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AN701

Designing High-Frequency DC-to-DC Converters With the Si9114A Switchmode Controller

Vishay Semiconductors 

AN701

Vishay Siliconix

Appendix B

FLYBACK CONVERTER TRANSFORMER DEĆ

SIGN

The flyback magnetic component is often referred to as a

transformer, but should in fact be viewed as an inductor with

energy storage capability.

Two modes of operation are possible. In discontinuous mode,

all the energy is transferred to the output(s). In continuous

mode, some energy remains in the inductor. Discontinuous

mode provides the advantages of a smaller inductor and the

absence of unexpected or difficult transfer modes in the control

mechanism. As energy transfer is complete, furthermore,

rectifier currents always fall to zero before reverse voltages are

applied. The result is lower rectifier switching losses.

For the sake of simplicity, it is recommended that only highly

experienced designers use continuous mode, further

information on which can be reviewed in some of the

publications listed in the reference section of this application

note. In this appendix, only the discontinuous mode is

explained.

Due to the discontinuous nature of the energy transfer

mechanism, flyback inductors are usually larger than the

transformers encountered in forward and other Buck

topologies, where only voltage transformation—and no

energy storage—is performed.

To design a Flyback Inductor, the following information must be

known:

Fsw

Dmax

n

VMIN

POUT

VOUT

operating switching frequency (usually 20 to

500 kHz)

maximum duty cycle (usually 50% in

Vishay Siliconix products)

target efficiency (usually 0.75 to 0.85)

minimum input voltage used

total output power

output voltage(s) required.

In this case, a design for a 24-V (16- to 30-V), 5-V, 2-A (10 W)

inductor will be demonstrated.

The first step is to determine the minimum primary inductance

from:

Lpmin

+

ǒVin

min

x

ton maxǓ2

2 x Pout

x

fsw

x

h

(18 IV x 2 ms)2 x 250 kHz x 0.75

Lpmin +

2 x 10 W

Lpmin + 12.2 mH [ 12 mH

The next step is to determine the peak primary current. For a

given inductor, the applied voltage will be:

di

V + L dt

Re-arranging yields:

Vin min x ton max

ipk +

Lpmax

18 V x 2 ms

ipk + 12 mH + 3 A

The R.M.S. value for a triangular waveform can be used to

calculate the power that will be dissipated in the MOSFET:

Ǹ ims + ipk x

dmax

3

Ǹ0.5

ims + 3 A x 3 + 1.22 A [ 1.2 A

To calculate the number of turns required for the 5-V output the

following quotation can be used:

NS

+ Np

ǒVO

) VFǓ(1–Dmax)

Vinmin Dmax

in this case, for Dmax = 0.5 and VF = 0.6 V for a 5 V output, the

secondary turns will be:

(5 V ) 0.6 V)(1–0.5)

NS + Np

16 V x 0.5

www.vishay.com

18

Document Number: 70575

16-Jan-01

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