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

Current regulations IEC-1000-3-2 impose strict requirements on the harmonic current content, which can be sent back to utility lines for all consumer electronic devices consuming more than 75W of power. To meet that requirement, the active Power Factor Correction (PFC) must be employed. The most common PFC solution, a boost converter, requires the use a full-bridge diode rectifier ahead of the boost converter in the circuit. This design greatly impairs the efficiency, as the two diodes in the bridge rectifier are in the direct power path for either positive or negative half-cycle of input ac line voltage. Hence, at low line ac voltages of 85V, this leads to 3% power loss, which together with power losses of the boost converter reduces overall efficiency to 93% or lower.

Thus, a very clear objective to increase the efficiency is to devise a converter which can eliminate a front-end full-bridge rectifier. But past attempts to develop such a converter failed to eliminate diode-bridge rectifiers, leading to the belief that such converter topology could not exist. Thus, some research publications now erroneously call boost PFC and double boost PFC converters and other variants single-stage PFC converters, simply not counting the front-end bridge rectifier as being always needed in front and considered an unavoidable front-part.

However, this is NOT the case as a new class of converters capable of the direct ac-dc conversion with PFC is indeed possible as introduced for the first time in this article. One member of this new class of single-stage ac-dc PFC converters is displayed in Figure 1a. The other converter topologies of this new single-stage PFC converter will be presented in the future.

BRIDGELESS PFC CONVERTER™

The new bridgeless PFC converter operates directly from the ac line and is the first true single-stage bridgeless ac-dc PFC converter.

To accomplish this objective, the new switching power conversion method, termed hybrid-switching, is needed (see sidebar: What is Power Factor (PF) and Why Low PF is bad). This method also leads to a rather unusual converter topology consisting of three switches only: one controllable switch S and two passive current rectifier switches CR1 and CR2 as seen in Figure 1a, which turn ON and turn OFF in response to the state of the main switch S for either positive or negative polarity of the input ac voltage.

The odd number of switches, three, is a distinctive characteristic of this converter with respect to all conventional switching converters, which always come with an even number of switches, such as 2, 4, 6 etc. This was dictated by the requirement of square-wave switching, often called PWM switching, using both inductive and capacitive energy transfers, which requires that the switches come in complementary pairs: when one ,switch is ON its complementary switch is OFF, and vice versa [1, 2 and 4].

Here, no such complementary switches exist, as one active switch S alone controls both diode switches, whose roles are changed automatically according to the polarity of the ac input voltage. For example, for the positive polarity of the ac input voltage, current rectifier CR1 conducts during the ON-time interval of switch S. Then for negative polarity of ac input voltage, the same current rectifier CR1 conducts during the OFF-time interval of switch S. The current rectifier CR2 also responds automatically to the state of the switch S and polarity of the input ac voltage. For the positive polarity it conducts during OFF-time interval of switch S, and for negative polarity it conducts during the ON-time interval of switch S.

Thus, the three switches operate at all times, for both positive and negative half-cycles of the input ac line voltage. Hence, this true Bridgeless PFC converter™ operates without the full-bridge rectifier since the converter topology itself performs an implied ac line rectification to result in DC output voltage for either polarity of input ac line voltage. Eliminating the full-bridge rectifier directly eliminates large losses especially for the low line of 85V.

As seen in Figure 1a, this topology consists also of an inductor in series with the input, the floating energy transferring capacitor, which act as a resonant capacitor for the part of the switching cycle, and a resonant inductor.

The active switch S on the primary side is modulated and operated at the switching frequency, which is three orders of magnitude higher than the line frequency, such as 50kHz switching frequency compared to a low ac line frequency of 50Hz or 60Hz. Note also that the duty ratio D is defined with respect to ON-time of the controlling switch S and all steady-state quantities, such as DC conversion ratios and DC current of inductor L will be expressed in terms of D.

The full-wave input line voltage and input line currents are sensed and sent as input to the bridgeless PFC IC controller, which in turns modulates the switch S on the primary side, so as to force the input line current to be proportional to the input line voltage to result in ideally desired Unity Power Factor as already illustrated in Figure 1b.

The line current iAC in that case is also sine-wave and is free from high frequency harmonics, thanks to the use of a small high frequency filter Lf, Cf at the ac input. The reality is rather different as the line voltage is quite distorted as described in the last section when an experimental 400W prototype was operated directly from such a distorted line but with unity power factor.

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