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LT3758

3758f

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 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 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:

LP MAX

SW MAX

OUT MAX

MAX

IN MIN

()

••

where

η 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:

LS MAX

D MAX

OUT MAX

()

where

D2 = 1 – DMAX – D3

the primary and secondary RMS currents are:

LP RMS

LP MAX

MAX

LS RMS

LS 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:

MAX

IN MAX

OUT MAX

••

•(

()

OUT MAX

+ )

••

()

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 MOS-

FET turn-off. This is increasingly prominent at higher load

currents, where more stored energy must be dissipated.

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.

The snubber resistor value (RSN) can be calculated by the

following equation:

OUT

SW PEAK

−

•

••

()

where VSN is the snubber capacitor voltage. A smaller

VSN results in a larger snubber loss. A reasonable VSN is

2 to 2.5 times of:

OUT

•

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