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NCP1217D133R2
1A 600V Ultrafast Rectifier
AND8032/D
Conducted EMI Filter
Design for the NCP1200
Prepared by: Christophe Basso
ON Semiconductor
INTRODUCTIONIf the NCP1200 easily lends itself to designing
Switch–Mode Power Supplies (SMPS) in a snap–shot, its
presence among other equipments necessitates an adequate
ElectroMagnetic Interference (EMI) input filter.
Fortunately, friendly methods such as SPICE exist to isolate
the guilty harmonics and make them stay below the limit.
This application note briefly describes the mechanism of
conducted EMI noise generation and depicts a way to
circumvent it.
The Bulk Capacitor is a Natural ShieldAs Figure 1a depicts, an SMPS is supplied by a network
made of a diode bridge rectifier and a bulk capacitor. Every
time the mains peak voltage exceeds the bulk capacitor
level, two diodes of the input bridge conducts for a period of
time given by the DC rail ripple: the larger the ripple, the
longer the conduction time (≈ 1–3 ms). As a result, Cbulk is
recharged
mains) while the SMPS load draws pulses at its high
switching frequency. During the refueling by the diodes,
Cbulk stores energy and delivers this energy during the time
the diodes are off (high series resistance, but still weakly
capacitive).
Input Bridge
Figure 1a. A typical component arrangement for an SMPS (here a FLYBACK).The current flowing through Cbulk is thus the ON time
drain current for a FLYBACK converter. Cbulk is
unfortunately not a perfect element and Figure 1a can be
refined into Figure 1b where parasitic elements appear: the
Equivalent Series Inductance (ESL) and the Equivalent
Series Resistor (ESR).
Input Bridge
DC Rail
