True Bridgeless PFC Converter Achieves Over 98% Efficiency, 0.999 Power Factor: Part 2

Aug 1, 2010 12:00 PM
Dr. Slobodan Cuk, President, TESLAco

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Last month, we described a patent-pending Bridgeless PFC converter based on a novel hybrid-switching method that eliminates the full-bridge rectifier altogether. The true single-stage power processing results in many advantages over conventional two-stage PFC converters with front-end full-bridge rectifiers, including efficiency over 98% achievable with appropriate switching devices, 0.999 Power Factor, reduced cost, size and weight, single low cost magnetics, and 100% utilization of all components for either positive or negative half-cycles of input line voltage. The Three-Phase Bridgeless PFC converter extension provides unity power factor operation, low total harmonic distortion, and eliminates the large output DC storage capacitor, yet results in small output ripple voltage. The bridgeless PFC converter of Fig. 1a operates directly of the AC line voltage, since the front-end full-bridge rectifier associated with conventional PFC converters is eliminated.

In Part 1, it was shown that this converter is capable of providing the dc voltage step-up for either polarity of the input voltage. Furthermore, it was also shown that the DC conversion ratio is the same for either polarity of the input voltage and equal to:

Thus, the equality of the step-up dc conversion gains as a function of duty ratio D of the controlling switch S for either polarity (1) is one important pre-requisite for a converter to operate as a Single-Stage AC-DC (Bridgeless) converter with PFC function.

In a clear departure from the previous attempts at bridgeless PFC conversion, all components, all three switches, input inductor L, resonant inductor Lr, and resonant capacitor Cr are 100% utilized as they take part in PFC operation for both positive as well as negative part of input line ac voltage so that there are never any idle components as is the case in conventional PFC converters.

PFC Control Mechanism

The Power Factor Correction is based on forcing the input line current iAC of the converter in Fig. 1b to become proportional and in phase to the input ac line voltage by use of the appropriate control circuit. One circuit, which can be utilized, comprises the conventional PFC IC controller to which current folding and voltage folding signal processing circuits are added externally, as illustrated in the block diagram of Fig. 1b.

The duty ratio modulation is used to control the average input current of the ac-dc switching converter of Fig. 1b; hence the bandwidth of this current feedback loop must be wide enough to force the average input current to be proportional to ac input voltage with minimal or no distortion. This is achieved by use of the switching frequency 1000 time higher than 60Hz, such as 60kHz.

On the other hand, a second voltage control loop should have a low bandwidth (lower than 60Hz) in order not to introduce the distortion in the input current and yet regulate the output voltage. Thus, the output dc voltage will be regulated although with a low bandwidth and will have a small ripple voltage provided an appropriate large size output storage capacitor C is used. Such a large output capacitor is needed in a single-phase AC line PFC converter to fill in for the difference in the pulsating input power and constant output DC power. This large output capacitor C requirement is completely removed when the Three-phase Bridgeless PFC converter is applied to a balanced three- phase ac line input.

Alternative PFC Control Methods

The forcing of input line current to be proportional to input line voltage is accomplished by controlling the duty ratio D of the controlling switch S in one of two ways:
1. Duty ratio modulation with constant switching frequency.
2. Constant ON-time and variable OFF time and therefore, variable switching frequency.
The ON-time interval starts at zero level. Since the two rectifiers limit the current flow to only one direction the resonant capacitor discharge interval is effectively limited to exactly one-half of the resonant period, that is:

D_RT_S = Tr/2 (2)
T_r = 1/f_r (3)

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