LT3957A データシートの表示（PDF） - Linear Technology

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LT3957A

Boost, Flyback, SEPIC and Inverting Converter with 5A, 40V Switch

Linear Technology 

LT3957A

APPLICATIONS INFORMATION

Multiple capacitors are often paralleled to meet ESR

requirements. Typically, once the ESR requirement is

satisfied, the capacitance is adequate for filtering and has

the required RMS current rating. Additional ceramic capaci-

tors in parallel are commonly used to reduce the effect of

parasitic inductance in the output capacitor, which reduces

high frequency switching noise on the converter output.

Boost Converter: Input Capacitor Selection

The input capacitor of a boost converter is less critical

than the output capacitor, due to the fact that the inductor

is in series with the input, and the input current wave-

form is continuous. The input voltage source impedance

determines the size of the input capacitor, which is typi-

cally in the range of 1μF to 100μF. A low ESR capacitor

is recommended, although it is not as critical as for the

output capacitor.

The RMS input capacitor ripple current for a boost con-

verter is:

IRMS(CIN) = 0.3 • ΔIL

FLYBACK CONVERTER APPLICATIONS

The LT3957A can be configured as a flyback converter

for the applications where the converters have multiple

outputs, high output voltages or isolated outputs. Due

to the 40V rating of the internal power switch, LT3957A

should be used in low input voltage flyback converters.

Figure 6 shows a simplified flyback converter.

The flyback converter has a very low parts 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 consequent

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 inductances and easy loop compen-

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

current and lower efficiency.

VIN +

SUGGESTED

RCD SNUBBER

–

CIN VSN

+

CSN RSN

DSN

NP:NS

LP

LS

D

ID +

COUT

+

VOUT

–

ISW

SW

LT3957A

GND

3957A F06

Figure 6. A Simplified Flyback Converter

Flyback Converter: Switch Duty Cycle and Turns Ratio

The flyback converter conversion ratio in the continuous

mode operation is:

VOUT = NS • D

VIN NP 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 = NS • D

VIN NP D2

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

NS

k = 1.5 ~ 2

3957afa

15

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