LT8331 datasheet(20/34 Pages) AD

20 / 34 page

LT8357

Rev. 0

For more information www.analog.com

The flyback converter conversion ratio in the discontinu-

ous mode operation is:

VOUT

VIN

•

According to the preceding equations, the user has rela-

tive freedom in selecting the switch duty cycle or turns

ratio to suit a given application. The selections of the

duty cycle and the turns ratio are somewhat iterative pro-

cesses, due to the number of variables involved. The user

can choose either a duty cycle or a turns ratio as the start

point. The following trade-offs should be considered when

selecting the switch duty cycle or turns ratio, to optimize

the converter performance. A higher duty cycle affects the

flyback converter in the following aspects:

• Lower MOSFET RMS current ISW(RMS), but higher

MOSFET VDS peak voltage

• Lower diode peak reverse voltage, but higher diode

RMS current ID(RMS)

• Higher transformer turns ratio (NP/NS)

The choice,

D+D2

(for discontinuous mode operation with a given D3) gives

the power MOSFET the lowest power stress (the prod-

uct of RMS current and peak voltage). However, in the

high output voltage applications, a higher duty cycle may

be adopted to limit the large peak reverse voltage of the

diode. The choice,

D+D2

(for discontinuous mode operation with a given D3)

gives the diode the lowest power stress (the product of

RMS current and peak voltage). An extreme high or low

duty cycle results in high power stress on the MOSFET

or diode, and reduces efficiency. It is recommended to

choose a duty cycle, D, between 20% and 80%.

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 9,

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 guar-

antee the converter is always in discontinuous mode

operation (choosing higher D3 allows the use of lower

inductance, 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 η is the converter efficiency. If the flyback con-

verter 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 9, the primary and secondary peak

currents are:

ILP(PEAK) = ISW(PEAK) = 2 • ILP(MAX)

ILS(PEAK) = ID(PEAK) = 2 • ILS(MAX)

LT8331 датащи(PDF) 20 Page - Analog Devices

LT8331 датащи(HTML) 20 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34

датащи скачать

ссылки URL