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

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

Find a downloadable version of this story in pdf format at the end of the story.

Until now, it was considered impossible to have an AC-DC converter with PFC and isolation features provided in a single power processing stage and without mandatory full-bridge rectifier. This is not the case anymore. The high performance of the non-isolated Bridgeless PFC converter (described in July 2010 Power Electronics Technology) with 0.999 Power Factor and 1.7%THD harmonic distortion is preserved with the only addition being an appropriately inserted isolation transformer. An Integrated Magnetics extension results in a single magnetic component, 3-switch configuration, compared to 14-switches and four magnetic components of the conventional Three-stage approach. As a result, this Single-stage solution has much improved efficiency of over 98%, compared to 90% of the Three-stage approach and offers simultaneously significant size and cost reductions.

The first limitation is found in the conventional Boost PFC converter shown in Fig. 1a, which can operate only from the rectified ac line as illustrated by the waveforms in Fig. 1b, thus resulting in two-stage power processing. An additional problem is that there is no simple and effective way to introduce the isolation in the conventional Boost converter of Fig. 1a.

As seen in Fig. 2a, one approach is to use a full-bridge extension of the Boost converter to introduce isolation, which is then controlled as PFC converter. Note, however, its complexity consisting of four transistors on the primary side and four diode rectifiers on the secondary side operating at the switching frequency of, for example, 100kHz with additional four diodes of input bridge rectifier operating at the line frequency of 50Hz or 60Hz resulting in total 12 switches. The line current will then have the superimposed input inductor ripple current at high switching frequency, which needs to be filtered out by an additional high frequency filter on ac line. The presence of 12 switches and their operation in the hard-switching mode results in high conduction and switching losses. The best efficiency reported with this two-stage approach is 87%, which also included additional switching devices to achieve resonant transitions and reduce switching losses.

This configuration has the start-up problem occurring due to step-up only DC voltage gain. Thus, additional circuitry is needed to pre-charge the output capacitor before start up of the converter. This problem does not exist in the Isolated Bridgeless PFC converter as described in later sections.

The most common approach for 1 kW or higher power, however, is to use a Three-stage power processing illustrated in Fig. 3a, in which the Bridge Rectifier on input is followed by the isolated full-bridge Boost PFC converter. In this case, a total of fourteen (14) switches is needed! The highest efficiency is up to 90%, so it is better then the two-stage approach. Therefore, this three-stage approach is utilized in practically all present applications for high power.

For medium and low power, an alternative approach with reduced number of switches is shown in Fig. 3b, in which the forward converter was used for the isolation stage. While the number of switches is reduced to ten, the problem is that four switching devices in the forward converter are still exposed to much increased voltage stresses on both primary side switches and secondary side switches when compared to the two-stage full-bridge solution. Note also that four magnetic pieces must be used as in full-bridge solution, when a input filter inductor is included.

NEW ISOLATED BRIDGELESS PFC CONVERTER

Clearly, the present AC-to-DC solutions with PFC and isolation use a configuration with three cascaded converters (bridge rectifier followed by two DC/DC converters) so that total power is processed in sequence three times, resulting in high power losses and low overall efficiency.

The Bridgeless PFC converter in Fig. 4a and its duty ratio modulation control of Fig. 4b were shown in July's Power Electronics Technology to operate directly from the ac line and without the need for the Full-Bridge rectifier in front. This converter has only three switches and operates on the basis of the new hybrid- switching method. Despite the use of a resonant inductor and a resonant capacitor, this hybrid switching method results in the DC voltage gain being dependent on the duty ratio only and having the same DC voltage gain as the ordinary boost converter for either positive input voltage or negative input voltage, hence providing an automatic ac line rectification without the need for a front-end bridge rectifier.

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