AN4205

Input stage of the AC/DC converter

Doc ID 023929 Rev 1

11/23

2.2

Output capacitor selection and post filter

The AC current in the output capacitor causes power dissipation on its series resistance

(ESR), resulting in a rise in temperature. Thus, it is important to not operate the output

capacitor beyond its AC current ripple rating, otherwise its lifetime will be very short. In

general, a low ESR capacitor is a good choice as it provides excellent energy storage and

improves the transient performance. For this project the requirement on ESR is very tight, so

that attenuates the ripple to the desired level.

Figure 4.

In this case we have chosen a capacitor supplier with:

Equation 27

Equation 28

Therefore,

Table 2.

Key transformer characteristics

Name

Symbol

Value

Inductance

2.18 mH ± 15%

93 µH ± 15%

35 µH ± 15%

Transformer ratio

4.85

Leakage inductance

3.5% nom

Parasitic capacitance

38 pF

Saturation current

0.45 A max

Switching frequency

100 kHz

L1

C6

C5

D5

STTH2R02U

AM16370v1

ESR

C

•

32

10

06

–

s

•

=

ESR

ripple

V

ripple

I

S peak

,

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

0.24

Ω

==

C

out

32

10

•

06

–

ESR

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

≥

135

μF

=