody wonders why I'm posting this.

EDIT - just to explain how I think it should work - Q1 and Q2 (using PWM) can generate (after filtering) a "smoothed" voltage that can vary between 0V and +V. To produce the first half cycle of a power AC waveform, Q4 turns on (Q3 off) and Q1/Q2 produced the PWM switching waveforms to make a sinewave from 0degrees to 180. For the 2nd half cycle, Q3 turns on (Q4 off) and Q1/Q2 produces an inverted sinewave voltage using the appropriate PWM timings.

Question:

  • Is there a problem that I'm unaware of in this type of design - maybe EMC emissions or "it just won't work stupid!"
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  • \$\begingroup\$ Maybe I am missing something (or you are pulling my peg-leg, because it was International Talk Like A Pirate Day yesterday, arrr, me buckos?-). Won't the load only have power flowing 1/2 the time at PWM frequency, arrr? I can understand that that reduces switching loses, but won't that halve the available power in an unpleasant way, arrr, ye lubber? (Corrected for lack of fitin' lingo, arrrr!) \$\endgroup\$ – gbulmer Sep 20 '14 at 17:32
  • \$\begingroup\$ They do get controlled like that, its benefits are easier seen in a 3phase inverter. I did come across that crazy challenge, the only way I saw of meeting that efficiency was with a resonant converter or other ZCS schemes \$\endgroup\$ – JonRB Sep 20 '14 at 17:53
  • \$\begingroup\$ @JonRB - do you have a link to it maybe? \$\endgroup\$ – Andy aka Sep 20 '14 at 18:26
  • \$\begingroup\$ I do, sort of. Such a scheme was used in an inverter I worked on years ago, we wrote a paper ( ieeexplore.ieee.org/xpl/… ) If I could provide a direct link I would. NOTE the sinus quality isn't as good as you could get \$\endgroup\$ – JonRB Sep 20 '14 at 18:42
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    \$\begingroup\$ My comment doesn't read right. I meant to say, the article Estimating MOSFET switching losses ..., might be a handy reference about switching loses for folks reading your question. There is a more to them than I had understood. Of course, you might have a better reference, and it might disappear too soon. \$\endgroup\$ – gbulmer Sep 21 '14 at 11:34
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Can it be done? Yes

Has it been done? Yes

Will it do as expected? half the switching losses? Yes & if care was taken over the right-leg device selection trading speed for conduction losses then you could further improve the powerCore losses.

Quick model with some REALLY badly optimised output filter & not really tuned, just to prove a point & 100kHz switching freq (10kHz appeared to provide reasonable output but an FFT would be required & varying loads: L,C, rect etc... )

Such a scheme does struggle at zero-crossing so the effect on THd would have to be evaluated and determined if it is an accepted limitation.

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