Datasheet

LT3957
16
3957f
APPLICATIONS INFORMATION
Figure 7 shows the waveforms of the fl yback converter
in discontinuous mode operation. During each switching
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
.
FLYBACK CONVERTER APPLICATIONS
The LT3957 can be confi gured as a fl yback 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, LT3797 should be used
in low input voltage fl yback converters. Figure 6 shows a
simplifi ed fl yback converter.
The fl yback 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 effi ciency due to
the high peak currents, high peak voltages and consequent
power loss. The fl yback converter is commonly used for
an output power of less than 50W.
The fl yback 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 effi ciency.
Figure 7. Waveforms of the Flyback Converter
in Discontinuous Mode Operation
3957 F07
I
SW
V
SW
I
D
t
DT
S
D2T
S
D3T
S
I
SW(MAX)
I
D(MAX)
T
S
Figure 6. A Simplifi ed Flyback Converter
N
P
:N
S
V
IN
C
IN
C
SN
V
SN
L
P
D
SUGGESTED
RCD SNUBBER
I
D
I
SW
3957 F06
GND
SW
LT3957
L
S
+
–
R
SN
D
SN
+
–
+
V
OUT
C
OUT
+
Flyback Converter: Switch Duty Cycle and Turns Ratio
The fl yback 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.
The fl yback 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 refl ected output voltage:
V
SN
=k•
V
OUT
•N
P
N
S
k = 1.5 ~ 2