stabilizer design issue

[Amin Khorsandi]

hi
we've designed a stabilizer for industrial purposes. the design details are these:
the output voltage is 220V 50Hz. a microcontroller generates a sine wave. the sine wave applied to an op-amp amplifier and then a push pull amplifier. there is a feedback from the output of push pull to the input of the op-amp.
the output of the push pull is connected to a transformer. the output appears on the output of transformer.
everything works well. the sine wave generates and can also supply loads. but the problem is with loads which have capacitance effect. this causes the output to phase shift of output voltage.
I don't know how to fix it. probably I have to change my feedback way or change the signal generation by a pll.
please give me a solution
thanks

 

[Audioguru]

Your circuit description sounds like an old fashioned sinewave analog inverter. It probably wastes as much power making heat as its output power.
A modern sinewave inverter uses Pulse-Width-Modulation (PWM) at a high frequency so that the output devices switch completely on and off (then there is very little heat produced) and the output has a high frequency filter to smooth the pulses into a pure sinewave. A capacitive load causes only a small phase shift. A PLL can correct the phase shift.

You could also rectify and filter the electricity mains into a high enough DC voltage and use it to drive PWM Mosfets so that you do not need a transformer at your output.

 

[Amin Khorsandi]

Your circuit description sounds like an old fashioned sinewave analog inverter. It probably wastes as much power making heat as its output power.
A modern sinewave inverter uses Pulse-Width-Modulation (PWM) at a high frequency so that the output devices switch completely on and off (then there is very little heat produced) and the output has a high frequency filter to smooth the pulses into a pure sinewave. A capacitive load causes only a small phase shift. A PLL can correct the phase shift.

You could also rectify and filter the electricity mains into a high enough DC voltage and use it to drive PWM Mosfets so that you do not need a transformer at your output.

you right.
it consumes lots of energy and makes heat. but the problem is; I'm using a +-24DC supply to my circuit driver. so I have to use a transformer to increase the voltage to a voltage between 200 to 260V AC. so using a transformer is undeniable (even if I use direct 220V ac input).
using switching mosfets sound good ...
would it be ok to increase 220V AC in to 260V then rectify the voltage and make a mosfet or igbt switching circuit?
another problem is to filter it. because the sine wave form is very important.

 

[Audioguru]

If your 220VAC never drops lower then when you rectify and filter it you get 309VDC. Use Mosfets to switch the 309V with a high frequency PWM that is modulated with the 50Hz sinewave. Then filter away the high frequency and the output will be a sinewave that is 218.5V RMS. The filter is very simple because the switching frequency is much higher than 50Hz.
The Mosfets will switch completely on and switch completely off so they are never linear and never get hot.

 

[Amin Khorsandi]

If your 220VAC never drops lower then when you rectify and filter it you get 309VDC. Use Mosfets to switch the 309V with a high frequency PWM that is modulated with the 50Hz sinewave. Then filter away the high frequency and the output will be a sinewave that is 218.5V RMS. The filter is very simple because the switching frequency is much higher than 50Hz.
The Mosfets will switch completely on and switch completely off so they are never linear and never get hot.

thanks
I will do it. bet still have a problem. I want to make output voltage between 200 to 250 volt ac. my input voltage is typically 220v. is it ok to use a transformer to increase the voltage and then rectify it to make 350V dc peak?

 

[Audioguru]

350VDC can be made from rectified and filtered 249VAC. If your input is typically 220VAC then you must make a custom transformer to stepup 220VAC to 249VAC.