SMPS

Your question is a little too vague and lacking in details for any but the most general of comments.

Regulation of a SMPS can be effected by the use of feedback. The output voltage and/or current (depending on the requirement) are used by a control circuit to determine the appropriate control input.

Depending on the type of SMPS, this will adjust either the pulse width (PWM type), switching frequency (variable-frequency type) or pulse position/timing (PPM type).

Normal control rules apply, which is to say that factors such as loop gain will strongly influence the stability of the regulation.

I hope you find this useful. If you want greater detail, you will need to repost with a few more details of your own.

Thanks for explanation,now I will tell you the exact design details,

I am using Fairchild's power Switch FSDM0565RE,whose switching freequency is 66KHz
my output ratings are 24V,1.2amp.
I started load from 120mA ,240mA,360mA......,1.2A .
I found that op voltage started to decrease after 480mA.
I observed vtg at VFB pin also , it is increasing ,But some how the TON Pulse width is unable to get wide To give sufficient output.

Tell me on which components I have to consentrate.