Compound Converter Topology for Better Power Factor Correction(PFC) in AC/DC Converters
To have an ideal Power Factor, an AC/DC converter should draw input current in proportion to the input voltage. That means no input energy is available during zero voltage crossings on the AC side. As a result, PFC methods with good output regulation must include energy storage, in some form, in order to continuously support the output. Conventionally, there are two basic approaches. The usual method is to precede the power converter with an additional power stage dedicated to PFC. CogniPower is patenting a simple, active topology that eliminates the separate PFC conversion stage in an AC/DC converter by intelligently controlling the use of a storage element.
To have an ideal Power Factor, an AC/DC converter should draw input current in proportion to the input voltage. That means no input energy is available during zero voltage crossings on the AC side. As a result, PFC methods with good output regulation must include energy storage, in some form, in order to continuously support the output. Conventionally, there are two basic approaches. The passive approach is to use a large enough filter to be effective at the line frequency, but the size and cost of that much inductive or capacitive filtering makes the passive approach less attractive. The usual method is to precede the power converter with an additional power stage dedicated to PFC. Adding the extra stage inevitably involves size, cost and efficiency penalties. CogniPower is patenting a simple, active topology that eliminates the separate PFC conversion stage in an AC/DC converter by intelligently controlling the use of a storage element.
In a Compound Converter, the majority of energy moves through only a single stage of power conversion. Improved efficiency is the direct result. If a power converter is to transfer the correct amount of energy, on the average, over an AC cycle, about half the time there will be a surplus available, and about half the time there will be a deficit. A compound converter directly services the output through a single power stage to the extent possible. During the period of surplus, excess energy is placed into a storage reservoir. During the period of deficit, supplemental energy from storage contributes to support the output. Using this approach, over 75% of the power can move through only a single stage of conversion. In the conventional two-stage PFC power supply, 100% of the power moves through both stages. Instead of an incremental efficiency improvement, the Compound Converter can reduce power losses by 35%, or more.
In a two-stage converter, the storage capacitor requires a high voltage rating. The storage element in a Compound Converter centers on a more convenient, lower voltage. Because the storage voltage can change over a larger range in a Compound Converter, a smaller capacity storage element provides equivalent support for the output, saving cost and space. Alternatively, if UPS function is desired, the storage element may be made arbitrarily large to support the output for extended periods.
The circuitry to implement a Compound Converter can be surprisingly simple. The block diagram above shows one version. The Control block slowly adjusts the ON time so that the voltage on the storage capacitor stays within limits. During a single cycle of the AC line, the ON time does not change a significant amount, resulting in a near-ideal Power Factor. The supplemental regulator can be a simple buck converter, a linear regulator, or it can be a Predictive Energy Balancing converter for smoothest operation. The synchronous rectifier block, SyncRect, is enabled by a control signal. When the regulation point is reached, the Main Regulator switch opens, and any remaining inductive current passes through the Auxiliary Flyback diode into the storage capacitor.
Because both the main and supplemental converters are capable of supporting the entire load, 100% overcurrent capability is built in. The supplemental converter improves transient response and can be used to reduce ripple. For best efficiency, the supplemental converter should regulate to a slightly lower voltage than the main regulator. The small resulting decrease in the output voltage during zero crossing is offset by a large increase in efficiency. With the slightly lowered supplemental regulation point, another benefit appears. The efficiency rises as the load is reduced. Since existing designs are challenged to meet low-load efficiency requirements, this characteristic is a most welcome change.
The waveforms above illustrate the flow of current in a Compound Converter. Because the voltage at the storage element stays roughly constant, the average power flowing out of storage to supply the supplemental regulator will approximately equal the average power flowing into storage.
The frequency of operation for a Compound Converter can be higher or lower, or adaptive, for even better efficiency. With variable clocking, zero-current switching and quasi-resonant topologies become practical. Multiple regulated outputs can be added with a minimum of additional complexity. Unisolated versions can be made even more simply.
The advantages of the Compound Converter can be enjoyed without engineering virtuosity. Since the benefits stem from the rearrangement of ordinary power converter elements, the components needed are inexpensive and generally available. If exemplary efficiency is desired, GaN switches on the AC side show their value. The two AC-side diodes can be eliminated when using GaN switches because of the absence of body diodes. With GaN, the majority of the power moves through only two semiconductor junctions, compared to five junctions for conventional two-stage PFC AC/DC converters with regulated outputs.
Better PFC, much better efficiency, smaller size, and lower cost add up to a compelling case. Combined with a low risk of implementation and the competitive advantage gained by using licensed proprietary technology, it is not surprising that Compound Converters are already going into production. Any AC/DC converter requiring PFC is a candidate for the CogniPower Compound Converter. Power for laptops, flat panel displays, LED lighting, and set top boxes are prime applications. As PFC is mandated for lower power AC/DC converters, the Compound Converter will find its way into even more places.
