Datasheet

LT3758/LT3758A
18
3758afd
ApplicAtions inForMAtion
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.
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
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 product
of RMS current and peak voltage). 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 between 20% and 80%.
Figure 7. A Simplified Flyback Converter
R
SENSE
N
P
:N
S
V
IN
C
IN
C
SN
V
SN
L
P
D
SUGGESTED
RCD SNUBBER
I
D
V
DS
I
SW
3758 F07
GATE
GND
LT3758
SENSE
L
S
M
+
–
+
–
R
SN
D
SN
+
–
+
C
OUT
+
Figure 8. Waveforms of the Flyback Converter
in Discontinuous Mode Operation
3758 F08
I
SW
V
DS
I
D
t
DT
S
D2T
S
D3T
S
I
SW(MAX)
I
D(MAX)
T
S
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 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
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