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Design Guide

The circuit operation is simple and based on biasing of transistor Q5 during fault conditions such that it

pulls down the voltage on the FLTR2 pin to ground. The bias current depends on how much overdrive

voltage is generated above the zener diode threshold. For proper circuit operation, it is recommended to

design for 4V overdrive in order to adequately bias the transistor. Therefore the zener diode should be

selected based on the expression,

(42)

where, V

is the zener diode threshold, N

and N

are the number of transformer auxiliary and secondary

turns respectively, and V

OVP

is the maximum specified output voltage.

13.7 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 38. Input EMI Filter

Conducted

Figure 38 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 (C13) 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. C13 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.

SNOA554C–October 2011–Revised May 2013 AN-2090 LM3448 -120VAC, 6W Isolated Flyback LED Driver

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