Interleaving PFC controller advances multi-kilowatt designs

Dallas—TI says its UCC28070 is the industry's first single-chip, interleaved power factor correction (PFC) controller for multi-kilowatt supplies in communications, server, and industrial systems. A complementary addition to the UCC28060 introduced earlier—the first single-chip transition-mode (TM) two-phase PFC controller—the UCC28070 continuous-conduction-mode (CCM) controller allows designers to meet more stringent PFC requirements (power factor greater than 0.9), greatly minimize input and output ripple, and cut electrical stresses on power supply components.

Typical single-phase CCM PFCs have high peak input current ripple, requiring a fairly substantial input filter. There's high peak output current ripple, too, and the power system's overall efficiency is relatively low at light loads. Paralleled CCM PFC stages working in single-phase for the same power level distribute the thermal load, and can provide basic on-off control over individual phases of the supply to improve efficiency at the lighter loads. But input and output ripple currents remain high.

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In that context, the UCC28070's two-phase output incorporates two PWMs (with matched average-current outputs) for 180-degree interleaving to cut input and output ripple—about 40 percent in peak output ripple for a 110-volt input, 1200-watt design operating in single-phase CCM and greater yet compared to single-phase TM designs. All else being equal, the output bulk capacitance in an interleaved design will be lower, as will thermal stresses and the physical size of the boost capacitor (typically by 25 percent in capacitor volume). Designers can also expect a physical size reduction in the boost inductor (about 32 percent). The UCC28070's optional programmable-frequency dithering mode allows designers to spread the switching frequency range in order to minimize electromagnetic interference (EMI) and further reduce capacitor size and filter costs.

The UCC28070's phase-management circuitry permits selective on-off control of individual phasing sections to maximize supply efficiency over its entire load range. Overall efficiency in a 1.2-kW system is thus up to 1.5 percent higher at 20 percent load to help meet The Green Grid, Climate Savers Computing Initiative, and Project Big Green energy initiatives. In a 240-watt supply under 20 percent load, this increase in efficiency results in a power savings of 27 percent, according to the company.

The UCC28070 offers output over-voltage protection with an open-loop detection feature to safeguard the system from common circuit-board failures. Each phase current is balanced with independent current-sensing circuitry to cut excess component heating. In addition, the UCC28070 provides under-voltage lockout, cycle-by-cycle peak current limiting, and a system over-temperature protection feature.

Availability : Sampling now, with volume production scheduled for January 2008. Suggested resale pricing in quantities of 100 units is $2.45. An evaluation board is also available; see
Datasheet : Click here.

Texas Instruments , 1-800-477-8924,

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