AN2747 datasheet(32/44 Pages) STMICROELECTRONICS

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PFC section design criteria

AN2747

32/44

PFC section design criteria

For PFC design criteria see AN 1875.

Full bridge design criteria

The design of full bridge section involves the inductor (buck) component and device

selection.

12.0.1

Inductor design

The design specs are:

lamp current

lamp ripple current

lamp voltage

lamp ripple voltage

The design steps are summarized below:

Fix the switching frequency fs and consequently the period Ts also is fixed. This

frequency for power application is in the range 20 kHz ÷ 100 kHz

Fix the duty cycle estimation

δ as

Fix the current ripple in the inductor L

Calculate the inductance value using the relation:

Equation 9

For the inductance calculation:

●

fs = 40 kHz, Ts = 25 s

●

Vlamp =100 V

●

Vbus = 400 V

●

δ = 0.25

●

∆I = 2 A

Using these parameters the inductance value is 0.93 mH. For this project an inductance of

0.8 mH is used.

12.1

Filter capacitor

To calculate the capacitor value that is in parallel to the lamp, the max current lamp ripple

must be considered. The lamp current ripple must be limited in order to avoid acoustic

resonance. In this application the current ripple was fixed at

∆Ilamp ± 5% of nominal current

out

input

---------------

lamp

bus

---------------

lamp

–

() T

⋅⋅

∆

-----------------------------------------------

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