# Do AC inverter H bridges ever get driven this way?

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: -

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!"
• 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!) Sep 20, 2014 at 17:32
• 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
– user16222
Sep 20, 2014 at 17:53
• @JonRB - do you have a link to it maybe? Sep 20, 2014 at 18:26
• 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
– user16222
Sep 20, 2014 at 18:42
• 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. Sep 21, 2014 at 11:34