Texas Instruments LM3448 -120VAC, 6W Isolated Flyback LED Driver Eval Board LM3448-120VFLBK/NOPB LM3448-120VFLBK/NOPB Datenbogen
Produktcode
LM3448-120VFLBK/NOPB
Design Guide
and rearranging with the previously calculated parameters,
(23)
provides an inductance L
CRIT
where the flyback converter will operate at the boundary of CCM for a
switching frequency f
SW
. In order to ensure DCM operation, a general rule of thumb is to pick a primary
inductance L
P
at 85% of the L
CRIT
value.
Transformer Geometries and Materials
The length of the gap necessary for energy storage in the flyback transformer can be determined
numerically; however, this can lead to non-standard designs. Instead, an appropriate A
numerically; however, this can lead to non-standard designs. Instead, an appropriate A
L
core value (a
value somewhere between 65nH/turns
2
and 160nH/turns
2
is a good starting point) can be chosen that will
imply the gap size. A
L
is an industry standard used to define how much inductance, per turns squared,
that a given core can provide. With the initial chosen A
L
value, the number of turns on the primary and
secondary are calculated,
(24)
(25)
Given the target operating frequency and the maximum output power, a core size can be chosen using the
vendor’s specifications and recommendations. This choice can then be validated by calculating the
maximum operating flux density given the core cross-sectional area A
vendor’s specifications and recommendations. This choice can then be validated by calculating the
maximum operating flux density given the core cross-sectional area A
e
of the chosen core,
(26)
With most common core materials, the maximum operating flux density should be set somewhere
between 250mT and 300mT. If the calculation is below this range, then A
between 250mT and 300mT. If the calculation is below this range, then A
L
should be increased to the next
standard value and the turns and maximum flux density calculations iterated. If the calculation is above
this range, then A
this range, then A
L
should be decreased to the next standard value and the turns and maximum flux
density calculations iterated. With the flux density appropriately set, the core material for the chosen core
size can be determined using the vendor’s specifications and recommendations. Note that there are core
materials that can tolerate higher flux densities; however, they are usually more expensive and not
practical for these designs. The rest of the transformer design can be done with the aid of the
manufacturer. There are calculated trade-offs between the different loss mechanisms and safety
constraints that determine how well a transformer performs. This is an iterative process and can ultimately
result in the choice of a new core or switching frequency range. The previous steps should reduce the
number of iterations significantly but a good transformer manufacturer is invaluable for completion of the
process.
size can be determined using the vendor’s specifications and recommendations. Note that there are core
materials that can tolerate higher flux densities; however, they are usually more expensive and not
practical for these designs. The rest of the transformer design can be done with the aid of the
manufacturer. There are calculated trade-offs between the different loss mechanisms and safety
constraints that determine how well a transformer performs. This is an iterative process and can ultimately
result in the choice of a new core or switching frequency range. The previous steps should reduce the
number of iterations significantly but a good transformer manufacturer is invaluable for completion of the
process.
Clamp
shows a large ringing (V
RING
) on the SW FET drain due to the leakage inductance of the
transformer and output capacitance of SW FET.
19
SNOA554C – October 2011 – Revised May 2013
AN-2090 LM3448 -120VAC, 6W Isolated Flyback LED Driver
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