Cross Coupling Between Power and Signal Traces on Printed Circuit Boards

Transcription

1 Cross Coupling Between Power and Signal Traces on Printed Circuit Boards Dr. Zorica Pantic-Tanner Edwin Salgado Franz Gisin San Francisco State University Silicon Graphics Inc. Silicon Graphics Inc Holloway Ave N. Shoreline Blvd., M./S N. Shoreline Blvd., M/S 946 San Francisco, CA Mountain View, CA Mountain View, CA Abstract Switching power supplies and other power switching circuits produce high frequency components in currents on power traces and power grounds in printed circuit boards (PCBs). These time-varying currents can couple energy (noise) to low level signal traces such as those normally encountered with the new low voltage high speed logic families and microprocessors. This can cause the system to malfunction or result in EMI emissions that exceed the levels allowed by regulatory agencies. [l] The effect of proximity and position of power switching circuits with respect to the signal traces will be examined. Sources of DC-DC Converter Noise Many state of the art microprocessors and ASIC integrated circuits are now powered by voltages that are less than the standard 5 VDC. Current technologies operate in the 3-4 volt region, and future devices will operate at voltages in the 2-3 volt range. With this reduced operating voltage, the susceptibility of the signal lines to noise increases. At the same time, these new operating voltages require on-board DC to DC converters that generate a substantial amount of switching noise. This switching noise can couple onto the signal lines, causing the system to malfunction. The noise generated can also have substantial harmonic content that can excite resonant structures within a system causing it to exceed EMC conduced and radiated emission limits. Normally, power is distributed via power and ground planes in printed circuit boards. But because many of the circuits connected to these low voltage microprocessors and ASCII still operate at a nominal 5 volts, a single power plane will not work in the newer dual-voltage systems. Adding an extra low voltage plane adds cost, so this is not always a viable solution. The lowest cost solution is to route the low voltage power for the micro-processors and ASICs through wide traces on the same layers that contain signals, which increases the coupling efficiency between the power traces and adjacent signal traces. In such cases, one can use traditional trace to trace crosstalk coupling models to analyze the problem. [2] As an example, consider the typical printed circuit board DC- DC converter shown in Figure 1. The radiated emissions from this converter can couple onto I/O traces that in turn are connected to long interface cables routed external to the shielded enclosure. If a sufficient amount of common mode noise is coupled onto these traces, the resultant emissions can exceed allowable limits. Referring to Figure 1, one can see that there is a noticeable resonance effect at about 170 MHz where the emissions are db higher. The noise generated by this DC-DC converter is shown in Figure 2. The switching frequency, F,, is about 1.2 MHz ( period, T = 833 nsec). Figure 3 shows an expansion in time of a single impulse, showing it to be a ringing type wave form that can be approximated by a damped sinusoid. Modeling the coupling Mechanism Figure 4 shows the model that was used to analyze different parameters of a representative coupling configuration. A power trace and two signal traces are located over a ground plane. The default parameters of the model are as follows: Substrate Thickness: Power Trace Width: Sig Trace Widths: Separation (power to T 1): Separation (Tl to T2): Dielectric Constant: 4.5 Unless otherwise stated, the termination impedances on all traces were matched to the applicable trace characteristic impedance. FDTD and MOM modeling methods were used to analyze the structure to determine what effects different physical and electrical parameters had on the amount of noise coupled from the power trace to the signal traces.[3] [4] Figure 5 shows the effect that termination impedance has on the crosstalk level. When source and load terminations are not matched to the characteristic impedance of the signal trace, noise coupled onto the signal traces can ring back and forth along the length of the trace. Using a simple trapezoidal pulse to illustrate this phenomena, one can see from Figure 5 that the induced noise is almost twice as high when unmatched termination impedances are used. Figure 6 shows the effect that trace separation has on the crosstalk level. As in previous example, a simple trapezoidal pulse is used to illustrate this phenomena. Referring to Figure /98/$ IEEE 624

2 6 one can see that the trace closes to the power trace has more noise induced than the trace that is farther away. Figure 7 shows the effects of frequency. In this case, a sine wave is used for the driving function since the effects of frequency are easier to show. Referring to Figure 7, one can see that as the frequency is increased from 20 MHz to 50 MHz, the amount of noise that is coupled onto the signal traces increases. In real life, however, the DC-DC switching noise does not always have a simple pulse or sinusoidal shaped envelope. For example, the DC-DC converter shown in Figure 1, exhibited a damped sinusoidal shaped envelope. The spectral response of such a waveform exhibits a resonance where the spectral energy is significantly higher than at other frequencies. Such a waveform can be mathematically represented by a damped sinusoid whose voltage, V,, can be expressed as References [l] For example, FCC in the United States and EN regulations in Europe. [2] Analysis and Modeling of Power Distribution Networks and Plane Structures in Multichip Modules and Pi%,, Frank Y. Yan, Timothy K, Postel, and Lawrence M. Rubin, 1995 IEEE International Symposium on Electromagnetic Compatibility. [3] LC FDTD Modeling Code (see for further details) [4] INCASES EMC Workbench [5] Fourier Transforms and Their Physical Applications, D. C. Champeney, 1973, Academic Press, Inc. Y =Ae(%in(w n r t) (Em 1) Referring to Figure 8a, the values of the constants were determined to be A = 0.75, c1 = 50*106, and w, = 2 n: 170*106. The sin term produces a natural resonant frequency near 170 MHz. The Fourier transform of Eqn 1 is shown in Figure 8b, and clearly shows the resonant peak. [5] In this case, a simple RLC resonant shunt circuit, tuned to 170 MHz can be used to filter out the noise at this frequency. In Figure 8b, the spectrum of the damped sinusoid (peak) and the impedance of the shunt RLC filter are overlayed, showing how the two can cancel each other out. Figure 8c shows the modeling results when the properly tuned RLC filter is inserted into the model. In this case, the actual damped sinusoid model is used as the noise source on the power trace rather than the simplified pulse or sinusoids used earlier. Referring to Figure 8, one can see that with the notch filter, the amount of noise coupled from the power trace to the signal trace is reduced significantly. Summary and Conclusions Based on the analysis described herein, crosstalk between power and adjacent signal traces can be reduced using one or more of the following techniques: - Matching source and load termination impedances - Increasing source-to-susceptor separation distance - Decreasing coupling length - Band-limiting (filtering) the frequency of the noise source 625

3 Figure 1: Radiated Emissions from DC-DC Converter Switching Noise Coupling onto I/ 0 Traces Figure 2: Switching Transients on Output Figure 3: Detail of Switching Transients Figure 4: Model used to Evaluate Coupling Mechanisms 626

4 Power Tl T2 Figure 5: Crosstalk Effects when Unmatched Terminations are used Figure 6: Effects of Trace Separation on Crosstalk Figure 7: Effects of Increasing Source Frequency 627

5 ! AsMat e &t( Figure 8: A shunt RLC Notch Filter between the power trace and ground that is tuned to w can be used to filter out the damped sinusoid noise