Design Information
www.ti.com
The maximum peak input current, occuring at the minimum AC voltage peak, determines the necessary
flyback transformer energy storage. As a general rule of thumb, the desired duty cycle at this worst-case
operating point should be specified near 0.5 to limit large conduction losses associated with high voltage
diodes. The maximum input current can be approximated by the maximum output power, expected
converter efficiency, and minimum input voltage. Note that there is also a 0.85 multiplier to account for the
fact that maximum power with a triac dimmer in-line is demanded at approximately 85% of the full
sinusoidal voltage waveform. Given the desired duty cycle, the maximum peak input current and
corresponding maximum peak primary current can be approximated:
(12)
Using the calculated turns ratio and the desired minimum switching frequency, the minimum necessary
primary inductance is calculated:
(13)
Switching Frequency Range
Given a primary inductance that meets the above constraint, the variable switching frequency has the
following limits:
(14)
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
L
core value
(160nH/turns2 is a good standard value to start with) 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:
(15)
Given the switching frequency range 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 of the chosen core.
(16)
With most common core materials, the maximum operating flux density should be set between 300mT and
3400mT. 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
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 always practical for these
designs.
12
AN-2150 LM3450A Evaluation Board
SNVA485B – June 2011 – Revised May 2013
Submit Documentation Feedback
Copyright © 2011–2013, Texas Instruments Incorporated
