LT8331 datasheet(16/30 Pages) AD

16 / 30 page

LT8331

Rev. C

For more information www.analog.com

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

•

1 − 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

According to the Absolute Maximum Ratings table, the SW

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

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

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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