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EMI Due to Electric Field Coupling on PCB Title EMI due to electric field coupling on PCB Author(s) Pong, MH; Lee, CM; Wu, X Pesc Record - Ieee Annual Power Electronics Specialists Citation Conference, 1998, v. 2, p. 1125-1130 Issued Date 1998 URL http://hdl.handle.net/10722/46075 Rights Creative Commons: Attribution 3.0 Hong Kong License EM1 Due to Electric Field Coupling on PCB M.H. Pong, C.M. Lee and X. Wu Department of Electrical & Electronic Engineering, Hong Kong University Pokfulam Road, HONG KONG Fax : +852-25598738, Phone : +852-28597099 E-mail : [email protected] ABSTRACT- In switching converter circuits, EM The effect is often suppressed by circuit isolation or noise can couple between PCB traces through the filtering. Non-conducted coupling comprises of far effect of electric field coupling. An experiment us- field coupling and near field coupling. Far field is also ing a flyback converter verifies the severity of this known as antenna effect by which noise radiates meters effect. Further experiments and field plots confirm away from the source. Near field includes electric field that a good PCB layout can significantly reduce coupling and magnetic field coupling in close vicinity. conducted EM1 due to unintentional E-field cou- In SMPS, electric field coupling has considerable pling. contribution to overall EM1 especially those circuits with high voltage transients. In practice, power elec- 1. INTRODUCTION tronics designers use filter and circuitry approach to suppress EMI. However, near field electric coupling Engineers in the field of switch mode power sup- between PCB traces in these circuits may by-pass fil- ply (SMPS) often encounter the problem of electro- ters. This paper proposes an alternative approach to magnetic interference (EMI). The switching nature of analyze such noise coupling effect. Electric field cou- SMPS make it an inevitable source of Electromagnetic pling between PCB traces is carefully examined. Ex- Interference (EMI). More and more countries have periments on a flyback converter circuit confm the established regulations against products that generate severity of near field coupling. Electric field emission EMI. Engineers and researchers are striving to reduce from PCB traces with elementary shapes are measured EM1 by various methods. and examined. The flyback converter is then re-layout From experience it is well known that circuit lay- after considering the effect of E-field coupling. Signifi- out design on a printed circuit board (PCB) plays a cant reduction in conducted noise is observed. very important part in reduction of EMI. A good circuit layout can very often solve EM1 problems without any Noise changes in circuit topology or components in the cir- Coupling I cuit. However, there is very little information available I I about the EM1 performance of a SMPS until the circuit Conducted Cou- Non-conducted is fully laid out on a PCB and tested. If EM1 issues are pling Coupling ignored until a problem is revealed by testing or in I normal operation, EM1 fixes tend to be applied at the Near Field test or even production stages of product development, """I -1 which can lead to solutions that are unsatisfactory, un- necessarily expensive, or both. Every EM1 problem comprises three elements: a source, a receiver and a mechanism by which noise is coupled from the source to the receiver as in Fig. 1. A Fig.2. Classification of Noise Coupling source refers to a noise generator with high dv/dt or Mechanism di/dt, and the receiver refers to a victim circuit or EM1 measurement equipment. 2. VERIFICATIONOF ELECTRICFIELD Coupling On PCB P Source A typical switch mode power supply contains four basic building blocks as shown in Fig.3 : 1. input filters and rectifier; 2. high-frequency inverter; Fig. 1. Basic Elements in EM1 Problems 3. output section; 4. control circuit. Coupling mechanism can be categorized into con- The input filters and rectifier rectifies and ducted coupling and non-conducted coupling (Fig.2). smoothes the a.c. voltage input from the power source. Conducted coupling refers to interference along a con- The voltage is then fed to the high frequency inverter ducting path that links up the source and the receiver. by which the line voltage is chopped to a square pulse 0-7803-4489-8/98/$10.000 1998 IEEE 1125 train and the period of pulses is controlled by a The control block is separated from the power circuit switching transistor. A transformer steps down the pul- because the low voltage controller is considered as a sating voltage to predefined level before going to be victim instead of a source. A screened cable links the the output section. The output section comprises of power transistor and the control circuit. The PCB lay- rectifiers and filters of various topologies, the major out is constructed in a 100" x 100" square board function of which is to convert the pulsating voltage to as shown in Fig.5. a d.c. voltage. The experiment is set up as shown in Fig.6. A Line Impedance Stabilization Network (LISN) connects High frequency between the a.c. mains and the input of the converter. Input rectifier inverter output A spectrum analyzer measures the high frequency section and filter noise emitted from the converter. This is the standard setup for conducted EM1 measurement. T AC 4 Mains Input 4 Flyback Converter circuit Spectrum Fig.3. Basic Building Blocks of SMPS Analyzer An experiment setup is constructed to verify the Fig.6. Experimental Setup of Conductor Noise severity of near field electric coupling. In this experi- Measurement ment, a typical flyback converter is chosen for EM1 measurement. The flyback converter comprises the Conducted EM1 measurement in Fig.S(a) shows four basic building block of a SMPS and the circuit the noise emission level exceeds the FCC limit of 48 design is shown in Fig.4. dBpV in frequency range 450 kHz to 30 MHz. In the next step the common choke CM1 is re- moved from the PCB assembly (Fig.7). This leaves an open circuit between the input surge suppresser RV1 and the bridge rectifier. Power is fed into the rectifier directly while the LISN is connected to the input pads as the previous setup. By circuitry approach, an open circuit represents a perfect filter as there is no con- ducting path for transmission of conducted noise. Fig.4. Schematic of Flyback Converter Noisy Trace I Fig.7. Conducted Noise Measurement after Circuit Isolation The measurement should be predicted for a very low noise level at the LISN as all conducted noise should have been curbed. However measurement re- veals results in a converse way. The noise level is still U1 high and outside the acceptance range of the FCC limit (see Fig.g(b)), although it has dropped compared to the measurement in Fig.X(a). The noise measured should mainly consist of non- Fig.5. PCB Layout of Flyback Converter conducted electric field coupled noise. Since the LISN 1126 is disconnected fiom the switching circuit, there should be no conducted coupling noise present in the spec- 1 2 trum. The two remaining coupling mechanisms are the categories of far field and near field non-conducted coupling. However the effective range of the far field coupling extends meters away from the circuit. So near field coupling should have contributed a very signifi- cant amount. Both electric and magnetic coupled noise appear in the noise spectrum. High dv/dt of power de- vices make electric field the dominate noise source. -1, Fig.9. Electric Field Coupling Model PEAK ................ In most practical cases, LUG ............ .. ....................................................................................... : ......... dR/ ............ VL33.0 dRpV and (1) can be simplified to ............................................................................................ .... ................ .............. ... STAKT 468 kHz STUP o8.w tin1 V, = jwRC,,V, (3 1 NRES an 9.0 knz UBU 30 kHr 3UP 1.09 SIC From equation (3) Cl2is an important factor. It (a) Normal Setup represents the effect of circuit layout in terms of elec- tric field coupling. In practice C,, is rather tedious to be derived analytically, especially in complicated PCB ......... PEflK ......... layout. Numerical method is more favorable in calcu- LOG ............................................................................................................ 10 ......... lating the coupling voltage Vn. dB/ ......... 4. ELECTRICFIELD PLOTS In order to examine the source of electric field in the flyback circuit discussed in section 2, an EM1 scan- ................................................................................................. ner system is used. The electric field close to the PCB ..........: 1: iJ ,*............. .. is plotted out and shown in Fig.10. The area of high START 458 fHz STEP ~6.~0wiz #RE8 EM 9.0 kHs 1‘BM 30 kHz *UP 1.03 sec emission concentrates on the primary side where there is high voltage swing of several hundred volts and high (b) Common Mode Choke is Removed dv/dt. In particular the trace connecting the drain of the switching MOSFET M1 has high emission [4] and this Fig.8. Spectrum of Conducted Noise Measured trace is marked up by an arrow in Fig. 10. The “noisy” trace shown in Fig.10 is likely to in- 3. ELECTRICFIELD COUPLING MECHANISM terfere conducted emission measurement at the power input terminals. In the layout shown in Fig.10, the, Electric field coupling between PCB traces can be “noisy “ trace is placed rather near to the input termi- represented by the model [l] shown in Fig. 9. Noise nals. Also it runs over considerable length across the source Vi is connected to conductor 1 and interference board and is likely to induce electric field coupling. noise couples to conductor 2 through capacitor C,,. The Note that by this layout electric field coupling can by- noise voltage picked up by conductor 2 is represented pass the filter elements in the primary circuit and di- by rectly links the “noisy trace” and the input terminals.
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