LT8331 datasheet(19/34 Pages) AD

19 / 34 page

LT8357

Rev. 0

For more information www.analog.com

The RMS input capacitor ripple current for a boost con-

verter is:

IRMS(CIN) = 0.3 • �IL

FLYBACK CONVERTER APPLICATIONS

The LT8357 can be configured as a flyback converter for

the applications where the converters have multiple out-

puts, high output voltages or isolated outputs. Figure 8

shows a simplified flyback converter using LT8357.

LT8357

GND

GATEP

GATEN

VIN

VOUT

SENSE

NP:NS

VSN

CIN

COUT

RSENSE

RSN

DSN

CSN

SUGGESTED

RCD SNUBBER

8357 F08

Figure 8. A Simplified Flyback Converter

The flyback converter has a very low part count for mul-

tiple outputs, and with prudent selection of turns ratio,

can have high output/input voltage conversion ratios with

a desirable duty cycle. However, it has low efficiency due

to the high peak currents, high peak voltages and con-

sequent power loss. The flyback converter is commonly

used for an output power of less than 50W.

The flyback converter can be designed to operate either

in continuous or discontinuous mode. Compared to con-

tinuous mode, discontinuous mode has the advantage of

smaller transformer inductance and easier loop compen-

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

current and lower efficiency. In the high output voltage

applications, the flyback converters can be designed

to operate in discontinuous mode to avoid using large

transformers.

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 9 shows the waveforms of the flyback converter

in discontinuous mode operation, where VSW, ISW and ID

represent primary switch node voltage, power MOSFET

current and output diode current, respectively. During

each switching period TS, three subintervals occur: DTS,

D2TS, D3TS. During DTS, power MOSFET M1 is on, and

output diode D1 is reverse-biased. During D2TS, M1 is off,

and LS is conducting current. Both LP and LS currents are

zero during D3TS.

8357 F09

ISW

VSW

DTS

D2TS

D3TS

ID(MAX)

Figure 9. Waveforms of the Flyback Converter in

Discontinuous Mode Operation

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