Now considering that the input current IIN(θ) of the converter is the average value of the inductor current in a

switching cycle results:

Equation 5

(5)

IINθCCM=IL,PKθ − ΔILθ2

Selecting the THD-CCM optimizer resistor equal to:

Equation 6

(6)

RTHD_CCM=KCCM RSLP

and replacing equations #2 and #4 in equation #3, after some calculations the inductor peak current results:

Equation 7

(7)

IL,PKθ = 1 RS  KM VOUT VC  VINθ +ΔILθ2

Finally replacing equation #7 in equation #5, the input current of the converter results:

Equation 8

(8)

IINθCCM= 1 RS  KM VOUT VC  VINθ

which is sinusoidal and in phase with the VIN(θ) input voltage (ideally zero-THD and unity-PF)

Considering the DCM operation (TR>0), through geometrical consideration, the input current IIN(θ) of the

converter can be expressed by:

Equation 9

(9)

IINθDCM=12 IL,PKθ TONθ+TFWθ

TONθ +TFWθ +TRθ

Equation 9 shows that the term TR>0 introduces distortion if IL,PK(θ) has a sinusoidal shape like in a standard

PFC. Referring to Figure 4, the sinusoidal voltage

VGθ =VC  K1  VINθVOUT isthenopportunelyshapedbythe

“THD-DCM” optimizer block, which generates the current reference voltage expressed by:

Equation 10

(10)

VCS_REFθDCM= VGθ TONθ +TFWθ +TRθ

TONθ +TFWθ

Replacing equation #1 in equation #10, and considering that in DCM operation the peak of the inductor current is

IL,PKθ =ΔILθ ,aftersomecalculationsresults:

Equation 11

(11)

IINθDCM= 1 RS  KM VOUT VC  VINθ

which is sinusoidal and in phase with the VIN(θ) input voltage (ideally zero-THD and unity-PF), and it has the

same gain like in CCM operation.

5.2

OFF-time modulator

The device embeds a novel OFF-time modulator which is able to achieve quasi-fixed switching frequency in

all operating conditions (CCM and DCM operation) and independent from the input/output voltage, the load

conditions and the converter’s parasitic as the existing modulators.

L4985

OFF-time modulator

DS13769 - Rev 3

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