LT3758 LINER [Linear Technology], LT3758 Datasheet - Page 19

• Manufacturer Part Number: LT3758
• Description: High Input Voltage, Boost, Flyback, SEPIC and Inverting Controller
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
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.
Flyback Converter: Switch Duty Cycle and Turns Ratio
The fl yback converter conversion ratio in the continuous
mode operation is:
Where N
Figure 8 shows the waveforms of the fl yback converter
in discontinuous mode operation. During each switching
period T
During DT
D2T
L
The fl yback converter conversion ratio in the discontinu-
ous mode operation is:
According to the preceding equations, the user has relative
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 processes, due
S
currents are zero during D3T
V
V
S
V
V
OUT
OUT
V
, M is off, and L
IN
IN
IN
S
+
S
=
=
, three subintervals occur: DT
/N
Figure 7. A Simplifi ed Flyback Converter
S
N
N
N
N
, M is on, and D is reverse-biased. During
C
P
S
P
S
P
IN
is the second to primary turns ratio.
•
•
V
D
1
SN
D
–
+
−
D
2
RCD SNUBBER
SUGGESTED
D
S
C
is conducting current. Both L
3758 F07
SN
LT3758
SENSE
R
GATE
D
GND
SN
SN
S
L
.
P
N
M
P
R
:N
I
SENSE
SW
S
+
–
V
L
DS
S
S
I
, D2T
D
D
+
S
+
–
, D3T
C
OUT
P
and
S
.
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 select-
ing the switch duty cycle or turns ratio, to optimize the
converter performance. A higher duty cycle affects the
fl yback converter in the following aspects:
• Lower MOSFET RMS current I
• Lower diode peak reverse voltage, but higher diode
• Higher transformer turns ratio (N
The choice,
(for discontinuous mode operation with a given D3) gives
the power MOSFET the lowest power stress (the product
of RMS current and peak voltage). The choice,
(for discontinuous mode operation with a given D3) gives
the diode the lowest power stress (the product of RMS
current and peak voltage). An extreme high or low duty
cycle results in high power stress on the MOSFET or diode,
and reduces effi ciency. It is recommended to choose a
duty cycle between 20% and 80%.
MOSFET V
RMS current I
D D
D D
+
+
D
D
Figure 8. Waveforms of the Flyback Converter
in Discontinuous Mode Operation
V
I
2
2
SW
DS
I
D
=
=
3
2
3
1
DS
I
D(MAX)
DT
peak voltage
S
D(RMS)
I
SW(MAX)
T
S
D2T
S
SW(RMS)
D3T
P
S
/N
S
)
, but higher
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