Dear satheeshvelu
Hi
consider , that you wanna design a simple forward converter. you will need a triangle wave generator. and then a band gap reference voltage and then some comparators and some compensation networks . and as you said for example uc3844 , it is current mode controller. so , you will need current controller and you will need ...... etc. it is really complicated and large circuit. ok , you can find all of them in the UC3844 . and you can select your desired PWM frequency with Rt and Ct selection according to it's table.
Best Wishes
Goldsmith
In general, a switching regulator includes most of the main components of a complete regulator including the power devices whereas a PWM controller includes only the control elements like an oscillator, reference voltage, error amp, phase control, etc. A PWM controller usually does not include the power output devices.
The PWM controller is the control section of a switching regulator but lacks
the power switch devices. Since these latter are very variable in attributes
by application, the logic level FET control signals are a good place to cut
the chain.