LM5018 datasheet(21/32 Pages) TI

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COUT2

VOUT2 =

IOUT2 x TON (MAX)

IL2

IOUT2

TON(MAX) x IOUT2

IL2

IOUT2

TON(MAX) x IOUT2

IL1

TON(MAX) x IOUT2 x N2/N1

OUT2

ON(MAX)

OUT1

0.16 V

LM5018

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SNVS787G – JANUARY 2012 – REVISED OCTOBER 2015

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 27 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 28.

(28)

Figure 18. Current Waveforms for COUT1 Ripple Calculation

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

value should be selected for COUT1 and/or COUT2.

8.2.2.2.7

Secondary Output Capacitor

A simplified waveform for secondary output current (IOUT2) is shown in Figure 19.

Figure 19. Secondary Current Waveforms for COUT2 Ripple Calculation

The secondary output current (IOUT2) is sourced by COUT2 during on-time of the buck switch, TON. Ignoring the

current transition times in the secondary winding, the secondary output capacitor ripple voltage can be calculated

using Equation 29.

(29)

For a 1:1 transformer turns ratio, the primary and secondary voltage ripple equations are identical. Therefore,

COUT2 is chosen to be equal to COUT1 (1 µF) to achieve comparable ripple voltages on primary and secondary

outputs.

If lower output voltage ripple is required, a higher value should be selected for COUT1 and/or COUT2.

8.2.2.2.8

Type III Feedback Ripple Circuit

Type III ripple circuit as described in Ripple Configuration is required for the Fly-Buck topology. Type I and Type

II ripple circuits use series resistance and the triangular inductor ripple current to generate ripple at VOUT and the

FB pin. The primary ripple current of a Fly-Buck is the combination or primary and reflected secondary currents

as illustrated in Figure 18. In the Fly-Buck topology, Type I and Type II ripple circuits suffer from large jitter as the

reflected load current affects the feedback ripple.

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