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The Buck converter with LC input filter operating in
discontinuous output current mode has a high power factor with a constant duty
cycle. A Buck converter in this operation mode can reduce the reverse recovery
loss of the freewheeling diode thus increase the efficiency. The operation,
power factor analysis and modeling of the converter are studied in this paper.
Experimental results are presented to verify the theoretical predictions.
The paper presents detailed comparisons between CRM
(critical conduction mode) and CCM (continuous conduction mode) control schemes
used for Boost PFC converter. The two schemes are analyzed and compared under
the chips of L6561 and UC 3854 which are commonly used for CRM and CCM
respectively. Both schemes are based on multiplier; however, the CCM is more
complex and needs more periphery components which increase the cost. The Boost
PFC under CRM is easier to be implemented. Nevertheless, the variable switch
frequency makes the system (including the power-stage inductor and capacitor)
hard to design. It seems that the CRM PFC is more attractive in low power
applications which only need to meet IEC61000-3-2
D standard. Some experiment results are also presented for the comparison.
By adding a suitable LC filter to the input of a Buck
converter, a high-power-factor buck converter is proposed. The converter can
operate in the discontinuous-output-current mode operation. A Buck converter in
this operation mode features simple control as the constant duty cycle PWM
used. The operation condition of the converter is studied. The validity of
analysis is verified by Simulation and Experimental results.
A novel high-power-factor Buck type converter with
average current control based on UC 3854 is proposed. The input current is
directly controlled by average current control scheme to deliver sinusoidal
input current and to gain a high power factor. The practical results, which
illustrate the proposed control philosophy, were obtained from a 120 W AC/DC
Buck type converter. The power factor can reach 0.97.