LT8301 datasheet(19/38 Pages) AD

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LT3757/LT3757A

Rev. G

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APPLICATIONS INFORMATION

Flyback Converter: Transformer Design for

Discontinuous Mode Operation

The transformer design for discontinuous mode of opera-

tion is chosen as presented here. According to Figure 8,

the minimum D3 (D3MIN) occurs when the converter

has the minimum VIN and the maximum output power

(POUT). Choose D3MIN to be equal to or higher than 10%

to guarantee the converter is always in discontinuous

mode operation (choosing higher D3 allows the use of low

inductances, but results in a higher switch peak current).

The user can choose a DMAX as the start point. Then, the

maximum average primary currents can be calculated by

the following equation:

ILP(MAX) = ISW(MAX) =

POUT(MAX)

DMAX • VIN(MIN) • η

where h is the converter efficiency.

If the flyback converter has multiple outputs, POUT(MAX)

is the sum of all the output power.

The maximum average secondary current is:

ILS(MAX) = ID(MAX) =

IOUT(MAX)

where:

D2 = 1 – DMAX – D3

the primary and secondary RMS currents are:

ILP(RMS) = 2 •ILP(MAX) •

DMAX

ILS(RMS) = 2 •ILS(MAX) •

According to Figure 8, the primary and secondary peak

currents are:

ILP(PEAK) = ISW(PEAK) = 2 • ILP(MAX)

ILS(PEAK) = ID(PEAK) = 2 • ILS(MAX)

The primary and second inductor values of the flyback

converter transformer can be determined using the fol-

lowing equations:

LP =

D2 MAX • V2 IN(MIN)• η

2 •POUT(MAX) • f

LS =

D22 •(VOUT + VD)

2 •IOUT(MAX) • f

The primary to second turns ratio is:

Flyback Converter: Snubber Design

Transformer leakage inductance (on either the primary

or secondary) causes a voltage spike to occur after the

MOSFET turn-off. This is increasingly prominent at higher

load currents, where more stored energy must be dis-

sipated. In some cases a snubber circuit will be required

to avoid overvoltage breakdown at the MOSFET’s drain

node. There are different snubber circuits, and Application

Note 19 is a good reference on snubber design. An RCD

snubber is shown in Figure 7.

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