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LT8331 Datasheet(HTML) 16 Page - Analog Devices

Part No. LT8331
Description  Low IQ Boost/SEPIC/Flyback/Inverting Converter with 0.5A, 140V Switch
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
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LT8331 Datasheet(HTML) 16 Page - Analog Devices

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LT8331
16
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
ID
t
DTS
D2TS
D3TS
ID(MAX)
TS
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
=
NS
NP
D
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
=
NS
NP
D
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
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:
NP
NS
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
LP
D
SUGGESTED
RCD SNUBBER
ID
ISW
8331 F06
GND
SW
LT8331
LS
+
RSN
DSN
+
+
VOUT
COUT
+


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