EMC For Dummies
EMC For Dummies Electromagnetic Compatibility Engineering by Henry W. Ott Designing for EMC The system approach considers EMC throughout the design. EMC is designed into– and not added onto– the product. 90% or more of the potential problems can be eliminated prior to initial testing. JHLin, EMC; EM Compatibility 2 US EMC Regulations JHLin, EMC; EM Compatibility 3 US EMC Regulations FCC A FCC B JHLin, EMC; EM Compatibility 4 US EMC Regulations LISN: Line Impedance Stabilization Network JHLin, EMC; EM Compatibility 5 US EMC Regulations JHLin, EMC; EM Compatibility 6 Typical Noise Path JHLin, EMC; EM Compatibility 7 Methods of Noise Coupling Conductively Coupled Noise – one of the most obvious, but often overlooked, ways to couple noise into a circuit is on a conductor. 1. The solution is to prevent the wire from picking up the noise or to remove the noise from it by filtering before it interferes with the susceptible circuit. 2. The noise conducted into a circuit on the power supply leads. 3. The noise coupled into or out of a shielded enclosure by the wires that pass through the shield. Common Impedance Coupling – occurs when currents from two different circuits flow through a common impedance. 1. This type of coupling usually occurs in the power and/or ground system. JHLin, EMC; EM Compatibility 8 Methods of Noise Coupling 2. In the power distribution circuit, JHLin, EMC; EM Compatibility 9 Use of Network Theory JHLin, EMC; EM Compatibility 10 Signal Grounds To reduce Vg ---- JHLin, EMC; Grounding Zg Ig 11 Signal Grounds Chassis Grounds Chassis ground is any conductor that is connected to the equipment’s metal enclosure. The key to minimizing noise and interference is to determine where and how to connect the signal ground to the chassis. It is important to establish a low-impedance connection between the chassis and the circuit ground in the I/O area of the board. Establishing a low-impedance connection between the circuit ground and the chassis in the I/O area is also advantageous with respect to radio frequency immunity. 12 Balancing JHLin, EMC; Balancing & Filtering 13 Power Supply Decoupling 2. The capacitor should serve as a short circuit across the frequency range over which the amplifier is capable of producing gain. JHLin, EMC; Balacing & Filtering 14 Ferrites Eliminate the interference of lowlevel circuits by the high-f communication noise. JHLin, EMC; Passive Components 15 Ferrites Impedance data for a typical ferrite core. JHLin, EMC; Passive Components 16 Ferrites JHLin, EMC; Passive Components 17 Ferrites Ferrite core used as common-mode chokes on a USB cable to suppress radiated emission JHLin, EMC; Passive Components 18 Apertures The amount of leakage from an aperture depends mainly on the following 3 items: 1. The maximum linear dimension, not area, of the aperture. 2. The wave impedance of the EM field. 3. The f of the field. JHLin, EMC; Shielding 19 Internal Noise Sources Ground noise is created when output of gate 1 switches from high to low. JHLin, EMC; Digital Ckt Grounding 20 Digital Circuit Ground Noise A grid spacing of 0.5 in. or less should be used in order to obtain the most significant reduction of ground noise. Ground grids have been used successfully on double-sided boards at frequencies up to a few tens of MHz. JHLin, EMC; Digital Ckt Grounding 21 Forward Differential-mode radiation from PCB Although these signal loops are necessary for circuit operation, their size and area must be controlled during the design process to minimize the radiation. JHLin, EMC; Digital Ckt Radiation 22 Forward Common-mode radiation from system cables Common-mode radiation is often harder to understand and control. JHLin, EMC; Digital Ckt Radiation 23 Differential-Mode Radiation Assuming the rise time equals the fall time n tr sin( ) sin(n d ) T I n 2 Id n d n tr T For a 50% duty cycle (d=0.5), the first harmonic has an amplitude I1 0.64 I and only odd harmonics are present. JHLin, EMC; Digital Ckt Radiation 24 Differential-Mode Radiation Differential-mode radiated emission envelope vs. frequency To minimize the emission, it is desirable to slow down the rise time of the signal as much as functionally possible. JHLin, EMC; Digital Ckt Radiation 25 Differential-Mode Radiation The spectrum for a 6-MHz, 4-ns rise time, 35-mA clock signal in a 10cm2 loop. JHLin, EMC; Digital Ckt Radiation 26 Controlling Differential-Mode Radiation Board Layout The most critical loops should be individually analyzed; however, most other noncritical loops can be controlled by just using good PCB layout practices. The most critical loops are those that operate at the highest frequency and where the signal is periodic. 27 Controlling Differential-Mode Radiation Frequency spectrum of the third harmonic of a 60-MHz clock with and without dithering Reducing the loop area, or providing canceling loops, only controls the differential-mode emission and has no effect on the commonmode emission. JHLin, EMC; Digital Ckt Radiation 28 Common-Mode Radiation For a short dipole antenna: 4 107 ( f I cm )sin E r (V/m) For a real dipole antenna, the current goes to zero at the open ends of the wire. In practice, a more uniform current distribution can be achieved if capacitor loaded or top-hat antenna: This configuration is approximated when the antenna (cable) connects to another piece of equipment. 29 Common-Mode Radiation The common-mode radiation can be controlled by: 1. Reducing the magnitude of the common-mode current 2. Reducing the frequency or harmonic content of the current 3. Reducing the antenna (cable) length The primary method of minimizing the common-mode radiation is to limit the common-mode current. Common-mode radiated emission envelope vs. frequency JHLin, EMC; Digital Ckt Radiation 30 Controlling Common-Mode Radiation Filtering of the I/O cables can be accomplished by adding a high impedance in series with the common-mode noise (e.g., a common-mode choke or ferrite core), or by providing a lowimpedance shunt (a capacitor) to divert the common-mode noise to “ground.” Separate I/O Grounds The I/O ground plane should have multiple connections to the enclosure to minimize its inductance and provide a low impedance connection. 31
© Copyright 2024