Sine wave power inverter

[walkura]

I was reading the posts in this topic and maybe i'm a bit late with my reply ,but i just found this forum .
The flux imbalance problem i saw mentioned before is something i know from smps designing .
Especialy in transistor push pull it can be a nasty subtle failure (with mosfets you have this problem already a lot less but i get back to that) .
The essence of flux imbalance is this you start at the 0 Gauss point ,gate 1 opens mosfet1 for x time transporting an amount of charge of x (m)Joule inducing a magnetic fieldstrength of x Gauss in you're core (depending on the frequency usualy between 1200 to 2000 +-) .
When for some reason there is a difference in
A : timing (Volt-second imbalance) better said (pulse 1 is shorter/longer then pulse 2 (technicaly impossible with 3525 etc. but homebrew program's might have different idea's)
B : while making you're ferrite you wound more on 1 of the windings .
In either of these cases pulse 1 will create a certain amount of Gauss and pulse 2 is not making exactly the same amount magnetic fieldstrength (youre core doenst get as much Gauss down as it went up).
The result is that you're core slowly accumulates that little remainder of fieldstrength after a x amount of pulses it runs into the flat part of the curve the core is so called saturated and according to Faradays law it can't support the magnetic field you're induction (ac resistance) drops to almost 0 Ohm destroying you're transistors .
From what i saw everyone here used mosfets ,in this aspect it suprises me a little you mention fluximbalance .
A normal transistor has a negative temperature coeeficient (this will increase the destruction speed in imbalanced transformers)
Mosfets however are the opposit when you have a imbalance one mosfet gets warmer its resistance increases and amount of joules delivered to the coil(core) go's down .
I hope this explained in normal human words the concept of flux imbalance .
For the people that want the more in depth story i would advice read the book from pressman called switch mode powersupply design .
This book offers in relative understandable language the concept and theory on +-15 topology's (not to mention good chapters on magnetics ,feedback & gatedriving etc. etc.)


Last summer i tryed a pwm sine inverter but lack of time froze the project for now .
Although i did have it working low power it never came out of experimental fase.
You may call me nuts but i did it with a st7lite25 + pushpull stage ,second order chebchev filter.
Next round will be sg3525 (for generation of 500 watt +- 310 volt) sync. rectification ,half bridge pwm stage + lc filter like you would expect in a class D amplifier .
(my last setup worked very well in low powers but what with 500 or 1000 watt ,how to filter?)

Good luck with youre inverters everyone