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Inverter spwm design and implementation to power mosfets

qwertyqwq

Member
Hi folks.
Im designing a inverter which is the low power for begin. The power mosfets will trigg with spwm signal which is made by compared sine wave and triangle waves.
I did spwm signal by comparing sine and triange waves. Main sine signal is 50Hz and the carrier signal is 22kHz.
The problem is trigging the power mosfets. As you know opamps cant drive power mosfets. There is a pic of spwm attched. At the begin ,the signal is zero and then its rising opamp Vcc voltage and at the very end its going back to zero. Between end and begin its never falls zero.
I cant use mosfet drivers cause of the spwm signal waveform.(after the beginning it never falls to zero voltage), And i cant use AND gate also beacause of the same reason.
Thats it. i tried to tell my problem as much as i can.
Which method should i use ?????
how can i apply this signal to mosfet(mosfets are irf44n)??
is that signal is good ??
should i decrease carrier signal frequancy ?

Thanx in advance ;)


IMG_4034123123123.png
 

qwertyqwq

Member
i forgot to tell im usin icl8038 pulse generator and triangle-sine waves are pretty good. is there any suitable pulse generator than icl8038 which you guys advice to me?
or should i use a microcontroller for creating spwm for inverter project ? which one is more suitable for pure sine wave projects ??
 

rjenkinsgb

Well-Known Member
Most Helpful Member
A PWM signal is "logic level", at whatever voltage is in use - 3.3V, 5V or 12V etc.

If you use a comparator between a high frequency triangle / sawtooth waveform and a low frequency waveform, you get a logic-level signal which is a PWM representation of that low level signal.
(As long as the input does not exceed the amplitude of the sawtooth carrier).

eg. http://www.thebox.myzen.co.uk/Tutorial/PWM_files/shapeimage_1.png

The logic level PWM signal can be used with any general mosfet driver IC.


You can also use an MCU and generate the PWM in software; again, the output will be logic levels.
 

qwertyqwq

Member
A PWM signal is "logic level", at whatever voltage is in use - 3.3V, 5V or 12V etc.

If you use a comparator between a high frequency triangle / sawtooth waveform and a low frequency waveform, you get a logic-level signal which is a PWM representation of that low level signal.
(As long as the input does not exceed the amplitude of the sawtooth carrier).

eg. http://www.thebox.myzen.co.uk/Tutorial/PWM_files/shapeimage_1.png

The logic level PWM signal can be used with any general mosfet driver IC.


You can also use an MCU and generate the PWM in software; again, the output will be logic levels.
first of all thanks for respons !
and secondly i figure it out the reason and i wanted to know anyone who has that problem like me.
the reason why is comparator which in this case LM324. it is pretty slow for this circuit and i replace it to TL074 low noise j-FET opamp. now its working pretty good especially according to the former. ;)
 

rjenkinsgb

Well-Known Member
Most Helpful Member
I'd suggest using a proper voltage comparator rather than an opamp, eg. an LM339 or LM393.
But, if a fast opamp works OK, that's fine.

You could also use a dedicated PWM IC as used in some switched mode power supplies.
 

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