AN2870 datasheet(28/41 Pages) STMICROELECTRONICS

28 / 41 page

Designing with L6585DE

AN2870

28/41

The mean value of the PFC frequency is calculated accordingly with Eq.13.

This capacitor may worsen the overall performance of the PFC stage: in fact the energy

stored in it may not be transferred to inductor when the input voltage is near zero. This is the

main reason of the introduction of the THD optimizer.

Other input circuitry

The EMI behavior of the circuit needs to be improved with a suitable EMI filter, a fuse with

inrush limiter can be introduced for improved reliability against burst and surge events and

finally also a surge suppressor (varistor) can be needed.

4.2

Ballast stage design

Resonant network and operating point design

The values of resonant inductor and resonant capacitor and the operating frequencies are

chosen in order to:

Supply the lamp with correct voltage and current during run mode

Maintain lamp voltage lower than Vpre during preheating mode

Develop a suitable high voltage across the lamp during the ignition

The resonant network when the lamp is off has a very high Q factor and the resonant

frequency is very close to the ideal resonant frequency of an LC resonator.

The relationship between lamp voltage and frequency can be easily found using the Fourier

transform and considering the fundamental harmonic of the square wave generated by the

half bridge.

Input voltage will be:

Equation 29

Useful parameters are resonant frequency f0, characteristic impedance Z0 and Q factor:

Equation 30

Where Vrun and Irun are respectively the lamp voltage and lamp current.

The suitable frequencies to obtain the desired operating parameters can be calculated as

follows:

out

bal

⎪

⎩

⎪

⎨

⎧

run

lamp

res

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