AN1889 datasheet(14/33 Pages) STMICROELECTRONICS

14 / 33 page

Output circuit

AN1889

14/33

Output circuit

For the purpose of the present note the transformer has been designed with just one output

power section (48 W) and one auxiliary output section to supply the driver. As already

mentioned in the fly-back operation, it is during turn-off that the energy stored in the primary

is transferred to the secondary. In a first approach we can neglect the energy absorbed by

the auxiliary output, so that almost all the peak secondary current, about 18 A at maximum

load, flows through the output capacitor, causing a voltage ripple due to the ESR of the

capacitor itself. In order to fix the output voltage ripple lower than 1 V, the ESR from the Ohm

law must be lower than 0.055

Ω.

Fixed a time constant of 100 µsec so that (ESR*Co=100*10

-6 s) the capacitor should be

higher than 1818 µF and taken also into account 20% margin, a Co of 2200 µF has been

selected.

Next step is to determine the output diode. For discontinuous mode operation ultra fast or

schottky diodes are recommended. The diode must be able to sustain the output voltage

plus the maximum reflected voltage according to the formula below:

Equation 21

Practical considerations impose to add a safety margin of about 20-25%. Good choice is to

consider also diodes with current capability about twice higher then the DC output current

Iout.

REV

out 1

inmax

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