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Design Guide

Figure 34. Input EMI Filter

Conducted

Figure 34 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. This combination of filter components 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. Reduction of dV/dt on switching edges and PCB layout iterations are

frequently necessary. 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.

13.4 Inrush Limiting and Damping

Inrush

With a forward phase dimmer, a very steep rising edge causes a large inrush current every cycle as

shown in Figure 35. Series resistance (R5, R6) can be placed between the filter and the TRIAC to limit the

effect of this current on the converter and to provide some of the necessary holding current at the same

time. This will degrade efficiency but some inrush protection is always necessary in any AC system due to

startup. The size of R5 and R6 are best found experimentally as they provide attenuation for the whole

system.

SNOA559B–October 2011–Revised May 2013 AN-2127 LM3448 A19 Edison Retrofit Evaluation Board

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