The Circuit Problem:
Solving the Circuit Problem
The section that's circled is a simple bridge rectifier with a potential divider stage in the form of a 100k pot. It rectifies and sends a sample feedback voltage from the inverter mains output to pin1 of the IC which identifies this feedback and accordingly controls the PWM of the IC and regulates it so that the output from the inverter never exceeds a predetermined limit as set by the 100k preset.
pin1 is the sensing input of the IC which responds and makes the PWMs narrower whenever the fed voltage from the 100k pot exceeds a certain predetermined limit.
Pin1 of IC SG3525/3524 actually forms one of the pinouts of the internal error amplifier opamp. The term error amplifier itself suggests that the opamp is assigned to sense and check a feedback sample voltage (error signal) from the inverter output and correct the output PWM width accordingly. This voltage is sensed with reference to the other pin of the IC (pin16) which is internally fixed at a reference voltage of 5.1V.
In case a rising feedback is detected, the potential at pin1 of the IC which is the sensing input of the error amplifier proportionately goes higher than the other complementing pin16 of the opamp creating a high at the output of the internal error opamp.
This high is utilized internally to modify or slim down the PWM frequency which in turn forces the mosfets to conduct with proportionately lower current, thus correcting the output voltage of the inverter automatically with respect to the feedback signal.
Parts list for the above shown SG 3525 inverter circuit with auto PWM voltage control feature