翻译原文-传导发射与传导敏感度

CHAPTER SIX Conducted Emissions and Susceptibility In this chapter we will investigate the mechanism by which emissions are generated and are conducted out of the product along the products ac power cord. Regulatory agencies impose limits on these conducted emissions because they are placed on the commercial power system net of the installation. The commercial power distribution system in an installation is a large array of wires connecting the various power outlets from which the other electronic systems in the installation receive their ac power. It therefore represents a large “antenna” system from which these conducted emissions can radiate quite effi ciently, causing interference in the other electronic systems of the installation. Thus the conducted emissions may cause radiated emis- sion, which may then cause interference. Ordinarily, the reduction of these con- ducted emissions is somewhat simpler than the reduction of radiated emissions since there is only one path for these emissions that needs to be controlled: the units power cord. However, it is important to realize that if a product fails to comply with the limits on conducted emissions, compliance with the limits on radiated emissions is a moot point! Therefore controlling conducted emissions of a product has equal priority with the control of radiated emissions. Once again, manufacturers of electronic products realize that simply complying with the regulatory limits on conducted and radiated emissions does not represent a complete design from the standpoint of EMC. A product must be reasonably insen- sitive to disturbances that are present on the power system net in order to ensure reliable operation of the product. For example, lightning may strike the power trans- mission lines that feed power to the installation. This may cause disturbances that range from a complete loss of commercial power (which no product is expected to withstand) to momentary power loss due to power system circuit breakers attempting to reclose (which a product is expected to withstand without loss of Introduction to Electromagnetic Compatibility, Second Edition, by Clayton R. Paul Copyright # 2006 John Wiley (b) a two-wire product. CONDUCTED EMISSIONS AND SUSCEPTIBILITY 9 the product into the correct holes of the power outlet. If the consumer plugged the product into the wrong holes of the power outlet, the chassis would be “hot” with respect to earth ground, setting up a clear shock hazard. Two-wire products invari- ably combat this problem by placing a 60-Hz transformer at the power entrance of the product, as shown in Fig. 6.5b. The chassis may be tied to the secondary side of the transformer, and would therefore not be directly connected to either the phase or the neutral conductor. The elimination of the green wire in this type of product is frequently thought to eliminate common-mode currents. This not necess- arily true, for reasons illustrated in Fig. 6.5b. Stray capacitances between the product chassis and the metallic walls of the test site act to provide the equivalent green-wire path back to the LISN (which must be bonded to the ground plane of the test site). Any common-mode voltage between the electronics of the product and the product frame will tend to drive these common-mode currents through this path. Stray capa- citances between primary and secondary of the transformer also exist. 6.2POWER SUPPLY FILTERS There are virtually no electronic products today that can comply with the conducted emission regulatory requirements without the use of some form of power supply fi lter being inserted where the power cord exits the product. Some products may appear not to contain a fi lter when there is, in fact, one present. An example is the use of a large 60-Hz transformer at the power input of the product in a two- wire product or when using a linear power supply. Properly designed transformers can provide inherent fi ltering, and so can, in some cases, obviate the need for an “intentional” fi lter. We will concentrate on the design of intentional power supply fi lters in this section. 6.2.1Basic Properties of Filters We will begin with a discussion of general fi lter properties. Electric fi lters occur throughout all branches of electrical engineering such as communications, signal processing, and automatic controls. There is a wealth of design information avail- able for these types of fi lters. The reader is cautioned that power supply fi lters that are intended to reduce conducted emissions are rarely designed using these traditional fi lter designs. Nevertheless, a discussion of these basic principles of the traditional fi lters serves a useful purpose in the illumination of certain basic principles that are common to all fi lters. Filters are typically characterized by their insertion loss (IL), which is typically stated in dB. Consider the problem of supplying a signal to a load as shown in Fig. 6.6a. A fi lter is inserted between the source and the load in order to prevent certain frequency components of the source from reaching the load, as shown in Fig. 6.6b. The load voltage without the fi lter inserted is denoted by VL,woand the load voltage with the fi lter inserted is denoted as VL,w, The insertion loss of the 6.2POWER SUPPLY FILTERS 10 fi lter is defi ned as ILdB 10 log10 PL,wo PL,w ? 10 log10 V2 L,wo=RL V2 L,w=RL ! 20 log10 VL,wo VL,w ? (6:7) Note that the voltages in this expression are not denoted with a caret (), and are therefore the magnitudes of the voltages. The insertion loss gives the reduction in the load voltage at the frequency of interest due to the insertion of the fi lter. Typi- cally, the insertion loss is displayed as a function of frequency. Some simple fi lters are shown in Fig. 6.7. These can be analyzed using the tech- niques discussed in Chapter 5. For example, let us determine the insertion loss of the simple lowpass fi lter of Fig. 6.7a. The load voltage without the fi lter can be easily determined from Fig. 6.6a as VL,wo RL RS RL VS(6:8) FIGURE 6.6 Defi nition of the insertion loss of a fi lter: (a) load voltage without the fi lter; (b) with the fi lter inserted. CONDUCTED EMISSIONS AND SUSCEPTIBILITY 11 The load voltage with the fi lter inserted is VL,w RL RS jvL RL VS RL RL RS 1 1 jvL=(RS RL) VS(6:9) The insertion loss is the ratio of (6.8) and (6.9): IL 20log101 jvL RS RL ? ? ? ? ? ? ? ? 20log10 ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi 1 (vt)2 q ? 10log101 (vt)2?(6:10) where t L RS RL (6:11) is the time constant of the circuit. A plot of the insertion loss would show 0 dB from dc to the 3 dB point ofv3dB 1/tand an increase at a rate of 20 dB/decade above that. Therefore the lowpass fi lter passes frequency components of the source from dc tov3dBand increasingly reduces the components at frequencies above that. For FIGURE 6.7 Four simple fi lters: (a) lowpass; (b) highpass; (c) bandpass; (d) bandreject. 6.2POWER SUPPLY FILTERS 12 frequencies above the 3 dB point the insertion loss expression simplifi es to IL ffi 10log10(vt)2?,v? 1 t 20log10vt 20log10 vL RS RL ? (6:12) Other fi lters are analyzed in a similar fashion. The example above has illustrated an important point: The insertion loss of a par- ticular fi lter depends on the source and load impedances, and therefore cannot be stated independently of the termination impedances. Most fi lter manufacturers provide frequency response plots of the insertion loss of a particular fi lter. Since the insertion loss of a fi lter is dependent on the source and load impedances, what value of source and load impedance is assumed in these specifi cations? The answer is rather obviousit is assumed that RS RL 50 V! This brings up another important point; how does this specifi cation of insertion loss based on 50 V source and load impedances relate to the fi lters performance in a conducted emission test? Consider the use of the fi lter in that test. The “load impedance” cor- responds to the 50 V impedances of the LISN between phase and green wire and between neutral and green wire. However, in a typical installation RLis the impe- dance seen looking back into the power distribution net. It is highly doubtful that this is 50 V! What is the “source impedance RS” in this useage? The answer is that we do not know, since this is the source impedance seen looking back into the products power input terminals. It is doubtful that this will be 50 V and further- more that it will be constant over the frequency range of the conducted emission test! So use of the manufacturers insertion loss data toassess the performance ofthe fi lter in a product may not give realistic results in a typical application. Furthermore there are two currents that must be reduced: common-mode and differential-mode. Filter manufacturers typically give separate insertion loss data for these currents. These data are obtained as shown in Fig. 6.8. For the differen- tial-mode insertion loss measurement the green-wire terminals are left unconnected and the phase and neutral wires form the circuit to be tested, as shown in Fig. 6.8a, since the differential-mode current, by defi nition, fl ows out the phase wire and returns via the neutral wire, and no differential-mode current returns on the green wire. For the common-mode test, the phase and neutral wires are tied together and form the test circuit with the green wire, as shown in Fig. 6.8b. Once again, the source and load impedances for each test are assumed to be 50V. 6.2.2A Generic Power Supply Filter Topology The most common power supply fi lter topology is some version of the generic fi lter topology shown in Fig. 6.9. The reader should note that this fi lter topology resembles CONDUCTED EMISSIONS AND SUSCEPTIBILITY 13 a Pi structure. The differential- and common-mode currents at the output of the product (usually the input to the products power supply) are denoted as ID and IC , whereas these currents at the input to the LISN (at the output of the fi lter) are denoted with primes asI0 Dand I0 C . The object of the fi lter is to reduce the unprimed FIGURE 6.8Insertion loss tests: (a) differential mode; (b) common mode. FIGURE 6.9 A typical power supply fi lter topology. 6.2POWER SUPPLY FILTERS 14 current levels to the primed levels such that the primed currents I0 D and I0 C give measured voltages VP 50(I0 C I0 D) (6:13a) VN 50(I0 C? I0 D) (6:13b) which are below the conducted emission limit at all frequencies in the frequency range of that limit. 6.2.3Effect of Filter Elements on Common- and Differential-Mode Currents A green-wire inductor LGW is included in the green wire between the fi lter output and the LISN input to block common-mode currents as discussed above. Capacitors between phase and neutral wires, CDLand CDR, are included to divert differential- mode currents. These are referred to as line-to-line capacitors. Capacitors that have insulation properties approved by safety agencies and are suitable for use as line-to-line capacitors are referred to as “X-caps.” The subscripts L and R denote “left” and “right” with regard to the side of the fi lter on which they are placed. Capacitors CCLand CCRare included between phase and green wire and between neutral and green wire to divert common-mode currents. These are referred to as line-to-ground capacitors. Capacitors that have insulation properties approved by safety agencies and are suitable for use as line-to-ground capacitors are referred to as “Y-caps.” The reason that different capacitors are needed for these tasks is due to safety considerations. For example, suppose that one of the line-to-ground Y-caps shorts out. If this capacitor happens to be connected to the phase wire, 120 V will be tied to the green wire, which is usually tied directly to the product frame, presenting an obvious shock hazard. Also, the safety agencies such as the Underwriters Laboratory (UL) in the United States specify the maximum leakage current that may fl ow through the line-to-ground capacitors at 60 Hz in order to minimize shock hazards due to these leakage currents. This provides an important constraint on the maximum value of the line-to-ground capacitors that may be used in the fi lter. Review Exercise 6.1Determine the maximum line-to-ground capacitance to satisfy a leakage current requirement of 150 mA. Answer: 3316 pF. Some fi lters include only the capacitors on the left or on the right, and some include both sets. Still other fi lters may, for example, include only CDLand CCR and omit CDRand CCL. Typical values of these capacitors are CD ffi 0:047mF and CC ffi 2200pF. Observe that the line-to-ground capacitors on the left, CCL, are in CONDUCTED EMISSIONS AND SUSCEPTIBILITY 15 parallel with the 50-V resistors of the LISN. Therefore, if their impedances at the frequency of interest are not signifi cantly lower than 50 V, then these capacitors will be ineffective in diverting the common-mode current. To judge whether the line-to-ground capacitors on the left will be effective, let us compute their impe- dance for typical values of CCL 2200 pF. The impedances of these capacitors will equal 50 V at 1.45 MHz, and so the capacitors CCLwill be effective in diverting common-mode currents from the LISN 50-V resistors only above this frequency! One fi nal element is typically includedthe common-mode choke represented by the coupled inductors. The self-inductances of each winding are represented by L and the mutual inductance is represented by M. Typically this element consists of two identical windings on a common ferrite core (that has suitable characteristics over the conducted emission frequency range), and so is similar to a transformer, as shown in Fig. 6.10a. Because the windings are identical and are wound tightly on the same core, the mutual inductance is approximately equal to the FIGURE 6.10Use of a common-mode choke to block common-mode conducted emissions: (a) physical construction and equivalent circuit; (b) equivalent circuit for differential-mode currents; (c) equivalent circuit for common-mode currents. 6.2POWER SUPPLY FILTERS 16 self-inductance, L ffi M, and as such has a coupling coeffi cient approaching unity: k M ffiffi ffiffi ffiffi ffiffi ffiffi L1L2 p ffi M L ffi 1(6:14) The purpose of the common-mode choke is to block common-mode currents. Ideally, the common-mode choke does not affect differential-mode currents. This was shown in Chapter 5, but it is worthwhile to repeat that here. Consider only differential-mode currents through the choke, as shown in Fig. 6.10b. Computing the voltage drop across one side of the choke gives V jvLID? jvMID jv(L ? M)ID(6:15) Therefore the element inserts an inductance L M in each lead with regard to differ- ential-mode currents. This is commonly referred to as the leakage inductance, and is due to the portion of the magnetic fl ux that leaks out the core and does not couple between the windings. Ideally this is zero, and the common-mode choke has no effect on differential-mode currents. We will see that this leakage inductance is not zero for actual chokes, and has an important role in the blockage of the differential-mode currents. Now consider the effect of the choke on common- mode currents, shown in Fig. 6.10c. Computing the voltage drop across one side of the choke gives V jvLIC jvMIC jv(L M)IC(6:16) Therefore the element inserts an inductance L M in each lead with regard to common-mode currents. Consequently, the common-mode choke tends to block common-mode currents. Typical values for the inductance are of order 10 mH. Thus the common-mode current impedance is jv(L M) 18,850 V at 150 kHz and 3.77 MV at 30 MHz. It is important to emphasize that these are ideal values. Parasitic capacitance between the windings as well as the type of core material strongly infl uence the frequency behavior of the choke. In addition, we have shown the frequency-dependent resistance, R(f), which is also presented to the common-mode currents by a ferrite core. It is important to reemphasize another important characteristic of the common- mode choke. In addition to the noise signal in the differential-mode current, there CONDUCTED EMISSIONS AND SUSCEPTIBILITY 17 is another componentthe high-level, 60 Hz power current. Typically this will be several amps. Currents of this level will easily saturate a ferrite core, and will there- fore reduce its permeability to values approaching that of air. The ability of the choke to block common-mode currents relies on large values of L and M being obtained, which in turn relies on having a large value of the relative permeability of the core. If the core material were saturated by the high-level, 60 Hz current, we would not obtain suffi ciently large inductance to provide blocking of the common-mode currents. On the other hand we do not wish to drop much of the power voltage across the choke. The fact that the differential-mode current fl ux tends to cancel in the core because of the way the windings are wound on the core means that, ideally, the choke does not provide any impedance to differen- tial-mode currents, and the choke is transparent to these currents (even at 60 Hz). Therefore the 60 Hz fl ux cancels in the core and does not saturate it. This is an additional benefi t of a common-mode choke. Let us now develop some equivalent circuits to represent the effect of the fi lter on the common- and differential-mode currents. We will assume a fi lter that is sym- metric with regard to phase and neutral. By this it is meant that the phasegreen- wire circuit and the neutralgreen-wire circuit are identical. This is true for the generic fi lter shown in Fig. 6.9 in that the line-to-ground capacitors between phase and the green wire are identical to those between neutral and green wire, and the self-inductances of both sides of th