True Bridgeless PFC Converter Achieves Over 98% Efficiency, 0.999 Power Factor

Jul 1, 2010 12:00 PM
Dr. Slobodan Cuk, President, TESLAco, Irvine, CA

HYBRID SWITCHING METHOD

The Bridgeless PFC converter ™ has two inductors, PWM inductor, L, and resonant inductor, Lr. As the enclosed analysis shows, the hybrid-switching method results in fundamentally different voltage excitation of the two inductors: PWM inductor L is excited as in PWM converters with a square-wave like voltage so that the inductor is flux-balanced over the entire switching period TS: positive flux during the ON-time interval and negative flux during OFF-time interval. Resonant inductor Lr is excited with a co-sinusoidal ac ripple voltage Δvr of the resonant capacitor Cr and is fully flux-balanced during ON-time interval only.

The large difference in the flux excitations of the inductor L and resonant inductor L is due to resonant inductor being exposed to ac ripple voltage of resonant capacitor during ON-time interval only, while inductor L is exposed to large voltage excursions during both ON-time and OFF-time interval consisting of the linear combination of large dc quantities, such as input and output dc voltages and resonant capacitor dc voltage. Consequently, the resonant inductor is much smaller than PWM inductor, yet its presence plays the crucial role in making single-stage, bridgeless PFC conversion possible.

Hybrid switching is a unique combination of the square-wave (PWM) switching and resonant switching. The PWM inductor exhibits PWM behavior during the whole switching period, while the resonant inductor exhibits resonant behavior during ON-time interval only. Finally, the resonant capacitor in conjunction with resonant inductor exhibits resonant behavior during ON-time interval but PWM behavior in conjunction with PWM inductor during the OFF-time interval. Thus, the resonant capacitor is common thread making such unique operation of two inductors possible, one in resonant mode the other in PWM mode.

Another clear distinguishing characteristic of hybrid switching is that there are three switches as opposed to two or four switches in the conventional PWM and other resonant converters. There is only one controlling active switch and two passive diode switches. Note that this new hybrid-switching method is operating in a completely different manner than the well-known conventional Square-wave Quasi-resonant converters [2], Series and Parallel resonant converters [3] and Converters with Resonant Switches [5] as well as all other presently know resonant methods.

DETAILED ANALYSIS

Here, only a simplified analysis is included which is sufficient to derive dc voltage conversion ratios for either positive or negative DC input voltage. The more comprehensive analysis based on an extension of State-Space Averaging Method [2] can also be employed to result in complete dc steady-state quantities for all the storage components and also the analytical solution for the dynamic (frequency response of the converter) for the purpose of control and regulation and stability analysis.

Clearly, the actual time domain waveforms of the resonant capacitor voltage and resonant inductor current have to be analyzed separately. This is done in a separate later section in which the solution the analytical time domain solution is found so that the derived analytical equations could be used for design. This actually proves that the method of State-Space Averaging is not applicable to resonant converters[2] and that is not limited to PWM converters only due to low ripple approximations (low ripple voltages and ripple currents) but that it has a more general applicability including the dynamic analysis of this new class of resonant converters based on Hybrid Switching Method.

First, we analyze the converter operation with respect to the converter in Figure 4a in which input voltage source is positive dc voltage and having the switch states as in Figure 4b. The linear switched network for ON-time interval is shown in Figure 4c and the linear switched network for OFF-time interval is shown in Figure 4d. To simplify the analysis, we will assume that the inductor L is very large, resulting in a constant input dc current I with negligible ac ripple current.

Continue to next page

Want to use this article? Click here for options!
© 2007 Penton Media, Inc.