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PFC chip for kilowatt-level PSUs

Back in June 2007, Texas Instruments introduced its UCC28060 singlechip two-phase power-factor controller for sub-1-kW powersupply applications. It has now followed that product offering with the UCC28070 for higher power, multi-kW systems such as communications infrastructure, industrial systems or server arrays. It is a two-phase, interleaved, average-currentmode controller that will help you build systems that achieve a power factor of over 0.9, reducing harmonics and ripple, improving energy efficiency and increasing reliability. The chip hosts two pulse-width modulators that operate 180° out of phase, for what TI calls Natural Interleaved operation. Existing designs may achieve higher power levels by adding more phases in parallel; without phase shifting, input and output ripple currents can remain high. Like the ’060, the new controller allows phases to be turned off at light-load conditions, reducing losses and increasing efficiency at fractional loading; however, using the 180° shift between phases allows you to split the choke function between two magnetic components, simplifying thermal management. It also lets you use smaller filtering components on input and output, with lower ripple-current ratings. In contrast to what you might expect, TI says that the total volume of two such inductors can be less than the one component that you would need in a single-phase design. You can also configure a system with multiple chips and more phases. The out-ofphase operation contributes to reducing input and output ripple, and you can also apply frequency dithering to vary the switching frequency; by so doing, you spread the spectrum of the switching noise and reduce the time-averaged noise at any given frequency, helping a design to meet conducted- and radiated-EMI limits. You can set the level and rate of the deviation of this dithering feature, and there is a facility to lock the switching frequency—which is up to 300 kHz—to an external or system clock. The chip comes with a full set of protection features. An output over-voltage scheme with open-loop detection protects against down-stream (in the power flow) component failures; the phase currents are balanced (in two-phase mode) with independent current sensing; and there are undervoltage- lockout, per-cycle peakcurrent- limit, and system-overtemperature protection features. TI also claims very good transient response and stability against AC-line disturbances for the architecture it has used in the ’070, which is a feed-forward control loop. A current synthesiser approximates the down-slope current in the off-time of the switched outputs; actual current is measured in the on-time. This gives effective control with minimal errors and no filtering ripple. TI sells the UCC28070 for $2.45 (100); the device is sampling now.

Texas Instruments, www.ti.com