Datasheet

LT3757/LT3757A
18
3757afe
For more information www.linear.com/LT3757
applicaTions inForMaTion
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.
Figure8 shows the waveforms of the flyback 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
.
The flyback converter conversion ratio in the discontinu-
ous mode operation is:
V
OUT
V
IN
=
N
S
N
P
•
D
D2
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
the converter performance. A higher duty cycle affects the
flyback converter in the following aspects:
• Lower MOSFET RMS current I
SW(RMS)
, but higher
MOSFET V
DS
peak voltage
• Lower diode peak reverse voltage, but higher diode
RMS current I
D(RMS)
• Higher transformer turns ratio (N
P
/N
S
)
The choice,
D
D+D2
=
1
3
(for discontinuous mode operation with a given D3) gives
the power MOSFET the lowest power stress (the prod-
uct of RMS current and peak voltage). However, in the
high output voltage applications, a higher duty cycle may
be adopted to limit the large peak reverse voltage of the
diode. The choice,
D
D+D2
=
2
3
(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 efficiency. It is recommended to
choose a duty cycle, D, between 20% and 80%.
Figure8. Waveforms of the Flyback Converter
in Discontinuous Mode Operation
3757 F08
I
SW
V
DS
I
D
t
DT
S
D2T
S
D3T
S
I
SW(MAX)
I
D(MAX)
T
S
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