27/35

AN628 APPLICATION NOTE

Input capacitor selection

The demo is not provided with complete dedicated EMI filter. At the input side, two parts compose the capacitor;

the first (Cf.) is placed across the AC input of the bridge and the second one (C1) is tied to the rectified mains.

The advantage, of this configuration is the minimization of the DC content in placing a low value after the mains

rectification (C1), just to filter the high frequency. The capacitor Cf placed in the AC side must be considered as

part of the EMI filter.

Output capacitor selection

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

Choosing Co = 220

µF (450V), the maximum rectified mains ripple is:

Sense resistor selection

360W) will be:

Power Mos

In the selection of the power switch, it has been preferred to share the thermal dissipation in two separate

TO220 packages. This is a good solution because the size of the heath sinkers can be limited.

The breakdown voltage is imposed = 500V.

Ω the formula for the dissipated power calculation are:

Adding the capacitive (about 2.5W) and the switching losses (as low as 2-3 W, thanks to the snubber) we can

estimate 10 to 12W total power dissipation at lower mains value.

Boost Diode

The 8A 600V chosen Turbuswitch fits well with the application. The power dissipated in the boost diode is about 1.4W.

Boost Inductor

The 0.55mH chosen inductor value allows a low ripple (23%) of its current; moreover, there is enough room (in

the industrialization phase) to reduce the switching frequency holding an acceptable current ripple (e.g. reduc-

ing the frequency to 75 kHz the current ripple will be held within 30%.

In this design, the coil has been realized with a gapped set-core ferrite E42*12*15.

The results that can expect, realizing the described circuit, has been tested. And the result are shortly reported

from Table 1 to Table 6.

The PCB and component Layout can be seen in figgs 25, 26 and 27 (The Gerber files of the PCB are available

on request).

V

∆

π

o

200

π V

⋅⋅

⋅

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

7.3V

==