UCC28056 datasheet(13/44 Pages) TI1

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UCC28056

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Feature Description (continued)

Assume that full power operation at minimum Line operation will be in CrM mode. In this case, the PFC inductor

value required to deliver maximum power from minimum Line can be calculated from the maximum switch ON

time (TONMAX0) using following expression:

(6)

Input power demand is expressed as the ratio of input power over maximum input power

(7)

Arranging this to express TON(α) time as a function of power demand gives:

(8)

Equation (8) represents the CrM/DCM TON control principle implemented by UCC28056. This equation is

quadratic in nature but UCC28056 employs the value of δONDCH(α) from previous cycles as the basis for

computing TON(α) for the current cycle. The process is much the same as solving an equation numerically by

iteration.

For light load operation the CrM/DCM controller is faced with a range of possible operating frequencies. At one

extreme, it can operate at high frequency with low current pulses in CrM mode (TDCM = 0). At the other extreme it

can operate, in DCM mode, at minimum frequency (TDCM = TDCMMax) with current pulses of maximum amplitude.

The controller can select a TDCM value that lies anywhere between these two extremes. Conduction loss will

normally dominate when operating at minimum operating frequency leading to reduced efficiency. Switching loss

will normally dominate when operating at maximum operating frequency (CrM) also leading to reduced efficiency.

Typically the most efficient operating frequency occurs when the pulse current amplitude is around one third of its

maximum value.

(9)

(10)

UCC28056 transitions from CrM to DCM operation when the peak inductor current across a Line Cycle drops

below ILPkOpt. While in DCM operation it adjusts the switching frequency to ensure that the peak inductor current

across a Line Cycle remains close to ILPkOpt for all Line and Load conditions. In this way, UCC28056 attempts to

maximize efficiency for all loads and for all Line voltages.

UCC28056 Datasheet(PDF) 13 Page - Texas Instruments