is the load parameter commonly used in DCM analyzes and
is the dc load resistance.
From (14), we conclude that the NLC controlled converter
always operates in the CCM if
and always in DCM if
(17)
For any load between these boundary values, the converter
goes from the DCM to the CCM and back to the DCM during
each half of the ac-line cycle.
The input current when the converter operates in the
DCM is given by (12), where the duty ratio is obtained by
solving
(18)
which follows from (6) imposed by the NLC controller and the
expression (12) for the input current in the CCM. A simulation
of the distorted input current waveform is shown in Fig. 3.
Total harmonic distortion (THD) of the input current as a
result of the DCM operation has been found as a function of
the load parameter for several . The considered range
for the conversion ratio is to , which allows
for universal-input operation. The results are shown in Fig. 4,
which can be used to select the converter parameters and/or
to ensure that the current THD is satisfactory. The worst
case THD is obtained for the maximum line voltage (maximum
) and the minimum load current (maximum , minimum
). Significant amounts of THD are obtained only if the load
is reduced much below the CCM/DCM boundary value. If we
consider as the worst case input-line voltage, the THD
is less than 10% for loads down to , which is one
tenth of the load when the converter starts operating
in the DCM during a portion of the ac-line cycle. Therefore,
current distortion in the DCM should not present a problem
in practical designs.