LT8331 datasheet(16/30 Pages) LINER

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LT8331

8331fa

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APPLICATIONS INFORMATION

Figure 7. Waveforms of the Flyback Converter

in Discontinuous Mode Operation

8331 F07

ISW

VSW

DTS

D2TS

D3TS

ID(MAX)

smaller transformer inductances and easy loop compen-

sation, and the disadvantage of higher peak-to-average

current and lower efficiency.

Flyback Converter: Switch Duty Cycle and Turns Ratio

The flyback converter conversion ratio in the continuous

mode operation is:

VOUT

VIN

•

− D

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

duty cycle.

Figure 7 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 Figure 6, the peak SW voltage is:

VSW(PEAK) = VIN(MAX) + VSN

where VSN is the snubber capacitor voltage. A smaller VSN

results in a larger snubber loss. A reasonable VSN is 1.5

to 2 times of the reflected output voltage:

VSN = k •

VOUT • NP

k = 1.5 ~ 2

AccordingtotheAbsoluteMaximumRatingstable,theSW

voltage Absolute Maximum value is 140V. Therefore, the

maximum primary to secondary turns ratio (for both the

continuous and the discontinuous operation) should be:

≤

140V

− VIN(MAX)

k • VOUT

Accordingtotheprecedingequations,theuserhasrelative

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 processes, 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 select-

ing the switch duty cycle or turns ratio, to optimize the

Figure 6. A Simplified Flyback Converter

NP:NS

VIN

CIN

CSN

VSN

SUGGESTED

RCD SNUBBER

ISW

8331 F06

GND

LT8331

–

RSN

DSN

–

VOUT

COUT

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