UCC28056CDBVT datasheet(17/55 Pages) TI2

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InMinPkL

ONMAX0

LMaxPkL

BST

LPkSOpt

LMaxPkL

2 I

3.5

ONMAX0

InMinPkL

Dem

ONMAX0

ONDCH

COMax

ONDCH

InPkL

T u G

InEqMin

ONDCH

InPkL

Dem

InMax

InEq

ONMAX0

InMinPkL

u BST

InEqMin

ONMAX0

2 L

UCC28056

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SLUSD37E – OCTOBER 2017 – REVISED NOVEMBER 2019

Product Folder Links: UCC28056

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Feature Description (continued)

Assume that full power operation at minimum Line operation is in CrM mode. Use Equation 6 to calculate the

PFC inductor value required to deliver maximum power from minimum Line.

where

•

TONMAX0 is the maximum switch ON time

(6)

Input power demand is an expression of the ratio of input power over maximum input power.

(7)

Equation 8 rearranges Equation 7 to express TON(θ) time as a function of power demand.

(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(θ) from previous cycles as the basis for computing TON(θ)

for the current cycle. The process is similar to solving an equation numerically by iteration.

A range of operating frequency options are available for CrM/DCM light-load operation. 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 anywhere between these two extremes. Conduction loss normally dominates when

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

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 approximately one third of the maximum

value.

(9)

(10)

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

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

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

attempts to maximize efficiency for all loads and for all Line voltages.

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