OUT2

ON(MAX)

OUT1

OUT1

N2

I

T

N1

V

67mV

C

æ

ö

´

´

ç

÷

è

ø

D

=

»

IN(MAX)

OUT

OUT

L1

SW

IN(MAX)

V

V

V

L1

119.3 H

I

f

V

-

=

´

=

m

D

´

L1

OUT1

OUT2

N2

I

0.15

I

I

2

0.1 A

N1

æ

ö

D

=

-

-

´

´

=

ç

÷

è

ø

LM5019

www.ti.com

SNVS788F – JANUARY 2012 – REVISED DECEMBER 2014

(19)

Using the maximum peak-to-peak inductor ripple current

given by Equation 20.

(20)

A higher value of 150 µH is selected to insure the high-side switch current does not exceed the minimum peak

current limit threshold.

8.2.2.2.6

Primary Output Capacitor

In a conventional buck converter the output ripple voltage is calculated as shown in Equation 21.

(21)

To limit the primary output ripple voltage

required.

Figure 18 shows the primary winding current waveform (IL1) of a Fly-Buck converter. The reflected secondary

winding current adds to the primary winding current during the buck switch off-time. Because of this increased

current, the output voltage ripple is not the same as in conventional buck converter. The output capacitor value

calculated in Equation 21 should be used as the starting point. Optimization of output capacitance over the entire

line and load range must be done experimentally. If the majority of the load current is drawn from the secondary

isolated output, a better approximation of the primary output voltage ripple is given by Equation 22.

(22)

A standard 1-µF, 25 V capacitor is selected for this design. If lower output voltage ripple is required, a higher

8.2.2.2.7

Secondary Output Capacitor

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