Current difference in DCM and CCM

[simbaliya]

It is said that I2R loss is lower in CCM than in DCM because of smaller current swing in CCM.

My puzzle is that although I2R loss is determined by I_rms, and small current swing does not necessarily mean low I_rms, then why for sure the I2R loss is higher in CMM?

 

[atripathi]

I think if you observe the current waveforms for both of these mode, it would be easy to understand and appreciate. In short:

Note that since all of the energy in the inductor discharges in each switching cycle during an interval shorter than the (1 – D) conduction time, the peak current in the diode must be higher in DCM than in CCM. If the peak current is higher in the diode in DCM, then the peak current will be higher in the inductor and in the switch as well. With higher peaks and the same or a shorter conduction time, as a rule of thumb, we can assume that for the same components, the RMS losses will be greater in a DCM converter than in an equivalent CCM converter.

Ref: https://focus.ti.com/download/trng/docs/seminar/Topic_3_Lynch.pdf