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LM5020 Reference Design -
Dual Output Flyback
Converter with isolated
outputs of 5V@0.2A and
12V@2.1A. Two LM2736Y's
provide an additional output
of 3.3V@0.5A and 5V@0.5A.
National Semiconductor
LM5020
Power Applications Design Center, Americas
May 2007
1.0 Design Specifications
Inputs Output #1 Output #2 Output #3 Output #4
VinMin=9V Vout1=5V Vout2=12V Vout3=3.3V Vout4=5V
VinMax=42V Iout1=0.2A Iout2=2.1A Iout3=0.5A Iout4=0.5A
2.0 Design Description
This design utilizes the LM5020 Current Mode PWM Con-
troller configured in a dual output Flyback converter. This
isolated design operates at 300kHz and uses a custom trans-
former to convert the input voltage, via flyback action, to an
output voltage level of 12V at 2.1A, as well as produce an
isolated 5V@0.2A rail. The relative low output power of this
design, and the fact that V
cc
is not required to power addi-
tional IC’s, allows the primary bias winding to be connected
to V
in
. The LM5020 has a minimum input voltage of 13V so
a start-up bias circuit was added to apply an external voltage
to V
cc
until the input voltage reached the proper threshold
level.
With total potential output power less than 30W, a custom
EFD20 core was chosen for this design. Because of the input
voltage range, a CCM to DCM transition tradeoff was made
to minimize losses associated with DCM operation. Radiated
EMI requirements necessitated the use of snubbers and other
filtering elements to minimize high energy signals from inter-
fering with adjacent circuitry.
The 12V rail also fed a 5V@0.5A rail, as well as an optional
3.3V@0.5A rail. The secondary side feedback components
are selected appropriately to ensure the voltage regulation
remains solid and that proper gain and phase margins are
maintained for stable operation and quick transient response.
© 2007 National Semiconductor Corporation www.national.com
LM5020 Reference Design - Dual Output Flyback Converter with isolated outputs of 5V@0.2A
and 12V@2.1A. Two LM2736Y's provide an additional output of 3.3V@0.5A and 5V@0.5A.LM5020
