Datasheet

LT3958
17
3958fa
For more information www.linear.com/LT3958
applicaTions inForMaTion
period T
S
, three subintervals occur: DT
S
, D2T
S
, D3T
S
.
During DT
S
, M is on, and D is reverse-biased. During
D2T
S
, M is off, and L
S
is conducting current. Both L
P
and
L
S
currents are zero during D3T
S
.
The flyback converter conversion ratio in the discontinu-
ous mode operation is:
V
OUT
V
IN
=
N
S
N
P
•
D
D2
According to Figure 6, the peak SW voltage is:
V
SW(PEAK)
= V
IN(MAX)
+ V
SN
where V
SN
is the snubber capacitor voltage. A smaller V
SN
results in a larger snubber loss. A reasonable V
SN
is 1.5
to 2 times of the reflected output voltage:
V
SN
= k •
V
OUT
• N
P
N
S
k = 1.5 ~ 2
According to the Absolute Maximum Ratings table, the SW
voltage Absolute Maximum value is 84V. Therefore, the
maximum primary to secondary turns ratio (for both the
continuous and the discontinuous operation) should be.
N
P
N
S
≤
84V − V
IN(MAX)
k • V
OUT
FLYBACK CONVERTER APPLICATIONS
The LT3958 can be configured as a flyback converter for the
applications where the converters have multiple outputs,
high output voltages or isolated outputs. 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
.
Figure 7. Waveforms of the Flyback Converter
in Discontinuous Mode Operation
3958 F07
I
SW
V
SW
I
D
t
DT
S
D2T
S
D3T
S
I
D(MAX)
T
S
Figure 6. A Simplified Flyback Converter
N
P
:N
S
V
IN
C
IN
C
SN
V
SN
L
P
D
SUGGESTED
RCD SNUBBER
I
D
I
SW
3958 F06
GND
SW
LT3958
L
S
+
–
R
SN
D
SN
+
–
+
V
OUT
C
OUT
+
Flyback Converter: Switch Duty Cycle and Turns Ratio
The flyback converter conversion ratio in the continuous
mode operation is:
V
OUT
V
IN
=
N
S
N
P
•
D
1− D
where N
S
/N
P
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
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