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

The bridgeless PFC converter has one component, the controlling switch S whose implementation is critical to the overall efficiency. From the description of the converter operation for positive and negative output voltages, it is clear that this switch S must have a two-quadrant switching characteristic operating in the first and third quadrant as illustrated in the definition of switch S in Figure 11a. In other words, switch S must for positive input voltage operate in first quadrant, but should also be able to sustain a full reverse voltage (third quadrant operation). Clearly, neither bipolar, nor MOSFET transistor can do that as they are not designed to sustain voltage in the third quadrant (MOSFET due to presence of the body diode). However, the Reverse Blocking Isolated Gate Bipolar Transistor (RBIGBT) can sustain the full reverse voltage as it is designed to do so and can sustain the full voltage of opposing polarity. Therefore, the switch S implementation uses two such RBIGBT devices in parallel.

Yet, in many practical applications, the MOSFET implementation is desired due to high switching frequency capability and low conduction losses. At present, a single MOSFET implementation is not possible due to built-in body-diode, so that switch S must be implemented by use of the two MOSFET devices connected in series at their sources and driven by a common floating gate drive circuit.

As the conduction losses are by far the dominant losses of the whole converter, it is expected that such switch implementation could raise the overall efficiency from the current 98% (see enclosed experimental measurements) to over 99% in near future.

CONVERTER START-UP

The dc gain characteristic of Equation (7) suggests that the converter would have the start-up problem as the dc gain characteristic is always greater than 1, since at start-up the output dc voltage is zero. The same conditions apply in the boost converter, which then requires use of in-rush current limiting circuitry. This converter, however, does not require such circuitry as its special mode of operation at low duty ratios permits a soft-start from zero output voltage.

Shown in Figure 12 with dashed lines is the ideal dc conversion gain characteristic given by Equation (7). The actual measured dc conversion characteristic shown in solid line, however, reveals the existence of the shaded region at very low duty ratios during which the dc conversion gain drops to zero. Therefore, effectively, the actual dc conversion gain is that of a step-down/step-up type. Thus, the output dc voltage can be started smoothly from zero dc output voltage and brought by duty ratio increase into a step-up dc conversion region without the need for in-rush current limiting circuitry.

PROTOTYPE VERIFICATION

A 400W prototype was built to verify the basic operation of the Bridgeless PFC converter and is shown in Figure 13a while the measurements of the line voltage and the line current recorded under Unity Power Factor control are illustrated in Figure 13b. Note that the line current is indeed proportional to the line voltage. However, there is a significant distortion in the line voltage. The distorted line voltage is the result of the many users connected to that line drawing the power at the peak of the AC line voltage resulting in the sag of the line voltage.

It is interesting to note that in that case, the objective is still not to draw the sinusoidal line current but instead the line current proportional to the line voltage. This will still result in PF=1, even though the line voltage is no longer sinusoidal but distorted. The power factor measured on this converter prototype is 0.999.

For questions regarding this article and to contact the author, readers should visit TESLAco's Web site at http://www.teslaco.com.

Footnote: Bridgeless PFC Converter TM and Single-Stage PFC Converter TM are trademarks of TESLAco

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