LT3757 View Datasheet(PDF) - Linear Technology
Part Name
Description
Manufacturer
LT3757 Datasheet PDF : 36 Pages
| |||
LT3757/LT3757A
Applications Information
Flyback Converter: Switch Duty Cycle and Turns Ratio
The flyback converter conversion ratio in the continuous
mode operation is:
VOUT = NS • D
VIN NP 1− D
where NS/NP is the second to primary turns ratio.
Figure 8 shows the waveforms of the flyback converter
in discontinuous mode operation. During each switching
period TS, three subintervals occur: DTS, D2TS, D3TS.
During DTS, M is on, and D is reverse-biased. During
D2TS, M is off, and LS is conducting current. Both LP and
LS currents are zero during D3TS.
The flyback converter conversion ratio in the discontinu-
ous mode operation is:
VOUT
VIN
=
NS
NP
•D
D2
VDS
ISW
ISW(MAX)
ID
ID(MAX)
DTS
D2TS
D3TS
TS
t
3757 F07
Figure 8. Waveforms of the Flyback Converter
in Discontinuous Mode Operation
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
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 ISW(RMS), but higher
MOSFET VDS peak voltage
• Lower diode peak reverse voltage, but higher diode
RMS current ID(RMS)
• Higher transformer turns ratio (NP/NS)
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). 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%.
3757afd
18
Share Link: 