AN4149 datasheet(33/43 Pages) STMICROELECTRONICS

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DocID023523 Rev 2

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AN4149

FOT PFC boost control loop

The transfer function of the compensated error amplifier is:

Equation 92

In order to calculate the values of the components of the compensation network, the

Venable K-Factor calculation is used, starting from the amount of phase margin needed at

the crossover frequency and the desired maximum 3rd harmonic distortion level D3.

Phase margin is defined as the difference between 180 ° and the actual phase lag at the

frequency where the open-loop gain is unity. The higher the phase margin, the more over-

damped the system is. As rule of thumb, to ensure fast transient response with a good level

of ringing, a phase margin of 60 ° has to be chosen.

The Venable K-factor is defined as:

Equation 93

The required E/A gain (at 2fL) in order to keep the distortion below the desired maximum 3

harmonic distortion (D3) is:

Equation 94

while the unity frequency gain is:

Equation 95

The frequency of the zero and the pole of the compensation network can be found with the

following two expressions, where Go is the unity gain factor (equation 91).













⋅

)

(

INV

3 =

732

180

cos

180

sin













⋅













⋅

)

(

out

ou t

COM P













⋅

−

⋅

018

380

011













⋅

−

min

LINE

⋅

763

732

018

⋅

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