UCC28056CDBVT datasheet(34/55 Pages) TI2

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u ¨

OutMax

COutRMSHF

DioRMSMax

Out Re g

1.15 A

OutMax

COutRMSLF

Out Re g

0.3 A

OutMax

COutRMSMax

DioRMSMax

Out Re g

1.19 A

Outpp

Out Re g

OSRe g

2 DSuThs

5.4%

Out

165W

115

1.43

u S u

OutMax

Line

Out Re g

Out

1.43

u S u

Out

Outpp

Out

Line

Out Re g

InRMS

sin

UCC28056

SLUSD37E – OCTOBER 2017 – REVISED NOVEMBER 2019

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9.2.2.2.4

Output Capacitor Selection

Power drawn by the PFC stage from the Line supply may be represented by the following expression.

(39)

Assuming a typical application, with constant load power, for some parts of the Line cycle excess power is drawn

from the supply and stored in the output capacitor. In other parts of the Line cycle load power exceeds input

power and this deficit must be supplied from the output capacitor. This process of energy transfer to an from the

output capacitor necessarily results in twice Line frequency output voltage ripple. The amplitude of this twice Line

frequency ripple depends only upon the ratio POut/COut and the Line frequency.

(40)

Choose an output capacitor value by prioritizing one of a number of application requirements:

•

Twice Line frequency output ripple voltage at maximum load.

•

Output voltage hold-up time after the Line supply has been disconnected.

•

Output voltage deviation as a result of a transient load step.

For this design example assume that the twice Line frequency output ripple voltage amplitude is less than 3% of

its regulation level. The POutMax/COut ratio required to achieve this can be calculated using Equation 41

(41)

Use Equation 42 to calculate the required capacitance value for this 165-W example design.

(42)

For best Line current total harmonic distortion (THD), the maximum output voltage ripple amplitude must satisfy

the condition presented in Equation 43. Satisfying this condition ensures that the error amplifier non-linear gain

does not activate due to extremes of the output voltage ripple.

(43)

Use Equation 44 to calculate the maximum RMS ripple current flowing in the output capacitor.

(44)

This current flowing into the output capacitor includes a switching frequency component (ICOutRMSHF) and a twice

Line frequency ripple component (ICOutRMSLF).

(45)

(46)

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