Conducted Interference

Transcription

1 Conducted Interference Tim Williams Elmac Services 1 This presentation discusses the nature of conducted interference with particular respect to the requirements of present-day commercial test standards. Equivalent circuits are presented and analysed so that the principle modes and routes of coupling can be understood in the general case. These principles can then be applied to a particular design in order to diagnose and fix interference coupling problems. 1

2 Conducted coupling Conducted RF emissions Mains Port Mandatory Signal Ports Occasional Conducted RF immunity Conducted transient immunity Usual Mandatory Usual Usual 2 Conducted interference can be classified according to whether it is incoming or outgoing, and whether it is coupled via the mains port or via signal ports. The vast majority of CISPR- and IEC-based EMC test standards require testing of all phenomena on the mains port; many require immunity testing on signal ports and more tests are being proposed. CISPR 22, for instance, now requires conducted RF emissions testing on telecommunications ports. 2

3 Mains emissions: equivalent circuit EUT L LISN V DM N V CM E 3 The basic equivalent circuit for conducted emissions testing on the mains port is shown here. The mains connection is represented by the AMN/LISN giving a defined RF impedance between live and earth, and between neutral and earth. The EUT contains both differential and common mode sources, generalised here as appearing in one case between live and neutral, and in the other case between both live and neutral with respect to earth. The connection to the mains is made via a length of cable which should also be included in the model for best accuracy, but is not included in this discussion. If the apparatus is Safety Class II there is no earth wire, but common mode signals can still return via the stray capacitance to the ground plane. 3

4 Differential mode sources EUT L Z DM V DM N Differential mode source voltage and impedance Typical source: switching current develops voltage across SMPS DC link 4 Differential mode sources appear between live and neutral connections without reference to the earth connection. In circuits with switch-mode power supplies or other power switching circuits the RF emissions are dominated by interference developed across the DC link to the switching devices. Although there will normally be a reservoir capacitor, the high di/dt through this capacitor will generate voltages at the harmonics of the switching frequency across its equivalent series impedance. Diode noise, if it is significant, will also appear in differential mode. 4

5 Common mode sources EUT Circuit PS V CM Common mode source voltage and impedance Typical sources: SMPS dv/dt and circuit noise via stray capacitance 5 Common mode sources are more complex. The common mode voltage appears between both live and neutral with respect to earth. Since the mains input is normally isolated from earth, it is usual for common mode coupling to be capacitive. The coupling is dominated by the interwinding capacitance of the isolating transformer and the stray capacitances of noise sources, both in the power supply (e.g. from heatsinks) and the operating circuit. These capacitances are referred to earth, either directly or via the enclosure if this is conductive. A well-shielded enclosure will minimise "leakage" of this capacitive coupling and hence reduced conducted emissions. Other impedances may appear in the coupling path: for instance the leakage inductance of the isolating transformer is in series with its interwinding capacitance and may give a series resonant peak in the MHz range. 5

6 The AMN/LISN 50µH V DM = V N - V L Mains supply to EUT 50Ω V N V L V CM = V N + V L 5Ω 6 The Line Impedance Stabilising Network (LISN) forms part of the overall equivalent circuit for testing. Its characteristics are defined in CISPR 16-1, which requires an impedance of 50Ω in parallel with 50µH + 5Ω between each phase (live and neutral) and the earth reference point. This reference is connected to the earth reference plane of the test setup. Voltages are measured across each line with respect to the earth reference and the higher voltage gives the test result. This test does not measure either common or differential mode directly, but each can be obtained by summing or differencing the two lines. 6

7 The mains filter 2 x C L CM C Y X C X filters differential mode C Y, L CM filter common mode 7 Application of a mains filter is usual in most cases of mains conducted interference. The typical mains filter includes components to filter both differential mode (C X ) and common mode (C Y, L CM ). Although a mains filter does not have an "input" or "output", it should be installed in the correct sense. Normally, the Y-capacitors will face the equipment in order to provide a capacitive divider with the common mode source capacitance. The X-capacitor will be more effective on the mains side of the choke, in order to take advantage of its leakage inductance. In general, capacitors should face the higher impedance and inductors should face the lower impedance. 7

8 Mains immunity: equivalent circuit Coupling/decoupling network circuit EUT injected interference external connections 8 A similar equivalent circuit can be devised for analysing mains-borne conducted immunity. In all the relevant tests, the interference is injected via a coupling/decoupling network (CDN) into all three of the mains lines, live, neutral and earth. Testing of the earth line is the major difference between these tests and the emissions tests. Common mode coupling is the principle path. The interwinding capacitance of the isolating transformer passes the interference into the operating circuit. Here it generates differential voltages across stray impedances at critical circuit nodes, thereby affecting operation. The return path for the interference current may include stray capacitance to the test earth reference plane, and/or it may return via external connections which are coupled to earth. Filtering all connections to the enclosure to ensure the current bypasses the circuit will provide the optimum solution. 8

9 Transient coupling (IEC ) Fast transient bursts 9 Fast transient burst testing couples the transients into each mains line capacitively via a CDN. The burst generator is referenced to the earth plane. The CDN does not have a well-defined source impedance; the burst generator is specified to be approximately 50Ω. 9

10 RF coupling (IEC ) R tot = 100Ω Modulated RF Z S = 50Ω 10 Conducted RF injection via a CDN on the mains port is very similar to fast transients, but the CDN is specified differently. All lines are tested simultaneously. The source impedance at the EUT port of the CDN is specified as 150Ω with respect to the earth reference plane. Modulated RF then appears at all conductors of the mains cable with this source impedance. The RF current flows through the EUT via both the power supply and the enclosure (if this is earthed), and returns by stray capacitance to the earth plane and by any external connections, which in this test are required to have a stabilised 150Ω impedance to the earth plane. 10

11 Signal port conducted emissions Circuit PS V N I CM EUT mains connection RF current measurement impedance stabilisation 11 Conducted emissions tests at the signal ports are likely to be increasingly required in the future. Various test methods may be applied, but all of these measure the common mode current or voltage being emitted from each port, into a cable whose common mode impedance is stabilised. The noise sources may be modelled as the contribution of all circuit noise referred between the port connection and the earth reference by stray capacitance, possibly mediated by the enclosure if this is conductive. The actual desired signal carried by the port will also contribute if its bandwidth is within the test spectrum and there is conversion of differential mode signal to common mode, either at the port or along the cable. 11

12 Signal port conducted immunity capacitive clamp Circuit PS CDN EUT RF Fast transient bursts 12 Immunity of signal ports is a frequent requirement of many test standards. Either continuous RF or transients may be involved. In either case common mode coupling is the norm. Although the interference is applied in common mode at the port, differences in internal stray impedances will convert this to differential interfering voltages at critical circuit nodes. Good PCB layout will minimise this conversion and hence improve the inherent circuit immunity. Further protection is provided by common mode filtering at each interface to divert the interference currents away from the circuit and into the chassis. The currents then return to the earth reference either directly (if a direct connection exists) or via stray capacitance from the chassis. 12

13 Summary interference may be conducted into or out of apparatus, in differential or common mode, as continuous RF or transients, on either or both of the mains supply and signal ports 13 Conducted interference is dealt with by filtering all appropriate interfaces and by proper shielding. Understanding the equivalent circuits for each coupling case allows the correct implementation of these measures. Tim Williams is with: Elmac Services, PO Box 111, Chichester, West Sussex, PO19 5ZS Tel Fax