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IL2

IOUT2

TON(MAX) x IOUT2

IL1

TON(MAX) x IOUT2 x N2/N1

OUT2

ON(MAX)

OUT1

67 mV

VOUT =

IL1

x f x COUT1

IN(MAX)

OUT

IN(MAX)

14.9 H

OUT1

OUT2

0.7 I

0.8 A

VOUT1

SW = .x RON

LM5017

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SNVS783I – JANUARY 2012 – REVISED OCTOBER 2015

8.2.2.2.4

Frequency Selection

Equation 24 is used to calculate the value of RON required to achieve the desired switching frequency.

(24)

Where K = 9 × 10–11

For VOUT1 of 10 V and fSW of 750 kHz, the calculated value of RON is 148 kΩ. A lower value of 130 kΩ is selected

for this design to allow for second order effects at high switching frequency that are not included in Equation 24.

8.2.2.2.5

Transformer Selection

A coupled inductor or a flyback-type transformer is required for this topology. Energy is transferred from primary

to secondary when the low-side synchronous switch of the buck converter is conducting.

The maximum inductor primary ripple current that can be tolerated without exceeding the buck switch peak

current limit threshold (0.7 A minimum) is given by Equation 25.

(25)

Using the maximum peak-to-peak inductor ripple current

ΔIL1 from Equation 25, the minimum inductor value is

given by Equation 26.

(26)

A higher value of 33 µH is selected to insure the high-side switch current does not exceed the minimum peak

current limit threshold. With this inductance, the inductor current ripple is

ΔIL1= 0.36 A at the maximum VIN.

8.2.2.2.6

Primary Output Capacitor

In a conventional buck converter the output ripple voltage is calculated as shown in Equation 27.

(27)

To limit the primary output ripple voltage

ΔVOUT1 to approximately 50 mV, an output capacitor COUT1 of 1.2 µF

would be required for a conventional buck.

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.

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