AN3032 datasheet(12/26 Pages) STMICROELECTRONICS

12 / 26 page

Ballast design

AN3032

12/26

Doc ID 16165 Rev 2

Taking into account a minimum half-bridge switching frequency (fswmin) of 40 kHz and a total

output power (PoutTOT) equal to 2*58 = 116 W, the input capacitor C4 can be determined by

the following equation.

Equation 14

C4 has been set to 470 nF x 2.

3.2.2

Output capacitor

The selection of the output bulk capacitor C1 depends on the DC output voltage, the

admitted overvoltage, the output power and the desired voltage ripple. With the following

values:

●

PFC output voltage VbusPFC = 420 V.

●

coefficient of the low frequency (twice the mains frequency (fmain) = 50 Hz) voltage

ripple r1 = 0.05.

the bulk capacitor can be calculated as:

Equation 15

To obtain the smallest possible ripple and good reliability, a commercial capacitor C1 of

47 µF, 450 V has been used.

3.2.3

Boost inductor

The inductance Lpfc is usually determined so that the minimum switching frequency (fmin pfc)

is greater than the maximum frequency of the internal starter to ensure correct TM

operation. Considering the minimum suggested value for the PFC section (fmin pfc) is 20 kHz

and that this last can occur at either the maximum VinrmsMax = 265 V or the minimum

VinrmsMin = 85 V mains voltage, the inductor value is defined by:

Equation 16

To margin from fmin pfc we have set fpfc to 40 kHz. In this condition, the lower value for the

inductor is determined by Vinrms = VinrmsMin and the result Lpfc = 0.5 mH with (as stated in

the PFCS pin description) a maximum ILmax of 4.75 A (using the inductor 1986-0002

manufactured by MAGNETICA).

min

outTOT

⋅

busPFC

main

busPFC

outTOT

⋅

(

)

busPFC

out

pfc

min

inrms

busPFC

inrms

pfc

⋅

−

⋅

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