RET
VREF
+
-
AC
RET
To VAC divider and passFET
LED
Driver
+
-
Y-cap across Xformer
C7
C1
C8
C9
C30
R8
R9
R56
L4
L1
R57
R39
R48
R62
R34
R36
C26
L2
C2
C3
C66
R2
R3
R47
D25
R51
Design Information
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ω
Z1
– Compensator Secondary Zero
ω
P3
– Compensator Primary HF Pole
T(s) – Total Loop Gain
Figure 21. Input EMI Filter
9.10 Input Filter
Background
Since the LM3450A 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.
Conducted
Figure 21
shows a typical filter used with an LM3450A design. To conform to conducted standards, a
fourth order filter (two second order stages) is implemented using shielded inductors (L1, L2, L4), an EMI
suppression X1/X2 film capacitor (C7), and a pulse-rated film capacitor (C1) which is also the primary PFC
capacitor sized previously. In addition to the basic filter components, damping is used to prevent excitation
of the resonant frequencies of the filter itself. The best practice for damping an EMI filter is to use an RC
damper network across each filter capacitor. The C of the damper should be set to be 3 times the filter
capacitor value. This EMI filter, if sized properly, can provide ample attenuation of the switching frequency
and lower order harmonics contributing to differential noise. The filter can be described as follows:
•
Stage 1 pole: L1+L4 and C7 gives 40db/decade roll-off
–
Stage 1 damping: C8||C9||C30 and R8||R9||R56
•
Stage 2 pole: L2 and C1 gives 40db/decade roll-off
–
Stage 2 damping: C2||C3||C66 and R2||R3||R47
Since L1 and L4 are symmetrically placed in both the line and neutral legs of the AC line, they help to
reduce common-mode noise also. It is sometimes necessary to place a high value resistance (R48, R51,
R62) across each inductor to prevent excitation of the SRF of the inductor which is usually at higher
frequencies. A Y1/Y2 film capacitor (C26) from the primary ground to the secondary ground is also
commonly used for reduction of common mode noise.
Radiated
Conforming to radiated EMI standards is much more difficult and is dependent on the entire system
including the enclosure. C26 will greatly 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. It can be a daunting task.
22
AN-2150 LM3450A Evaluation Board
SNVA485B – June 2011 – Revised May 2013
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