LT8331 datasheet(17/30 Pages) AD

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LT8331

Rev. C

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APPLICATIONS INFORMATION

the converter performance. A higher duty cycle affects the

flyback converter in the following aspects:

Lower switch RMS current ISW(RMS), but higher switch

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.

The maximum duty cycle (DMAX) occurs when the con-

verter 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 • ΔISW

(

) • 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.

Given an operating input voltage range, and having cho-

sen the operating frequency and ripple current in the pri-

mary winding, the primary winding inductance can be

calculated using the following equation:

L =

VIN(MIN)

ΔISW • fOSC

• DMAX

The primary winding peak current is the switch current

limit (maximum 0.7A). The primary and secondary maxi-

mum RMS currents are:

ILP(RMS) ≈

POUT(MAX)

DMAX • VIN(MIN) • η

ILS(RMS) ≈

IOUT(MAX)

1 − DMAX

Based on the preceding equations, the user should design/

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

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 (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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