CCM, DCM and CrCM in buck-
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This GIF is the switch current and voltage in a forward converter operating in continuous conduction mode. Note that the slope on Id is the inductor current ramping up from an initial value. At lighter loads, the ‘pedestal’ initial value approaches zero. When it hits zero, any lower load causes the converter to operate in discontinuous conduction mode DCM.
In forward converters, the magnetising current of the transformer adds a little to the rising current slope.
This GIF is the inductor current in a forward or buck converter with a diode as the lower switch. In CCM the current is a sawtooth with a DC offset which is the output current.
In CrCM, the valley of the sawtooth just touches zero when the DC offset is lower.
With even lower DC offset (load current) there is a dead time between switch and diode conduction where the current stays at zero. Note that for any load current in CCM or CrCM, the on and off times are constant. In DCM, the on and off times are both shorter, depending on load current. This signifies a fundamental change in control loop response.