42007 Datasheet, PDF (1/6 Pages) Fairchild Semiconductor – Safety, EMI and RFI Considerations

June 1996
Application Note 42007
by Daryl Sugasawara
and Jens Paetau
Safety, EMI and RFI Considerations
INTRODUCTION
The filtering of conducted and radiated noise is an
intricate part of the design of a power supply or DC to DC
converter. This Application Note addresses the origin of
generated EMI and RFI and the best ways to control them.
The topics discussed will be:
s Background on Safety Agencies and Documents
s EMI filter design for higher power Switch Mode
Power Supplies
s EMI sources and ways to control them
s PCB layout considerations
s EMI and RFI Filter considerations for low power
boost converters; ML4861 design example
SAFETY AGENCIES AND BACKGROUND
The main objective of the national and international safety
regulatory agencies is to provide the user with a safe and
quality product which is not going to interfere with other
electronic equipment. These safety agencies and standards
are different depending on the country they originated.
Equipment manufacturers that would like to sell their
product in these countries first need to get their product
approved by the corresponding safety agencies. Most
power supply manufacturers use the IEC (International
Electro Technical Commission), VDE (Verband Deustcher
Electrotechnik), or UL (Underwriters Laboratories) and
CSA (Canadian Standards Association) as their base to the
majority of the world’s safety requirements. Regarding the
radiated and conducted interference levels acceptable in
VDE 0871 FREQUENCY
100
FCC Class A
80
VDE 0871/A,C
VDE 0875/N
60
VDE 0871/B
VDE 0875/N-12
40
FCC Class B
20
0.01
0.1
1.0
10
100
FREQUENCY (MHz)
the United States and internationally are FCC’s Docket
20780 and VDE’s 0806. One point to consider is that
both, FCC and VDE standards exclude sub assemblies
from compliance to these rules. This is understandable
since power supply radiation and conduction
characteristics can vary depending on different system
level loading. Therefore the final product, where the
switching power supply is to be used, must also comply to
EMI and RFI specifications. Both agencies require
manufacturers to minimize the radiated and conducted
interference of their equipment which is connected to the
AC mains and employs high frequency digital circuitry.
VDE has subdivided its RFI regulations into two categories
a) 0-10KHz unintentional high frequency generation
(VDE 0875, VDE 0879)
b) 10KHz-30MHz intentional high frequency
generation (VDE 0871, VDE 0872)
The FCC includes all electronic devices which generate
signals at a rate greater than 10KHz. The FCC and VDE
regulations closely follow each other. The FCC class A
specification covers business, commercial and industrial
environments, while FCC B covers residential
environments only. The main difference as can be seen in
figure 1 is the frequency span covered by both agencies.
The VDE frequency range for EMI and RFI emissions
covers a spectrum from 10KHz to 30MHz, while FCC’s
frequency span covers only the range of 450KHz to
30MHz.
EMI FILTER DESIGN IN SWITCH MODE
POWER SUPPLIES
How is EMI generated? Technically, EMI is generated by a
varying electric or magnetic field and transmitting them by
means of conductive, inductive or capacitive coupling,
through free space or a combination of these means.
Switching power supplies are one of the worst sources for
EMI and RFI generation because of their inherent current
voltage waveforms and very fast switching times.
Switching transistors, MOSFETs, diodes, transformers, and
inductors are the main source of RFI generation. The
common mode noise generated by switching is a problem
in large computer systems and can be controlled with an
input filter between line, neutral and chassis. The
differential noise, like transient response, is a function of
the output filter capacitors and filter chokes.
Figure 1. FCC and VDE Compliance Curves
1
REV. 1.0 10/25/2000