AN628 APPLICATION NOTE

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Input capacitor

The input capacitor, placed across the rectified mains, must be considered as part of the EMI filter. The advan-

tage, in placing this part after the mains rectification, is the shunt effect for the high frequency current in order

to avoid it to flow throws the diodes of the bridge due to the poor recovery characteristic.

On the other side, the value of this capacitor must to be held as low as possible because the inherent DC voltage

content affects the harmonic distortion.

With 220nF, the high frequency is filter enough and the introduced DC level can be considered not significant

at reasonable load.

Output capacitor

For the output capacitor selection, it can be consider just the output voltage ripple.

Choosing 100

µF/450Vthr 100/120Hz ripple is ± 8Vac

Instead, if the pre regulated voltage bus must ensure enough energy for Hold-up requirements (i.e. the energy

is delivered to a power supply system), the Coot value will be increased to around 180

µF.

Sense resistor

The sense resistance (Rest) is selected considering both, the signal level and the power dissipation parameters.

Using ±70m

Ω, the sense signal is good enough to be managed by the current loop. On the other side, the max-

imum power dissipation will be:

Where Alarms max. = 2.50A

Power Mos

The Mosfet breakdown voltage is imposed; Bvdss

in to account the conduction power dissipation.

Adding the switching (and the capacitive) losses we can estimate 8W to 10W total power dissipation.

Boost Diode

The continuous current mode of operation, suggest using an Ultra-fast reverse recovery diode. The STMicro-

electronics TURBOSWITCH™ family offers a good solution for this kind of application.

Boost Inductor

The inductor design starts defining the L value that is a function of the switching frequency and the accepted

current ripple. In this design, we suggest an inductor value L = 0.75mH that can be realized using an ET3411

gapped set-core ferrite.