Single-Stage Isolated Bridgeless PFC Converter Achieves Over 98% Efficiency, 0.999 Power Factor

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

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Another Isolated Bridgeless PFC converter (patents pending) with pulsating input current and based on a novel Hybrid-switching method eliminates the front-end full-bridge rectifier, yet provides isolation in a Single-stage power processing, resulting in performance, size, and cost advantages over conventional Three- stage, bridge-type PFC converters.

The single-stage isolated bridgeless PFC Converter with pulsating input current attains improved efficiency of over 98% (achievable with appropriate switching devices) compared to 89% of the Three-stage approach, offering simultaneously significant size and cost reductions, while maintaining the near unity power factor of 0.999 and 1.7% total harmonic distortion. The single-stage converter uses just a single magnetic part (the isolation transformer) and three switches, compared with compared to four magnetic pieces and 14-switches of the conventional Three-stage approach. This pulsating version requires only a single resonant capacitor and no input inductor, when compared to the Isolated Bridgeless Converter presented in October 2010 issue of Power Electronics Technology.

The new non-isolated Bridgeless PFC converter with pulsating input current of Fig. 1a operates directly of the ac line and without the need for a Full-Bridge rectifier in front. Despite the use of a resonant inductor and a resonant capacitor, thanks to the novel hybrid-switching method, the DC voltage gain depends on the duty ratio only and NOT on resonant component values or the load current. Furthermore, the converter's DC voltage gain is the same for either positive input voltage or negative input voltage. Hence, it provides automatic ac line rectification, without the need for a bridge rectifier.

Although the converter topology resembles that of conventional flyback converter, its voltage gain characteristics in the operating step-up region is entirely different and not the flyback converter type with DC voltage gain of D/ (1-D), but instead of the boost converter type with DC voltage gain 1/(1-D).

The isolated version of the converter is obtained simply by replacing replacing a single inductance L in converter of Fig. 1a with a two-winding isolation transformer with NP:NS turns ratio, as illustrated in the Single-Stage Isolated Bridgeless PFC converter Fig. 1b. The duty ratio control of the power stage with the help of Bridgeless PFC IC Controller and high frequency input filter shown in Fig. 1b results in clean sinusoidal input current drawn in phase with sinusoidal input line voltage and proportional to it, resulting in unity power factor. The measured power factor on experimental 400W prototype was 0.999 for 110V ac, 60Hz line.

BRIDGELESS PFC CONVERTER OPERATION

A very clear objective to increase the efficiency is to devise a converter, which can eliminate a front-end full-bridge rectifier. Past attempts failed to eliminate diode-bridge rectifiers, leading to the belief that the solution to such a goal may be “impossible” and that such converter topology could not exist. This is shown to be no longer the case as the July and August issues of Power Electronics Technology demonstrated a non-isolated True Bridgeless PFC converter and the October issue introduced an Isolated Bridgeless PFC converter based on it. Here, another True Bridgeless PFC converter topology (non-isolated and isolated ones ) is introduced which is even simpler, such as the pulsating input current converter topology of Fig. 1a.

This converter topology comprises only three switches: one controllable switch S and two passive current rectifier switches CR1 and CR2 as seen in Fig. 1a, which turn-ON and turn-OFF in response to the state of the main controlling switch S for either positive or negative polarity of the input ac voltage.

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