Hallo Harun,

I'm not quite sure whether I understood your post in detail. So, please forgive me, when I do not hit the problem...

You measure temperature T of your application. Duty cycle then is calculated by:

Duty cycle = (SP-T) / PB x 100% + 50%

Heating is done the following:
Measure temperature. Only if measured temperature T is less than SP, heating pulse is applied. So, when T is higher than SP ignore heating, wait for one period doing nothing. But when T is less than SP, calculate duty cycle and apply according pulse for one period.
After this period go to beginning.

This is a very easy algorithm, but results in some ripple. Algorithm can still made easier by using constant duty cycle.

Bye,
Kai