Switching Power Supplies A - Z

DC-DC Converter Design and Magnetics

So for example, if the ESR of the output capacitor is 10 Ω, we get the dissipation

P

SW

= I

RMS_OUT

2

× ESR = 0.08

2

× 10 = 0.064 W (any topology)

For the boost and the buck-boost, we need to use

I

RMS_OUT

= I

O

×











D

MAX

+

r

DMAX

2

12

1 − D

DMAX

(boost and buck-boost)

Note that the general output capacitor selection procedure is as follows:

The rule-of-thumb is to pick an output capacitor with a ripple current rating equal to or

greater than the worst-case RMS capacitor current calculated above. Its voltage rating is

usually picked to be at least 20 to 50% higher than what it will see in the application (i.e. V

O

for all topologies). ...