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LT8331

8331fa

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APPLICATIONS INFORMATION

converter performance. A higher duty cycle affects the

flyback converter in the following aspects:

LowerswitchRMScurrentISW(RMS),buthigherswitch

VSW peak voltage

Lower diode peak reverse voltage, but higher diode

RMS current ID(RMS)

Higher transformer turns ratio (NP/NS)

It is recommended to choose a duty cycle between 20%

and 80%.

Flyback Converter: Maximum Output Current

Capability and Transformer Design

The maximum output current capability and transformer

design for continuous conduction mode (CCM) is chosen

as presented here.

Themaximumdutycycle(DMAX)occurswhentheconverter

has the minimum VIN:

DMAX =

VOUT •

















VOUT •















 + VIN(MIN)

Due to the current limit of its internal power switch, the

LT8331 should be used in a flyback converter whose

maximum output current (IO(MAX)) is:

IO(MAX) ≤

VIN(MIN)

VOUT

• DMAX • 0.5A − 0.5 • ∆ISW

(

) • η

where

η (< 1.0) is the converter efficiency. Minimum

possible inductor value and switching frequency should

also be considered since they will increase inductor ripple

current ∆ISW.

The transformer ripple current ∆ISW has a direct effect on

the design/choice of the transformer and the converter’s

output current capability. Choosing smaller values of ∆ISW

increases the output current capability, but requires large

primary and secondary inductances and reduces the cur-

rent loop gain (the converter will approach voltage mode).

Accepting larger values of ∆ISW allows the use of low

primary and secondary inductances, but results in higher

input current ripple, greater core losses, and reduces the

output current capability. It is recommended to choose a

∆ISW of approximately 0.2A to 0.3A.

Givenanoperatinginputvoltagerange,andhavingchosen

the operating frequency and ripple current in the primary

winding,theprimarywindinginductancecanbecalculated

using the following equation:

VIN(MIN)

∆ISW • fOSC

• DMAX

Theprimarywindingpeakcurrentistheswitchcurrentlimit

(maximum 0.7A). The primary and secondary maximum

RMS currents are:

ILP(RMS) ≈

POUT(MAX)

DMAX • VIN(MIN) • η

ILS(RMS) ≈

IOUT(MAX)

− DMAX

Basedontheprecedingequations,theusershoulddesign/

choose the transformer having sufficient saturation and

RMS current ratings.

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

overvoltagebreakdownattheMOSFET’sdrainnode.There

are different snubber circuits (such as RC snubber, RCD

snubber, etc.) and Application Note 19 is a good reference

on snubber design. An RCD snubber is shown in Figure 6.

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

following equation:

RSN = 2 •

V2SN − VSN • VOUT •

I2SW(PEAK) • LLK • fOSC

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