LT8301 datasheet(18/38 Pages) AD

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LT3757/LT3757A

Rev. G

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APPLICATIONS INFORMATION

Flyback Converter: Switch Duty Cycle and Turns Ratio

The flyback converter conversion ratio in the continuous

mode operation is:

VOUT

VIN

•

1– D

where NS/NP is the second to primary turns ratio.

Figure 8 shows the waveforms of the flyback converter

in discontinuous mode operation. During each switching

period TS, three subintervals occur: DTS, D2TS, D3TS.

During DTS, M is on, and D is reverse-biased. During

D2TS, M is off, and LS is conducting current. Both LP and

LS currents are zero during D3TS.

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,

+ 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,

+ 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%.

Figure 8. Waveforms of the Flyback Converter

in Discontinuous Mode Operation

3757 F08

ISW

VDS

DTS

D2TS

D3TS

ISW(MAX)

ID(MAX)

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