Once the PWM IC received the voltage it will output a signal to drive the transistor (or FET) and produces a changing in magnetic field in the transformer primary winding. The changing magnetic field induces voltage in the secondary windings. Each of these AC voltage produced by the secondary windings is then rectified, filtered, and regulated to produce a clean DC voltage. One of the main DC output voltage is the B+ that supply to flyback transformer (for TV and Monitor Circuit)
The output from the B+ voltage supply is then connected, through a “feedback” loop (which consist of optoisolator ic and an error amplifier TL431 IC), back to the PWM IC. When the voltage from the B+ supply rises or drop a bit, the PWM IC will act to correct the output.
If you still do not understand the above explanation, please do not be discourage because you can always buy technical books and schematic diagrams and read it till you get the whole idea of how a SMPS work. You can ask a repair friend or even surf the internet for a better and easy explanation.
Here I would like you to download a free SMPS article by Sencore and I found it to be a great help for you who are still struggle on how SMPS work and how to troubleshoot when it fails. You must ask your self what is the purpose and its function of the components in the SMPS circuit and how to check them if they fail. Find out on your own the function of these components in SMPS circuit:
Bridge rectifier,
Filter capacitor,
Start up resistors
Chopper/Power FET
Pulse Width Modulation (PWM IC)
Current sense resistor
Switch mode power transformer
Optoisolator/optocoupler
Error Amplifier IC (TL431)
Secondary diodes
Secondary filter capacitors
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