Datasheet

L1
C6
R6
V+
LINE
NEUTRAL
C5
L2
R4
R12
C8
R5
D2
R9
C7
VR1
F1
C9 C10
R10
C4
Design Guide
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where, V
Z
is the zener diode threshold, N
A
and N
S
are the number of transformer auxiliary and secondary
turns respectively, and V
OVP
is the maximum specified output voltage.
12.7 Input Filter
Background
Since the LM3448 is used for AC to DC systems, electromagnetic interference (EMI) filtering is critical to
pass the necessary standards for both conducted and radiated EMI. This filter will vary depending on the
output power, the switching frequencies, and the layout of the PCB. There are two major components to
EMI: differential noise and common-mode noise. Differential noise is typically represented in the EMI
spectrum below approximately 500kHz while common-mode noise shows up at higher frequencies.
Figure 35. Input EMI Filter
Conducted
Figure 35 shows a typical filter used with this LM3448 flyback design. In order to conform to conducted
standards, a fourth order filter is implemented using inductors and "X" rated AC capacitors. If sized
properly, this filter design can provide ample attenuation of the switching frequency and lower order
harmonics contributing to differential noise. A "Y" rated AC capacitor (C1) from the primary ground to the
secondary ground is also critical for reduction of common-mode noise (refer to "Evaluation Board
Schematic". This combination of filters along with any necessary damping can easily provide a passing
conducted EMI signature.
Radiated
Conforming to radiated EMI standards is much more difficult and is completely dependent on the entire
system including the enclosure. C1 will also help reduce radiated EMI; however, reduction of dV/dt on
switching edges and PCB layout iterations are frequently necessary as well. Consult available literature
and/or an EMI specialist for help with this. Several iterations of component selection and layout changes
may be necessary before passing a specific radiated EMI standard.
Interaction with Dimmers
In general input filters and forward phase dimmers do not work well together. The TRIAC needs a
minimum amount of holding current to function. The converter itself is demanding a certain amount of
current from the input to provide to its output, and the input filter is providing or taking current depending
upon the dV/dt of the capacitors. The best way to deal with this problem is to minimize filter capacitance
and increase the regulated hold current until there is enough current to satisfy the dimmer and filter
simultaneously.
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AN-2091 LM3448 - 230VAC, 6W Isolated Flyback LED Driver SNOA555CApril 2011Revised May 2013
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