In this control scheme, time ratio Ton/T (duty ratio) is varied. This is realized by two different ways called Constant Frequency System and Variable Frequency System as described below:
Constant Frequency System
In this scheme, on-time is varied but chopping frequency f is kept constant. Variation of Ton means adjustment of pulse width, as such this scheme is also called pulse-width-modulation scheme.
Variable Frequency System
In this technique, the chopping frequency f is varied and either (i) on-time Ton is kept constant or (ii) off-time Toff is kept constant. This method of controlling duty ratio is also called Frequency-modulation scheme.
